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Article contents

Self and identity.

• Sanaz Talaifar Sanaz Talaifar Department of Psychology, University of Texas at Austin
•  and  William Swann William Swann Department of Psychology, University of Texas
• https://doi.org/10.1093/acrefore/9780190236557.013.242
• Published online: 28 March 2018

Active and stored mental representations of the self include both global and specific qualities as well as conscious and nonconscious qualities. Semantic and episodic memory both contribute to a self that is not a unitary construct comprising only the individual as he or she is now, but also past and possible selves. Self-knowledge may overlap more or less with others’ views of the self. Furthermore, mental representations of the self vary whether they are positive or negative, important, certain, and stable. The origins of the self are also manifold and can be considered from developmental, biological, intrapsychic, and interpersonal perspectives. The self is connected to core motives (e.g., coherence, agency, and communion) and is manifested in the form of both personal identities and social identities. Finally, just as the self is a product of proximal and distal social forces, it is also an agent that actively shapes its environment.

• self-concept
• self-representation
• self-knowledge
• self-perception
• self-esteem
• personal identity
• social identity

Introduction

The concept of the self has beguiled—and frustrated—psychologists and philosophers alike for generations. One of the greatest challenges has been coming to terms with the nature of the self. Every individual has a self, yet no two selves are the same. Some aspects of the self create a sense of commonality with others whereas other aspects of the self set it apart. The self usually provides a sense of consistency, a sense that there is some connection between who a person was yesterday and who they are today. And yet, the self is continually changing both as an individual ages and he or she traverses different social situations. A further conundrum is that the self acts as both subject and object; it does the knowing about itself. With so many complexities, coupled with the fact that people can neither see nor touch the self, the construct may take on an air of mysticism akin to the concept of the soul (Epstein, 1973 ).

Perhaps the most pressing, and basic, question psychologists must answer regarding the self is “What is it?” For the man whom many regard as the father of modern psychology, William James, the self was a source of continuity that gave individuals a sense of “connectedness” and “unbrokenness” ( 1890 , p. 335). James distinguished between two components of the self: the “I” and the “me” ( 1910 ). The “I” is the self as agent, thinker, and knower, the executive function that experiences and reacts to the world, constructing mental representations and memories as it does so (Swann & Buhrmester, 2012 ). James was skeptical that the “I” was amenable to scientific study, which has been borne out by the fact that far more attention has been accorded to the “me.” The “me” is the individual one recognizes as the self, which for James included a material, social, and spiritual self. The material self refers to one’s physical body and one’s physical possessions. The social self refers to the various selves one may express and others may recognize depending on the social setting. The spiritual self refers to the enduring core of one’s being, including one’s values, personality, beliefs about the self, etc.

This article focuses on the “me” that will be referred to interchangeably as either the “self” or “identity.” We define the self as a multifaceted, dynamic, and temporally continuous set of mental self-representations. These representations are multifaceted in the sense that different situations may evoke different aspects of the self at different times. They are dynamic in that they are subject to change in the form of elaborations, corrections, and reevaluations (Diehl, Youngblade, Hay, & Chui, 2011 ). This is true when researchers think of the self as a sort of scientific theory in which new evidence about the self from the environment leads to adjustments to one’s self-theory (Epstein, 1973 ; Gopnick, 2003 ). It is also true when researchers consider the self as a narrative that can be rewritten and revised (McAdams, 1996 ). Finally, self-representations are temporally continuous because even though they change, most people have a sense of being the same person over time. Further, these self-representations, whether conscious or not, are essential to psychological functioning, as they organize people’s perceptions of their traits, preferences, memories, experiences, and group memberships. Importantly, representations of the self also guide an individual’s behavior.

Some psychologists (e.g., behaviorists and more recently Brubaker & Cooper, 2000 ) have questioned the need to implicate a construct as nebulous as the self to explain behavior. Certainly an individual can perform many complex actions without invoking his or her self-representations. Nevertheless, psychologists increasingly regard the self as one of the most important constructs in all of psychology. For example, the percentage of self-related studies published in the field’s leading journal, the Journal of Personality and Social Psychology , increased fivefold between 1972 and 2002 (Swann & Seyle, 2005 ) and has continued to grow to this day. The importance of the self becomes evident when one considers the consequences of a sense of self that is interrupted, damaged, or absent. Epstein ( 1973 ) offers a case in point with an example of a schizophrenic girl meeting her psychiatrist:

Ruth, a five year old, approached the psychiatrist with “Are you the bogey man? Are you going to fight my mother? Are you the same mother? Are you the same father? Are you going to be another mother?” and finally screaming in terror, “I am afraid I am going to be someone else.” [Bender, 1950 , p. 135]

To provide a more commonplace example, children do not display several emotions we consider uniquely human, such as empathy and embarrassment, until after they have developed a sense of self-awareness (Lewis, Sullivan, Stanger, & Weiss, 1989 ). As Darwin has argued ( 1872 / 1965 ), emotions like embarrassment exist only after one has a developed a sense of self that can be the object of others’ attention.

The self’s importance also is evident when one considers that it is a pancultural phenomenon; all individuals have a sense of self regardless of where they are born. Though the content of self-representations may vary by cultural context, the existence of the self is universal. So too is the structure of the self. One of the most basic structural dimensions of the self involves whether the knowledge is active or stored.

Forms of Self-Knowledge

Active and stored self-knowledge.

Although i ndividuals accumulate immeasurable amounts of knowledge over their lifespans, at any given moment they can access only a portion of that knowledge. The aspects of self-knowledge held in consciousness make up “active self-knowledge.” Other terms for active self-knowledge are the working self-concept (Markus & Kunda, 1986 ), the spontaneous self-concept (McGuire, McGuire, Child, & Fujioka, 1978 ), and the phenomenal self (Jones & Gerard, 1967 ). On the other hand, “stored self-knowledge” is information held in memory that one can access and retrieve but is not currently held in consciousness. Because different features of the self are active versus stored at different times depending on the demands of the situation, the self can be quite malleable without eliciting feelings of inconsistency or inauthenticity (Swann, Bosson, & Pelham, 2002 ).

Semantic and Episodic Representations of Self-Knowledge

People possess both episodic and semantic representations of themselves (Klein & Loftus, 1993 ; Tulving, 1983 ). Episodic self-representations refer to “behavioral exemplars” or relatively brief “cartoons in the head” involving one’s past life and experiences. For philosopher John Locke, the self was built of episodic memory. For some researchers interested in memory and identity, episodic memory has been of particular interest because it is thought to involve re-experiencing events from one’s past, providing a person with content through which to construct a personal narrative (see, e.g., Eakin, 2008 ; Fivush & Haden, 2003 ; Klein, 2001 ; Klein & Gangi, 2010 ). Recall of these episodic instances happens together with the conscious awareness that the events actually occurred in one’s life (e.g., Suddendorf & Corballis, 1997 ).

Episodic self-knowledge may shed light on the individual’s traits or preferences and how he or she will or should act in the future, but some aspects of self-knowledge do not require recalling any specific experiences. Semantic self-knowledge involves memories at a higher level abstraction. These self-related memories are based on either facts (e.g., I am 39 years old) or traits and do not necessitate remembering a specific event or experience (Klein & Lax, 2010 ; Klein, Robertson, Gangi, & Loftus, 2008 ). Thus, one may consider oneself intelligent (semantic self-knowledge) without recalling that he or she achieved stellar grades the previous term (episodic self-knowledge). In fact, Tulving ( 1972 ) suggested that the two types of knowledge may be structurally and functionally independent of each other. In support of this, case studies show that damage to the episodic self-knowledge system does not necessarily result in impairment of the semantic self-knowledge system. Evaluating semantic traits for self-descriptiveness is associated with activation in brain regions implicated in semantic, but not episodic, memory. In addition, priming a trait stored in semantic memory does not facilitate recall of corresponding episodic memories that exemplify the semantic self-knowledge (Klein, Loftus, Trafton, & Fuhrman, 1992 ). The tenuous relationship between episodic and semantic self-knowledge suggests that only a portion of semantic self-knowledge arises inductively from episodic self-knowledge (e.g., Kelley et al., 2002 ).

Recently some researchers have questioned the importance of memory’s role in creating a sense of identity. For example, at least when it comes to perceptions of others, people perceive a person’s identity to remain more intact after a neurodegenerative disease that affects their memory than one that affects their morality (Strohminger & Nichols, 2015 ).

Conscious and Nonconscious Self-Knowledge (Sometimes Confused With Explicit Versus Implicit)

Individuals may be conscious, or aware, of aspects of the self to varying degrees in different situations. Indeed sometimes it is adaptive to have self-awareness (Mandler, 1975 ) and other times it is not (Wegner, 2009 ). Nonconscious self-representations can influence behavior in that individuals may be unaware of the ways in which their self-representations affect their behavior (Greenwald & Banaji, 1995 ). Some researchers have even suggested that individuals can be unconscious of the contents of their self-representations (e.g., Devos & Banaji, 2003 ). It is important to remember that consciousness refers primarily to the level of awareness of a self-representation, rather than the automaticity of a given representation (i.e., whether the representation is retrieved in an unaware, unintentional, efficient, and uncontrolled manner) (Bargh, 1994 ).

A key ambiguity in recent work on implicit self-esteem is defining its criterial attributes. One view contends that the nonconscious and conscious self reflect fundamentally distinct knowledge systems that arise from different learning experiences and have independent effects on thought, emotion, and behavior (Epstein, 1994 ). Another perspective views the self as a singular construct that may nevertheless show diverging responses on direct and indirect measures of self due to factors such as the opportunity and motivation to control behavioral responses (Fazio & Twoles-Schwen, 1999 ). While indirect measures such as the Implicit Association Test do not require introspection and as a result may tap nonconscious representations, this is an assumption that should be supported with empirical evidence (Gawronski, Hofmann, & Wilbur, 2006 ).

Issues of direct and indirect measurement are a key consideration in research on the implicit and explicit self. Indirect measures of self allow researchers to infer an individual’s judgment about the self as a result of the speed or nature of their responses to stimuli that may be more or less self-related (De Houwer & Moors, 2010 ). Some researchers have argued that indirect measures of self-esteem are advantageous because they circumvent self-presentational issues (Farnham, Greenwald, & Banaji, 1999 ), but other researchers have questioned such claims (Gawronski, LeBel, & Peters, 2007 ) because self-presentational strivings can be automatized (Paulhus, 1993 ).

Recent findings have raised additional questions regarding the validity of some key assumptions regarding research inspired by interest in implicit self-esteem (for a more optimistic take on implicit self-esteem, see Dehart, Pelham, & Tennen, 2006 ). Although near-zero correlations between individuals’ scores on direct and indirect measures of self (e.g., Bosson, Swann, & Pennebaker, 2000 ) are often taken to mean that nonconscious and conscious self-representations are distinct, other factors, such as measurement error and lack of conceptual correspondence, can cause these low correlations (Gawronski et al., 2007 ). Some researchers have also taken evidence of negligible associations between measures of implicit self-esteem and theoretically related outcomes to mean that such measures may not measure self-esteem at all (Buhrmester, Blanton, & Swann, 2011 ). A prudent strategy is thus to consider that indirect measures reflect an activation of associations between the self and other stimuli in memory and that these associations do not require conscious validation of the association as accurate or inaccurate (Gawronski et al., 2007 ). Direct measures, on the other hand, do require validation processes (Strack & Deutsch, 2004 ; Swann, Hixon, Stein-Seroussi, & Gilbert, 1990 ).

Global and Specific Self-Knowledge

Self-views vary in scope (Hampson, John, & Goldberg, 1987 ). Global self-representations are generalized beliefs about the self (e.g., I am a worthwhile person) while specific self-representations pertain to a narrow domain (e.g., I am a nimble tennis player). Self-views can fall anywhere on a continuum between these two extremes. Generalized self-esteem may be thought of as a global self-representation at the top of a hierarchy with individual self-concepts nested underneath in specific domains such as academic, physical, and social (Marsh, 1986 ). Individual self-concepts, measured separately, combine statistically to form a superordinate global self-esteem factor (Marsh & Hattie, 1996 ). When trying to predict behavior it is important not to use a specific self-representation to predict a global behavior or a global self-representation to predict a specific behavior (e.g., Donnellan, Trzesniewski, Robins, Moffitt, & Caspi, 2005 ; Swann, Chang-Schneider, & McClarty, 2007 ; Trzesniewski et al., 2006 ).

Actual, Possible, Ideal, and Ought Selves

The self does not just include who a person is in the present but also includes past and future iterations of the self. In addition, people tend to hold “ought” or “ideal” beliefs about the self. The former includes one’s beliefs about who they should be according to their own and others’ standards while the latter includes beliefs about who they would like to be (Higgins, 1987 ). In a related vein, possible selves are the future-oriented positive or negative aspects of the self-concept, selves that one hopes to become or fears becoming (Markus & Nurius, 1986 ). Some research has even shown that distance between one’s feared self and actual self is a stronger predictor of life satisfaction than proximity between one’s ideal self and actual self (Ogilvie, 1987 ). Possible selves vary in how far in the future they are, how detailed they are, and how likely they are to become an actual self (Oyserman & James, 2008 ). Many researchers have studied the content of possible selves, which can be as idiosyncratic as a person’s imagination is. The method used to measure possible selves (close-ended versus open-ended questions) will affect which possible selves are revealed (Lee & Oyserman, 2009 ). The content of possible selves is also socially and contextually grounded. For example, as a person ages, career-focused possible selves become less important while health-related possible selves become increasingly important (Cross & Markus, 1991 ; Frazier, Hooker, Johnson, & Kaus, 2000 ).

Researchers have been interested in not just the content but the function of possible selves. Thinking about successful possible selves is mood enhancing (King, 2001 ) because it is a reminder that the current self can be improved. In addition, possible selves may play a role in self-regulation. By linking present and future selves, they may promote desired possible selves and avoid feared possible selves. Possible selves may be in competition with each other and with a person’s actual self. For example, someone may envision one possible self as an artist and another possible self as an airline pilot, and each of these possible selves might require the person to take different actions in the present moment. Goal striving requires employing limited resources and attention, so working toward one possible self may require shifting attention and resources away from another possible self (Fishbach, Dhar, & Zhang, 2006 ). Possible selves may have other implications as well. For example, Alesina and La Ferrara ( 2005 ) show how expected future income affects a person’s preferences for economic redistribution in the present.

Accuracy of Self-Knowledge and Feelings of Authenticity

Most individuals have had at least one encounter with an individual whose self-perception seemed at odds with “reality.” Perhaps it is a friend who believes himself to be a skilled singer but cannot understand why everyone within earshot grimaces when he starts singing. Or the boss who believes herself to be an inspiring leader but cannot motivate her workers. One potential explanation for inaccurate self-views is a disjunction between episodic and semantic memories; the image of grimacing listeners (episodic memory) may be quite independent of the conviction that one is a skilled singer (semantic memory). Of course, if self-knowledge is too disjunctive with reality it ceases to be adaptive; self-views must be moderately accurate to be useful in allowing people to predict and navigate their worlds. That said, some researchers have questioned the desirability of accurate self-views. For example, Taylor and Brown ( 1988 ) have argued that positive illusions about the self promote mental health. Similarly, Von Hippel and Trivers ( 2011 ) have argued that certain kinds of optimistic biases about the self are adaptive because they allow people to display more confidence than is warranted, consequently allowing them to reap the social rewards of that confidence.

Studying the accuracy of self-knowledge is challenging because objective criteria are often scarce. Put another way, there are only two vantage points from which to assess a person: self-perception and the other’s perception of the self. This is true even of supposedly “objective” measures of the self. An IQ test is still a measure of intelligence from the vantage point of the people who developed the test. Both vantage points can be subject to error. For example, self-perceptions may be biased to protect one’s self-image or due to self-comparison to an inappropriate referent. Others’ perceptions may be biased because of a lack of cross-situational information about the person in question or lack of insight into that person’s motives. Because of the advantages and disadvantages of each vantage point, self-reports may be better for assessing some traits (e.g., those low in observability, like neuroticism) while informant reports may be better for others (such as traits high in observability, like extraversion) (Vazire, 2010 ).

Furthermore, self-perceptions and others’ perceptions of the self may overlap to varying degrees. The “Johari window” provides a useful way of thinking about this (Luft & Ingham, 1955 ). The window’s first quadrant consists of things one knows about oneself that others also know about the self (arena). The second quadrant includes knowledge one has about the self that others do not have (façade). The third quadrant consists of knowledge one does not have about the self but others do have (blindspot). The fourth quadrant consists of information about the self that is not known to oneself or to others (unknown).

Which of these quadrants contains the “true self”? If I believe myself to be kind, but others do not, who is right? Which is a reflection of the “real me”? One set of attempts to answer this question has focused on perceptions of authenticity. The authentic self (Johnson & Boyd, 1995 ) is alternatively termed the “true self” (Newman, Bloom, & Knobe, 2014 ), “real self” (Rogers, 1961 ), “intrinsic self” (Schimel, Arndt, Pyszczynski, & Greenberg, 2001 ), “essential self” (Strohminger & Nichols, 2014 ), or “deep self” (Sripada, 2010 ). Recent research has addressed both what aspects of the self other people describe as belonging to a person’s true self and how individuals judge their own authenticity.

Though authenticity has long been the subject of philosophical thought, only recently have researchers begun addressing the topic empirically, and definitional ambiguities abound (Knoll, Meyer, Kroemer, & Schroeder-Abe, 2015 ). Some studies use unidimensional measures that equate authenticity to feeling close to one’s true self (e.g., Harter, Waters, & Whitesell, 1997 ). A more elaborate and philosophically grounded approach proposes four necessary factors for trait authenticity: awareness (the extent of one’s self-knowledge, motivation to expand it, and ability to trust in it), unbiased processing (the relative absence of interpretative distortions in processing self-relevant information), behavior (acting consistently with one’s needs, preferences, and values), and relational orientation (valuing and achieving openness in close relationships) (Kernis, 2003 ). Authenticity is related to feelings of self-alienation (Gino, Norton, & Ariely, 2010 ). Being authentic is also sometimes thought to be equivalent to low self-monitoring (Snyder & Gangestad, 1982 )— someone who does not alter his or her behavior to accommodate changing social situations (Grant, 2016 ). Authenticity and self-monitoring, however, are orthogonal constructs; being sensitive to environmental cues can be compatible with acting in line with one’s true self.

Although some have argued that the ability to behave in a way that contradicts one’s feelings and mental states is a developmental accomplishment (Harter, Marold, Whitesell, & Cobbs, 1996 ), feelings of authenticity have been associated with many positive outcomes such as positive self-esteem, positive affect, and well-being (Goldman & Kernis, 2002 ; Wood, Linley, Maltby, Baliousis, & Joseph, 2008 ). One interesting line of research examines the interaction of authenticity, power, and well-being. Power can increase feelings of authenticity in social interactions (Kraus, Chen, & Keltner, 2011 ), and that increased authenticity in turn can result in higher well-being (Kifer, Heller, Peruvonic, & Galinsky, 2013 ). Another line of research examines the relationship between beliefs about authenticity (at least in the West) and morality. Gino, Kouchaki, and Galinsky ( 2015 ) suggest that dishonesty and inauthenticity share a similar source: dishonesty involves being untrue to others while inauthenticity involves being untrue to the self.

Metacognitive Aspects of Self

Valence and importance of self-views.

Self-knowledge may be positively or negatively valenced. Having more positive self-views and fewer negative ones are associated with having higher self-esteem (Brown, 1986 ). Both bottom-up and top-down theories have been used to explain this association. The bottom-up approach posits that the valence of specific self-knowledge drives the valence of one’s global self-views (e.g., Marsh, 1990 ). In this view, someone who has more positive self-views in specific domains (e.g., I am intelligent and attractive) should be more likely to develop high self-esteem overall (e.g., I am worthwhile). In contrast, the top-down perspective holds that the valence of global self-views drive the valence of specific self-views such that someone who thinks they are a worthwhile person is more likely to view him or herself as attractive and intelligent (e.g., Brown, Dutton, & Cook, 2001 ). The reasoning is grounded in the view that global self-esteem develops quite early in life and thus determines the later development of domain-specific self-views.

A domain-specific self-view can vary not only in its valence but also in its importance. Domain-specific self-views that one believes are important are more likely to affect global self-esteem than those self-views that one considers unimportant (Pelham, 1995 ; Pelham & Swann, 1989 ). As James wrote, “I, who for the time have staked my all on being a psychologist, am mortified if others know much more psychology than I. But I am contented to wallow in the grossest ignorance of Greek” ( 1890 / 1950 , p. 310). Of course not all self-views matter to the same extent for all people. A professor of Greek studies is likely to place a great deal of importance on his knowledge of Greek. Furthermore, changes to features that are perceived to be more causally central than others are believed to be more disruptive to identity (Chen, Urminsky, & Bartels, 2016 ). Individuals try to protect their important self-views by, for example, surrounding themselves with people and environments who confirm those important self-views (Chen, Chen, & Shaw, 2004 ; Swann & Pelham, 2002 ) or distancing themselves from close friends who outperform them in these areas (Tesser, 1988 ).

Certainty and Clarity of Self-Views

Individuals may feel more or less certain about some self-views as compared to others. And just as they are motivated to protect important self-views, people are also motivated to protect the self-views of which they are certain. People are more likely to seek (Pelham, 1991 ) and receive (Pelham & Swann, 1994 ) feedback consistent with self-views that are highly certain than those about which they feel less certain. They also actively resist challenges to highly certain self-views (Swann & Ely, 1984 ).

Another construct related to certainty is self-concept clarity. Not only are people with high self-concept clarity confident and certain of their self-views, they also are clear, internally consistent, and stable in their convictions about who they are (Campbell et al., 1996 ). The causes of low self-concept clarity have been theorized to be due to a discrepancy between one’s current self-views and the social feedback one has received in childhood (Streamer & Seery, 2015 ). Both high self-concept certainty and self-concept clarity are associated with higher self-esteem (Campbell, 1990 ).

Stability of Self-Views

The self is constantly accommodating, assimilating, and immunizing itself against new self-relevant information (Diehl et al., 2011 ). In the end, the self may remain stable (i.e., spatio-temporally continuous; Parfit, 1971 ) in at least two ways. First, the self may be stable in one’s absolute position on a scale. Second, there may be stability in one’s rank ordering within a group of related others (Hampson & Goldberg, 2006 ).

The question of the self’s stability can only be answered in the context of a specified time horizon. For example, like personality traits, self-views may not be particularly good predictors of behavior at a given time slice (perhaps an indication of the self’s instability) but are good predictors of behavior over the long term (Epstein, 1979 ). Similarly, more than others, some people experience frequent, transient changes in state self-esteem (Kernis, Cornell, Sun, Berry, & Harlow, 1993 ). Furthermore, there is a difference between how people perceive the stability of their self-views and the actual stability of their self-knowledge. Though previous research has explored the benefits of perceived self-esteem stability (e.g., Kernis, Paradise, Whitaker, Wheatman, & Goldman, 2000 ; Kernis, Grannemann, & Barclay, 1989 ), a recent program of research on fixed versus growth mindsets explores the benefits of the malleability of self-views in a variety of domains (Dweck, 1999 , 2006 ). For example, teaching adolescents to have a more malleable (i.e., incremental) theory of personality that “people change” led them to react less negatively to an immediate experience of social adversity, have lower overall stress and physical illness eight months later, and better academic performance over the school year (Yeager et al., 2014 ). Thus, believing that the self can be unstable can have positive effects in that one negative social interaction, or an instance of poor performance is not an indication that the self will always be that way, and this may, in turn, increase effort and persistence.

Organization of Self-Views

Though we have already touched on some aspects of the organization of self (e.g., specific self-views nested within global self-views), it is important to consider other aspects of organization including the fact that some self-views may be more assimilated within each other. Integration refers to the tendency to store both positive and negative self-views together, and is thought to promote resilience in the face of stress or adversity (Showers & Zeigler-Hill, 2007 ). Compartmentalization refers to the tendency to store positive and negative self-views separately. But both integration and compartmentalization can have positive and negative consequences and can interact with other metacognitive aspects of the self, like importance. For example, compartmentalization has been associated with higher self-esteem and less depression among people for whom positive components of the self are important (Showers, 1992 ). On the other hand, for those whose negative self-views are important, compartmentalization has been associated with lower self-esteem and higher depression.

Origins and Development of the Self

Developmental approaches.

Psychologists have long been interested in when and how infants develop a sense of self. One very basic question is whether selfhood in infancy is comparable to selfhood in adulthood. The answer depends on definitions of selfhood. For example, some researchers have measured and defined selfhood in infants as the ability to self-regulate and self-organize, which even animals can do. This definition bears little resemblance to selfhood in adulthood. Nativist and constructivist debates within developmental psychology (and language development in particular) that grapple with the problem of the origins of knowledge also have implications for understanding origins of the self. A nativist account (e.g., Chomsky, 1975 ) that considers the human mind to be innately constrained to formulate a very small set of representations would suggest that the mind is designed to develop a self. Information from the environment may form specific self-representations, but a nativist account would posit that the structure of the self is intrinsic. A constructivist account would reject the notion that there are not enough environmental stimuli to explain the development of a construct like the self unless one invokes a specific innate cognitive structure. Rather, constructivists might suggest that a child develops a theory of self in the same way scientific theories are developed (Gopnik, 2003 ).

Because infants and children cannot self-report their mental states as adults can, psychologists must use other methods to study the self in childhood. One method involves studying the development of children’s use of the personal pronouns “me,” “mine,” and “I” (Harter, 1983 ; Hobson, 1990 ). Another method that is used cross-culturally, mirror self-recognition (Lewis & Brooks-Gunn, 1979 ; Lewis & Ramsay, 2004 ), has been associated with brain maturation (Lewis, 2003 ) and myelination of the left temporal region (Carmody & Lewis, 2006 , 2010 ; Lewis & Carmody, 2008 ). Pretend play, which occurs between 15 and 24 months, is an indication that the self is developing because it requires the toddler’s ability to understand its own and others’ mental states (Lewis, 2011 ). The development of self-esteem has also historically been difficult to study due to a lack of self-esteem measures that can be used across the lifespan. Recently, psychologists have developed a lifespan self-esteem scale (Harris, Donnellan, & Trzesniewski, 2017 ) suitable for measuring global self-esteem from ages 5 to 93.

The development of theory of mind, the understanding that others have minds separate from one’s own, is also closely related to the development of the self. For example, people cannot make social comparisons until they have developed the required cognitive abilities, usually by middle childhood (Harter, 1999 ; Ruble, Boggiano, Feldman, & Loebl, 1980 ). Finally, developmental psychology is also useful in understanding the self beyond childhood and into adolescence and beyond. Adolescence is a time where goals of autonomy from parents and other adults become particularly salient (Bryan et al., 2016 ), adolescents experiment with different identities to see which fit best, and many long-term goals and personal aspirations are established (Crone & Dahl, 2012 ).

Biological Approaches

Biological approaches to understanding the origins of the self consider neurological, genetic, and hormonal underpinnings. These biological underpinnings are likely evolutionarily driven (Penke, Denissen, & Miller, 2007 ). Neuroscientists have debated the extent to which self-knowledge is “special,” or processed differently than other kinds of knowledge. What is clear is that no brain region by itself is responsible for our sense of self, but different aspects of the self-knowledge may be associated with different brain regions. Furthermore, the same region that is implicated in self-related processing can also be implicated in other types of processing (Ochsner et al., 2005 ; Saxe, Moran, Scholz, & Gabrieli, 2006 ). Specifically, the medial prefrontal cortex (mPFC) and the posterior cingulate cortex (PCC) have been associated with self-related processing (Northoff Heinzel, De Greck, Bermpohl, Dobrowolny, & Panksepp, 2006 ). But meta-analyses have found that the mPFC and PCC are recruited during the processing of both self-specific and familiar stimuli more generally (e.g., familiar others) (Qin & Northoff, 2011 ).

Twin studies of personality traits can shed light on the genetic bases of self. For example, genes account for about 40% to 60% of the population variance in self-reports of the Big Five personality factors (for a review, see Bouchard & Loehlin, 2001 ). Self-esteem levels also seem to be heritable, with 30–50% of population variance accounted for by genes (Kamakura, Ando, & Ono, 2007 ; Kendler, Gardner, & Prescott, 1998 ).

Finally, hormones are unlikely to be a cause of the self but may affect the expression of the self. For example, testosterone and cortisol levels interact with personality traits to predict different levels of aggression (Tackett et al., 2015 ). Differences in levels of and in utero exposure to certain hormones also affect gender identity (Berenbaum & Beltz, 2011 ; Reiner & Gearhart, 2004 ).

Intrapsychic Approaches

Internal processes, including self-perception and introspection, also influence the development of the self. One of the most obvious ways to develop knowledge about the self (especially when existing self-knowledge is weak) is to observe one’s own behavior across different situations and then make inferences about the aspects of the self that may have caused those behaviors (Bem, 1972 ). And just as judgments about others’ attributes are less certain when multiple possible causes exist for a given behavior, the same is true of one’s own behaviors and the amount of information they yield about the self (Kelley, 1971 ).

Conversely, introspection involves understanding the self from the inside outward rather than from the outside in. Though surprisingly little thought (only 8%) is expended on self-reflection (Csikszentmihalyi & Figurski, 1982 ), people buttress self-knowledge through introspection. For example, contemporary psychoanalysis can increase self-knowledge, even though an increase in self-knowledge on its own is unlikely to have therapeutic effects (Reppen, 2013 ). Writing is one form of introspection that does have psychological and physical therapeutic benefits (Pennebaker, 1997 ). Research shows that brief writing exercises can result in fewer physician visits (e.g., Francis & Pennebaker, 1992 ), and depressive episodes (Pennebaker, Kiecolt-Glaser, & Glaser, 1988 ), better immune function (e.g., Esterling, Kiecolt-Glaser, Bodnar, & Glaser, 1994 ), and higher grade point average (e.g., Cameron & Nicholls, 1998 ) and many other positive outcomes.

Experiencing the “subjective self” is yet another way that individuals gain self-knowledge. Unlike introspection, experiencing the subjective self involves outward engagement, a full engagement in the moment that draws attention away from the self (e.g., Csikszentmihalyi, 1990 ). Being attentive to one’s emotions and thoughts in the moment can reveal much about one’s preferences and values. Apparently, people rely more on their subjective experiences than on their overt behaviors when constructing self-knowledge (Andersen, 1984 ; Andersen & Ross, 1984 ).

Interpersonal Approaches

At the risk of stating the obvious, humans are social animals and thus the self is rarely cut off from others. In fact, many individuals would rather give themselves a mild electric shock than be alone with their thoughts (Wilson et al., 2014 ). The myth of finding oneself by eschewing society is dubious, and one of the most famous proponents of this tradition, transcendentalist Henry David Thoreau, actually regularly entertained visitors during his supposed seclusion at Walden (Schulz, 2015 ). As early as infancy, the reactions of others can lay the foundation for one’s self-views. Attachment theory (Bowlby, 1969 ; Hazan & Shaver, 1994 ) holds that children’s earliest interactions with their caregivers lead them to formulate schemas about their lovability and worth. This occurs outside of the infant’s awareness, and the schemas are based on the consistency and responsiveness of the care they receive. Highly consistent responsiveness to the infant’s needs provide the basis for the infant to develop feelings of self-worth (i.e., high global self-esteem) later in life. Though the mechanisms by which this occur are still being investigated, it may be that self-schemas developed during infancy provide the lens through which people interpret others’ reactions to them (e.g., Hazan & Shaver, 1987 ). Note, however, that early attachment relationships are in no way deterministic: 30–45% of people change their attachment style (i.e., their pattern of relating to others) across time (e.g., Cozzarelli, Karafa, Collins, & Tagler, 2003 ).

Early attachment relationships provide a working model for how an individual expects to be treated, which is associated with perceptions of self-worth. But others’ appraisals of the self are also a more direct source of self-knowledge. An extremely influential line of thought from sociology, symbolic interactionism (Cooley, 1902 ; Mead, 1934 ), emphasized the component of the self that James referred to as the social self. He wrote, “a man has as many social selves as there are individuals who recognize him and carry an image of him in their mind” ( 1890 / 1950 , p. 294). The symbolic interactionists proposed that people come to know themselves not through introspection but rather through others’ reactions and perceptions of them. This “looking glass self” sees itself as others do (Yeung & Martin, 2003 ). People’s inferences about how others view them become internalized and guide their behavior. Thus the self is created socially and is sustained cyclically.

Research shows, however, that reflected appraisals may not tell the whole story. While it is clear that people’s self-views correlate strongly with how they believe others see them, self-views are not necessarily perfectly correlated with how people actually view them (Shrauger & Schoeneman, 1979 ). Further, people’s self-views may inform how they believe others see them rather than the other way around (Kenny & DePaulo, 1993 ). Lastly, individuals are better at knowing how people see them in general rather than knowing how specific others view them (Kenny & Albright, 1987 ).

Though others’ perceptions of the self are not an individual’s only source of self-knowledge, they are an important source, and in more than one way. For example, others’ provide a reference point for “social comparison.” According to Festinger’s social comparison theory ( 1954 ), people compare their own traits, preferences, abilities, and emotions to those of similar others, making both upward and downward comparisons. These comparisons tend to happen spontaneously and effortlessly. The direction of the comparison influences how one views and feels about the self. For example, comparing the self to someone worse off boosts self-esteem (e.g., Helgeson & Mickelson, 1995 ; Marsh & Parker, 1984 ). In addition to increasing self-knowledge, social comparisons are also motivating. For instance, those undergoing difficult or painful life events can cope better when they make downward comparisons (Wood, Taylor, & Lichtman, 1985 ). When motivated to improve the self in a given domain, however, people may make upward comparisons to idealized others (Blanton, Buunk, Gibbons, & Kuyper, 1999 ). Sometimes individuals make comparisons to inappropriate others, but they have the ability (with mental effort) to undo the changes made to the self-concept as a result of this comparison (Gilbert, Giesler, & Morris, 1995 ).

Others can influence the self not only through interactions and comparisons but also when an individual becomes very close to a significant other. In this case, according to self-expansion theory (Aron & Aron, 1996 ), as intimacy increases, people experience cognitive overlap between the self and the significant other. People can acquire novel self-knowledge as they subsume attributes of the close other into the self.

Finally, the origins and development of the self are interpersonally influenced to the extent that our identities are dependent on the social roles we occupy (e.g., as mother, student, friend, professional, etc.). This will be covered in greater detail in the section on “The Social Self.” Here it is important simply to recognize that as the social roles of an individual inevitably change over time, so too does their identity.

Cultural Approaches

Markus and Kitayama’s seminal paper ( 1991 ) on differences in expression of the self in Eastern and Western cultures spawned an incredible amount of work investigating the importance of culture on self-construals. Building on the foundational work of Triandis ( 1989 ) and others, this work proposed that people in Western cultures see themselves as autonomous individuals who value independence and uniqueness more so than connectedness and harmony with others. In contrast to this individualism, people in the East were thought to be more collectivist, valuing interdependence and fitting in. However, the theoretical relationship between self-construals and the continuous individualism-collectivism variable have been treated in several different ways in the literature. Some have described individualism and collectivism as the origins of differences in self-construals (e.g., Gudykunst et al., 1996 ; Kim, Aune, Hunter, Kim, & Kim, 2001 ; Singelis & Brown, 1995 ). Others have considered self-construals as synonymous with individualism and collectivism (Oyserman, Coon, & Kemmelmeier, 2002 ; Taras et al., 2014 ) or have used individualism-collectivism at the individual level as an analog of the variable at the cultural level (Smith, 2011 ).

However, in contrast to perspectives that treat individualism and collectivism as a unidimensional variable (e.g., Singelis, 1994 ), individualism and collectivism have also been theorized to be multifaceted “cultural syndromes” that include normative beliefs, values, and practices, as well as self-construals (Brewer & Chen, 2007 ; Triandis, 1993 ). In this view, there are many ways of being independent or collectivistic depending on the domain of functioning under consideration. For example, a person may be independent or interdependent when defining the self, experiencing the self, making decisions, looking after the self, moving between contexts, communicating with others, or dealing with conflicting interests (Vignoles et al., 2016 ). These domains of functioning are orthogonal such that being interdependent in one domain does not require being interdependent in another. This multidimensional picture of individual differences in individualism and collectivism is actually more similar to Markus and Kitayama’s ( 1991 ) initial treatment aiming to emphasize cultural diversity and contradicts the prevalent unidimensional approach to cultural differences that followed.

Recent research has pointed out other shortcomings of this dichotomous approach. There is a great deal of heterogeneity among the world’s cultures, so simplifying all culture to “Eastern” and “Western” or collectivistic versus individualistic types may be invalid. Vignoles and colleagues’ ( 2016 ) study of 16 nations supports this. They found that neither a contrast between Western and non-Western, nor between individualistic and collectivistic cultures, sufficiently captured the complexity of cross-cultural differences in selfhood. They conclude that “it is not useful to characterize any culture as ‘independent’ or ‘interdependent’ in a general sense” and rather advocate for research that identifies what kinds of independence and interdependence may be present in different contexts ( 2016 , p. 991). In addition, there is a great deal of within culture heterogeneity in self-construals For example, even within an individualistic, Western culture like the United States, working-class people and ethnic minorities tend to be more interdependent (Markus, 2017 ), tempering the geographically based generalizations one might draw about self-construals.

Another line of recent research on the self in cultural context that has explored self-construals beyond the East-West dichotomy is the study of multiculturalism and individuals who are a member of multiple cultural groups (Benet-Martinez & Hong, 2014 ). People may relate to each of the cultures to which they belong in different ways, and this may in turn have important effects. For example, categorization, which involves viewing one cultural identity as dominant over the others, is associated positively with well-being but negatively with personal growth (Yampolsky, Amiot, & de la Sablonnière, 2013 ). Integration involves cohesively connecting multiple cultures within the self while compartmentalization requires keeping one’s various cultures isolated because they are seen to be in opposition. Each of these strategies has different consequences.

Finally, the influence of religion remains significant in many parts of the world (Georgas, van de Vijver, & Berry, 2004 ; Inglehart & Baker, 2000 ), and so religion is also an important source of differences in self-construal. These religious traditions provide answers to the question of how the self should relate to others. For example, Buddhism emphasizes the interdependence of all things and thus agency does not necessarily reside in individual actors. Moreover, for Buddhists the boundaries between the self and the other are insignificant, and in fact the self is thought to be impermanent (see Garfield, Nichols, Rai, Nichols, & Strohminger, 2015 ).

Motivational Properties of the Self

Need for communion, agency, and coherence.

Understanding what motivates people is one of social psychology’s core questions, and a variety of motives have been proposed. Three motives that are particularly important to self-processes are the need for communion (belonging and interpersonal connectedness), the need for agency (autonomy and competence), and the need for coherence (patterns and regularities). The needs for communion and agency are the foundations of many aspects of social behavior (Baumeister & Leary, 1995 ; Wiggins & Broughton, 1991 ). Among attitude researchers, constructs similar to communion and agency (i.e., warmth and competence) represent the two basic dimensions of attitudes (e.g., Abele & Wojciszke, 2007 ; Fiske, Cuddy, & Glick, 2007 ; Judd, James-Hawkins, Yzerbyt, & Kashima, 2005 ). Of even more relevance to the self, communion and agency correspond with the dual forms of self-esteem (e.g., Franks & Marolla, 1976 ; Gecas, 1971 ). That is, self-esteem can be broken down into two components: self-liking and self-competence (Tafarodi & Swann, 2001 ). Self-competence is an evaluation of one’s ability to bring about a desired outcome while the need for communion is an evaluation of one’s goodness, worth, and lovability. Each of these dimensions of self-esteem predicts unique outcomes (e.g., Bosson & Swann, 1999 ; Tafarodi & Vu, 1997 ).

Those who do not fulfill their communion needs have poorer physical outcomes such as relatively poor physical health, weakened immune functioning, and higher mortality rates (House, Landis, & Umberson, 1988 ; Uchino, Cacioppo, & Kiecolt-Glaser, 1996 ). As far as psychological outcomes, people who lack positive connections with others also experience greater loneliness (Archibald, Bartholomew, & Marx, 1995 ; Newcomb & Bentler, 1986 ), while those with rich social networks report higher levels of happiness and life satisfaction (Diener, Suh, Lucas, & Smith, 1999 ). People’s sense of autonomy also contributes to psychological well-being (Ryff, 1989 ) and encourages people to strive for high performance in domains they care about. Autonomy strivings can also be beneficial in that they contribute to people’s need for self-growth (e.g., Heine, Kitayama, & Lehman, 2001 ; Taylor, Neter, & Wayment, 1995 ).

Finally, a great deal of support exists for the notion that people have a fundamental need for psychological coherence or the need for regularity, predictability, meaning, and control (Guidano & Liotti, 1983 ; Heine, Proulx, & Vohs, 2006 ). Coherence is a distinct from consistency because it refers specifically to the consistency between a person’s enduring self-views and the other aspects of their psychological universe (English, Chen, & Swann, 2008 ). The coherence motive may be even more basic than the needs for communion and agency (Guidano & Liotti, 1983 ; Popper, 1963 ). That is, self-views serve as the lenses through which people perceive reality, and incoherence degrades the vision of reality that these lenses offer.

When people feel that their self-knowledge base is incoherent, they may not know how to act, and guiding action is thought to be the primary purpose of thinking in the first place (James, 1890 / 1950 ).

Self-Enhancement and Self-Verification Motives

Drawing on Prescott Lecky’s ( 1945 ) proposition that chronic self-views give people a strong sense of coherence, self-verification theory posits that people desire to be seen as they see themselves, even if their self-views are negative. Self-views can guide at least three stages of information processing: attention, recall, and interpretation. In addition, people act on the preference for self-confirmatory evaluations ensuring that their experiences reinforce their self-views. For example, just as those who see themselves as likable seek out and embrace others who evaluate them positively, so too do people who see themselves as dislikable seek out and embrace others who evaluate them negatively (e.g., Swann, Pelham, & Krull, 1989 ). The theory suggests that people both enter and leave relationships that fail to satisfy their self-verification strivings (Swann, De La Ronde, & Hixon, 1994 ), even divorcing people who they believe have overly positive appraisals of them (for a review, see Kwang & Swann, 2010 ). People may also communicate their identities visually through “identity cues” that enable others to understand and react accordingly to that identity (Gosling, 2008 ). People seek verification of their specific as well as global (self-views). They are especially inclined to seek self-verifying evaluations for self-views that are certain or important (Pelham & Swann, 1994 ; Swann & Pelham, 2002 ).

For the 70% of individuals with globally positive self-views (e.g., Diener & Diener, 1995 ), self-verification may look like self-enhancement strivings (Brown, 1986 ) in that it will compel people to seek and prefer positive feedback about the self. In fact, even people with negative self-views tend to self-enhance when they do not have the cognitive resources available to reflect on their self-views and compare it to the feedback available (Swann, Hixon, Stein-Seroussi, & Gilbert, 1990 ). In addition, people have a tendency to self-enhance before they self-verify (Swann et al., 1990 ). Other evidence for self-enhancement includes the tendency for people to view themselves as better than average, though this may be most likely for ambiguous traits that can describe a wide variety of behaviors because the evidence that people use to make self-evaluations is idiosyncratic (Dunning, Meyerowitz, & Holzberg, 1989 ).

It is important to remember in discussions of self-verification and self-enhancement that people do not seek to see themselves as they actually are but rather as they see themselves . As mentioned in the section on accuracy, this self-view may overlap to varying degrees with “reality” or others’ perceptions of the self.

The Social Self

Identity negotiation.

People’s self-views influence the kinds of relationships they will engage in, and people can take on numerous identities depending on the situation and relationship. Identity negotiation theory (Swann & Bosson, 2008 ) suggests that relationship partners establish “who is who” via ongoing, mutual, and reciprocal interactions. Once people establish a “working consensus” for what roles each person will take in the relationship (e.g., Swann & Bosson, 2008 ), their agreed-upon expectations help disconnected individuals collaborate toward common obligations and goals, with some commitment to each other. Identity negotiation processes help define relationships and serve as a foundation for organized social activity. The identities that people negotiate tend to align with their chronic self-views. People follow these identity-negotiating processes, albeit largely unintentionally, during each of several successive stages of social interaction. Identities only survive to the extent that they are nourished and confirmed by the social environment, so negotiating identities in relationships is one way an individual ensures the survival of their self-views.

Personal and Social Self-Knowledge

Researchers have historically distinguished between two types of identity: personal and social. Personal identity refers to those features of the self that distinguish us from others while social identity refers to features of the self that are a source of commonality with others, such as group memberships. Once formed, social identities have a powerful influence on thought and behavior (Tajfel, 1981 ; Tajfel & Turner, 1979 ). Social category memberships can influence a person’s self-definition as much or more than idiosyncratic personal attributes (Ray, Mackie, Rydell, & Smith, 2008 ). One version of social identity theory posits that people enter groups that they view as both positive and distinctive to bolster their self-views (e.g., Abrams & Hogg, 1988 ). Evidence shows that people display a strong ingroup bias, or tendency to favor their own group relative to outgroups (e.g., Brewer & Kramer, 1985 ; Tajfel, Billig, Bundy, & Flament, 1971 ). This bias, along with the outgroup homogeneity effect whereby people see outgroup members as more similar than ingroup members (Linville & Jones, 1980 ) facilitates people’s ability to dehumanize members of outgroups. Dehumanization, perceiving a person as lacking in human qualities, then allows for the justification and maintenance of intergroup prejudice and conflict (Cortes, Demoulin, Rodriguez, Rodriguez, & Leyens, 2005 ; Vaes, Paladino, Castelli, Leyens, & Giovanazzi, 2003 ).

Self-categorization theory, in contrast to emphasizing motivation as in social identity theory, stresses the perceptual processes that lead humans to categorize the world into “us” and “them” (Turner, 1985 ; Turner, Hogg, Oakes, Reicher, & Wetherell, 1987 ). Other approaches argue that social identities reduce uncertainty (e.g., Hogg, 2007 , 2012 ), make the world more coherent (e.g., Ellemers & Van Knippenberg, 1997 ), or protect people from the fear of death (Castano, Yzerbyt, Paladino, & Sacchi, 2002 ). Though these approaches emphasize cognitive aspects of group membership, group-related emotions are also an important component of social identity. For instance, intergroup emotions theory proposes that a person’s emotional reactions toward other social groups can change in response to situationally induced shifts in self-categorization (Mackie, Maitner, & Smith, 2009 ).

Whatever the nature of the motive that causes people to identify with groups, although group memberships are critical for survival, they can also place people in grave danger when they motivate extreme action on behalf of the group. Research on identity fusion, which occurs when the boundaries between one’s personal and social identities become porous, shows how strong alignment with a group can lead to fighting and dying for that group at great personal cost (Whitehouse, McQuinn, Buhrmester, & Swann, 2014 ). This occurs when people come to view members of their social group as family (Swann et al., 2014 ).

Some research has investigated how personal and social identities are cognitively structured (Reid & Deaux, 1996 ). The segregation model of identity assumes that social and personal attributes are distinct (Trafimow, Triandis, & Goto, 1991 ) while the integration model suggests that identities and attributes coexist in a limited set of cognitive structures. Deaux, Reid, Mizrahi, and Cotting ( 1999 ) suggest that what constitutes social versus personal identity should not be determined by the attribute itself but rather the function it is serving (i.e., connecting the self to other people or distinguishing the self from other people). Similarly, optimal distinctiveness theory (Brewer, 1991 ) argues that individuals have an inherent drive to identity with groups but an equally important drive to maintain their individuality. To cope, they strive to find a balance between these opposing forces by finding an identity that supports both the individual’s need for autonomy and affiliation.

For most people, gender and ethnicity are important social identities, and there is variation in the strength of people’s identification with these groups (Luhtanen & Crocker, 1992 ). In terms of how gender affects the expression of the self, girls are often socialized to prioritize the qualities that align them to others, while boys are taught to prioritize the qualities that distinguish and differentiate them from others (e.g., Spence, Deaux, & Helmreich, 1985 ). Moreover, women’s self-esteem tends to be connected more to their relational qualities, while men’s self-esteem is linked to their independent qualities (Josephs, Markus, & Tafarodi, 1992 ).

Though society has made great strides in allowing men and women to embrace identities of their own choosing (e.g., Cotter, Hermsen, & Vanneman, 2004 ), traditional social expectations about what it means to be a man or a women persist. For example, gender stereotypes have remained constant over the past thirty years even as women have made significant professional and political gains (Haines, Deaux, & Lofaro, 2016 ). These stereotypes remain entrenched for men as well. England ( 2010 ) argues that for the gender revolution to be complete, not only should traditionally male professions and domains be open to women but traditionally female domains should be increasingly occupied by men. This would help move society closer to attaining gender equality while signaling that traditionally female-dominated roles are equally valued.

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Why Identity Matters and How It Shapes Us

Sanjana is a health writer and editor. Her work spans various health-related topics, including mental health, fitness, nutrition, and wellness.

Dr. Sabrina Romanoff, PsyD, is a licensed clinical psychologist and a professor at Yeshiva University’s clinical psychology doctoral program.

Verywell / Zoe Hansen

Defining Identity

• What Makes Up a Person's Identity?

Identity Development Across the Lifespan

The importance of identity, tips for reflecting on your identity.

Your identity is a set of physical, mental, emotional, social, and interpersonal characteristics that are unique to you.

It encapsulates your core personal values and your beliefs about the world, says Asfia Qaadir , DO, a child and adolescent psychiatrist at PrairieCare.

In this article, we explore the concept of identity, its importance, factors that contribute to its development , and some strategies that can help you reflect upon your identity.

Your identity gives you your sense of self. It is a set of traits that distinguishes you from other people, because while you might have some things in common with others, no one else has the exact same combination of traits as you.

Your identity also gives you a sense of continuity, i.e. the feeling that you are the same person you were two years ago and you will be the same person two days from now.

Asfia Qaadir, DO, Psychiatrist

Your identity plays an important role in how you treat others and how you carry yourself in the world.

What Makes Up a Person's Identity?

These are some of the factors that can contribute to your identity:

• Physical appearance
• Physical sensations
• Emotional traits
• Life experiences
• Genetics 
• Health conditions
• Nationality
• Race  
• Social community 
• Peer group 
• Political environment
• Spirituality
• Sexuality 
• Personality
• Beliefs 
• Finances 

We all have layers and dimensions that contribute to who we are and how we express our identity.

All of these factors interact together and influence you in unique and complex ways, shaping who you are. Identity formation is a subjective and deeply personal experience.

Identity development is a lifelong process that begins in childhood, starts to solidify in adolescence, and continues through adulthood.

Childhood is when we first start to develop a self-concept and form an identity.

As children, we are highly dependent on our families for our physical and emotional needs. Our early interactions with family members play a critical role in the formation of our identities.

During this stage, we learn about our families and communities, and what values are important to them, says Dr. Qaadir. 

The information and values we absorb in childhood are like little seeds that are planted years before we can really intentionally reflect upon them as adults, says Dr. Qaadir.

Traumatic or abusive experiences during childhood can disrupt identity formation and have lasting effects on the psyche.

Adolescence

Adolescence is a critical period of identity formation.

As teenagers, we start to intentionally develop a sense of self based on how the values we’re learning show up in our relationships with ourselves, our friends, family members, and in different scenarios that challenge us, Dr. Qaadir explains.

Adolescence is a time of discovering ourselves, learning to express ourselves, figuring out where we fit in socially (and where we don’t), developing relationships, and pursuing interests, says Dr. Qaadir.

This is the period where we start to become independent and form life goals. It can also be a period of storm and stress , as we experience mood disruptions, challenge authority figures, and take risks as we try to work out who we are.

As adults, we begin building our public or professional identities and deepen our personal relationships, says Dr. Qaadir.

These stages are not set in stone, rather they are fluid, and we get the rest of our lives to continue experiencing life and evolving our identities, says Dr. Qaadir.

Having a strong sense of identity is important because it:

• Creates self-awareness: A strong sense of identity can give you a deep sense of awareness of who you are as a person. It can help you understand your likes, dislikes, actions, motivations, and relationships.
• Provides direction and motivation: Having a strong sense of identity can give you a clear understanding of your values and interests, which can help provide clarity, direction, and motivation when it comes to setting goals and working toward them.
• Enables healthy relationships: When you know and accept yourself, you can form meaningful connections with people who appreciate and respect you for who you are. A strong sense of identity also helps you communicate effectively, establish healthy boundaries, and engage in authentic and fulfilling interactions.
• Keeps you grounded: Our identities give us roots when things around us feel chaotic or uncertain, says Dr. Qaadir. “Our roots keep us grounded and help us remember what truly matters at the end of the day.”
• Improves decision-making: Understanding yourself well can help you make choices that are consistent with your values, beliefs, and long-term goals. This clarity reduces confusion, indecision, and the tendency to conform to others' expectations, which may lead to poor decision-making .
• Fosters community participation: Identity is often shaped by cultural, social, political, spiritual, and historical contexts. Having a strong sense of identity allows you to understand, appreciate, and take pride in your cultural heritage. This can empower you to participate actively in society, express your unique perspective, and contribute to positive societal change.

On the other hand, a weak sense of identity can make it more difficult to ground yourself emotionally in times of stress and more confusing when you’re trying to navigate major life decisions, says Dr. Qaadir.

Dr. Qaadir suggests some strategies that can help you reflect on your identity:

• Art: Art is an incredible medium that can help you process and reflect on your identity. It can help you express yourself in creative and unique ways.
• Reading: Reading peoples’ stories through narrative is an excellent way to broaden your horizons, determine how you feel about the world around you, and reflect on your place in it.
• Journaling: Journaling can also be very useful for self-reflection . It can help you understand your feelings and motivations better.
• Conversation: Conversations with people can expose you to diverse perspectives, and help you form and represent your own.
• Nature: Being in nature can give you a chance to reflect undisturbed. Spending time in nature often has a way of putting things in perspective.
• Relationships: You can especially strengthen your sense of identity through the relationships around you. It is valuable to surround yourself with people who reflect your core values but may be different from you in other aspects of identity such as personality styles, cultural backgrounds, passions, professions, or spiritual paths because that provides perspective and learning from others.

American Psychological Association. Identity .

Pfeifer JH, Berkman ET. The development of self and identity in adolescence: neural evidence and implications for a value-based choice perspective on motivated behavior . Child Dev Perspect . 2018;12(3):158-164. doi:10.1111/cdep.12279

Hasanah U, Susanti H, Panjaitan RU. Family experience in facilitating adolescents during self-identity development . BMC Nurs . 2019;18(Suppl 1):35. doi:10.1186/s12912-019-0358-7

Dereboy Ç, Şahin Demirkapı E, et al. The relationship between childhood traumas, identity development, difficulties in emotion regulation and psychopathology . Turk Psikiyatri Derg . 2018;29(4):269-278.

Branje S, de Moor EL, Spitzer J, Becht AI. Dynamics of identity development in adolescence: a decade in review . J Res Adolesc . 2021;31(4):908-927. doi:10.1111/jora.12678

Stirrups R.  The storm and stress in the adolescent brain .  The Lancet Neurology . 2018;17(5):404. doi:10.1016/S1474-4422(18)30112-1

Fitzgerald A. Professional identity: A concept analysis . Nurs Forum . 2020;55(3):447-472. doi:10.1111/nuf.12450

National Institute of Standards and Technology. Identity .

By Sanjana Gupta Sanjana is a health writer and editor. Her work spans various health-related topics, including mental health, fitness, nutrition, and wellness.

Research: What Is American Identity and Why Does It Matter?

Why Does the American Identity Matter?

The most important reason for understanding American identity is related to white racial identification. It may not be prevalent in U.S. political attitudes, but it’s still an issue. A survey from 2012 asked white respondents to indicate if whiteness represented the way they thought of themselves most of the time, as opposed to identifying themselves as Americans . One fifth of the survey’s white respondents said that they preferred the term white to American when identifying themselves.

How to Analyze American Identity

• There’s no such thing as a universal identity, especially for an omni-cultural country such as the USA.
• Everyone has their own understanding of what it means to be American today, as citizens come from different religious, ethnic, ideological, and geographical backgrounds.
• Explaining the concept of American identity calls for an inclusive approach based on solidarity.
• Depending on how you discuss the concept, an academic essay may require arguments on modern-day immigration and immigrant policies. How do they fit within the common understanding of American identity?

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Relative Identity

Identity is often said to be a relation each thing bears to itself and to no other thing (e.g., Zalabardo 2000). This characterization is clearly circular (“no other thing”) and paradoxical too, unless the notion of “each thing” is qualified. More satisfactory (though partial) characterizations are available and the idea that such a relation of absolute identity exists is commonplace. Some, however, deny that a relation of absolute identity exists. Identity, they say, is relative: It is possible for objects \(x\) and \(y\) to be the same \(F\) and yet not the same \(G\) (where \(F\) and \(G\) are predicates representing kinds of things—apples, ships, passengers—rather than merely properties of things—colors, shapes). In such a case “same” cannot mean absolute identity. For example, the same person might be two different passengers, since one person may be counted twice as a passenger. If to say that \(x\) and \(y\) are the same person is to say that \(x\) and \(y\) are persons and are (absolutely) identical, and to say that \(x\) and \(y\) are different passengers is to say that \(x\) and \(y\) are passengers and are (absolutely) distinct, we have a contradiction. Others maintain that while there are such cases of “relative identity”, there is also such a thing as absolute identity. According to this view, identity comes in two forms: trivial or absolute and nontrivial or relative (Gupta 1980). These maverick views present a serious challenge to the received, absolutist doctrine of identity. In the first place, cases such as the passenger/person case are more difficult to dismiss than might be supposed (but see below, §3). Secondly, the standard view of identity is troubled by many persistent puzzles and problems, some of recent and some of ancient origin. The relative identity alternative sheds considerable light on these problems even if it does not promise a resolution of them all.

A word about notation. In what follows, lower-case italic letters “\(x\)”, “\(y\)”, etc., are used informally either as variables (bound or free) or as (place-holders for) individual constants. The context should make clear which usage is in play. Occasionally, for emphasis or in deference to logical tradition, other expressions for individual constants are employed. Also, the use/mention distinction is not strictly observed; but again the context should resolve any ambiguity.

1. The Standard Account of Identity

2.1 the paradox of change, 2.2 chrysippus’ paradox, 2.3 the paradox of 101 dalmatians, 2.4 the paradox of constitution, 2.5 the ship of theseus paradox, 2.6 church’s paradox, 2.7 the theological paradox, 3. relative identity, 4.1 the paradox of change, 4.2 chrysippus’ paradox, 4.3 the paradox of 101 dalmatians, 4.4 the paradox of constitution, 4.5 the ship of theseus paradox, 4.6 church’s paradox, 4.7 the theological paradox, 5. absolute identity, 6. objections and replies, other internet resources, related entries.

[Note: The following material is somewhat technical. The reader may wish to casually review it now and return to it as needed, especially in connection with §5. The propositions \(\text{Ref}, \text{LL}, \text{Ref}', \text{LL}', \text{NI}\), and \(\text{ND}\) are identified in the present section and are referred to as such in the rest of the entry.]

Identity may be formalized in the language \(L\) of classical first-order logic (FOL) by selecting a two-place predicate of \(L\), rewriting it as “=”, and adopting the universal closures of the following two postulates:

where the formula \(\phi(x)\) is like the formula \(\phi(y)\) except for having occurrences of \(x\) at some or all of the places \(\phi(y)\) has occurrences of \(y\) (see Enderton 2000, for a precise definition). Ref is the principle of the reflexivity of identity and LL ( Leibniz’ Law ) is the principle of the indiscernibility of identicals . It says in effect that identical objects cannot differ in any respect. The other characteristic properties of identity, symmetry \((x = y \rightarrow y = x)\), and transitivity \((x = y \amp y = z \rightarrow x = z)\), may be deduced from Ref and LL. Any relation that is reflexive, transitive, and symmetric is called an “equivalence relation”. Thus, identity is an equivalence relation satisfying LL. But not all equivalence relations satisfy LL. For example, the relation x and y are the same size is an equivalence relation that does not satisfy LL (with respect to a rich language such as English).

Let \(E\) be an equivalence relation defined on a set \(A\). For \(x\) in \(A\), \([x]\) is the set of all \(y\) in \(A\) such that \(E(x, y)\); this is the equivalence class of x determined by E . The equivalence relation \(E\) divides the set \(A\) into mutually exclusive equivalence classes whose union is \(A\). The family of such equivalence classes is called “the partition of \(A\) induced by \(E\)”.

Now let \(A\) be a set and define the relation \(I(A,x,y)\) as follows: For \(x\) and \(y\) in \(A\), \(I(A,x,y)\) if and only if for each subset \(X\) of \(A\), either \(x\) and \(y\) are both elements of \(X\) or neither is an element of \(X\). This definition is equivalent to the more usual one identifying the identity relation on a set \(A\) with the set of ordered pairs of the form \(\langle x,x\rangle\) for \(x\) in \(A\). The present definition proves more helpful in what follows.

Suppose for the moment that we do not assign any special interpretation to the identity symbol. We treat it like any other two-place predicate. Let \(M\) be a structure for \(L\) and assume that Ref and LL are true in \(M\). Call the relation defined in \(M\) by the conjunction of Ref and LL “indiscernibility” (see Enderton 2000, for the definition of definability in a structure). There are three important points to note about the relationship between indiscernibility, and the relation \(I(A,x,y)\). First, indiscernibility need not be the relation \(I(A,x,y)\) (where \(A\) is the domain of the structure). It might be an equivalence relation \(E\) having the property that for some elements \(u,v\), of the domain, \(E(u,v)\) holds, although \(I(A,u,v)\) fails. Secondly, there is no way to “correct for” this possibility. There is no sentence or set of sentences that could be added to the list beginning with Ref and LL that would guarantee that indiscernibility coincides with \(I(A,x,y)\). This fact is usually expressed by saying that identity is not a first-order or “elementary” relation. (For a proof, see Hodges 1983.) However, in a language such as set theory (as usually interpreted) or second-order logic, in which there is a quantifier “all \(X\)” permitting quantification over all subsets of a given set, \(I(A,x,y)\) is definable (Freund 2019).

Third, given any structure \(M\) for \(L\) in which Ref and LL are true, there is a corresponding structure \(\textit{QM}\), the “quotient structure” determined by \(M\), in which indiscernibility does coincide with \(I(A,x,y)\). \(\textit{QM}\) is obtained in roughly the following way: Let the elements of \(\textit{QM}\) be the equivalence classes \([x]\), for elements \(x\) of \(M\) determined by indiscernibility in \(M\). If \(F\) is a one-place predicate true in \(M\) of some object \(x\) in \(M\), then define \(F\) to be true of \([x]\) in \(\textit{QM}\), and similarly for many-place predicates and constants. It can then be shown that any sentence true in \(M\) is true in \(\textit{QM}\), and vice versa. The existence of quotient structures makes it possible to treat the identity symbol as a logical constant interpreted in terms of \(I(A,x,y)\). There is in fact in general no other way to guarantee that Ref and LL will hold in every structure. (As Quine [1970] points out, however, a finite language will always contain a predicate satisfying Ref and LL in any structure; cf. Hodges 1983.) The alternative, however, is to view FOL with Ref and LL (FOL = ) as a proper theory in whose models (structures in which Ref and LL hold) there will be an equivalence relation \(E\) such that if \(E(x,y)\) holds, then \(x\) and \(y\) will be indiscernible with respect to the defined subsets of the domain. But we cannot in general assume that every subset of the domain is definable. If the domain is infinite, \(L\) runs out of defining formulas long before the domain runs out of subsets. Nonetheless, a strong metatheorem asserts that any set of formulas that has a model, has a countable (finite or denumerable) model. This means that the difference between indiscernibility and \(I(A,x,y)\) is minimized at least to the extent that, for a sufficiently rich language such as \(L\), the valid formulas concerning indiscernibility (i.e., the formulas true in every model of what is termed below “the pure \(L\)-theory with identity”) coincide with the valid formulas concerning \(I(A,x,y)\). (See Epstein 2001 for a sketch of a proof of this fact.) This is not to say, however, that there isn’t a significant difference between identity qua indiscernibility and identity qua \(I(A,x,y)\) (see below). Both points of view—that FOL = is a proper theory and that it is a logic—may be found in the literature (Quine 1970). The latter is the more usual view and it will count here as part of the standard account of identity.

Assume that \(L'\) is some fragment of \(L\) containing a subset of the predicate symbols of \(L\) and the identity symbol. Let \(M\) be a structure for \(L'\) and suppose that some identity statement \(a = b\) (where \(a\) and \(b\) are individual constants) is true in \(M\), and that Ref and LL are true in \(M\). Now expand \(M\) to a structure \(M'\) for a richer language—perhaps \(L\) itself. That is, assume we add some predicates to \(L'\) and interpret them as usual in \(M\) to obtain an expansion \(M'\) of \(M\). Assume that Ref and LL are true in \(M'\) and that the interpretation of the terms \(a\) and \(b\) remains the same. Is \(a = b\) true in \(M'\)? That depends. If the identity symbol is treated as a logical constant, the answer is “yes”. But if it is treated as a non-logical symbol, then it can happen that \(a = b\) is false in \(M'\). The indiscernibility relation defined by the identity symbol in \(M\) may differ from the one it defines in \(M'\); and in particular, the latter may be more “fine-grained” than the former. In this sense, if identity is treated as a logical constant, identity is not “language relative”; whereas if identity is treated as a non-logical notion, it \(is\) language relative. For this reason we can say that, treated as a logical constant, identity is “unrestricted”. For example, let \(L'\) be a fragment of \(L\) containing only the identity symbol and a single one-place predicate symbol; and suppose that the identity symbol is treated as non-logical. The formula

is then true in any structure for \(L'\) in which Ref and LL are true. The reason is that the unique one-place predicate of \(L'\) divides the domain of a structure into those objects it satisfies and those it does not. Hence, at least two of any group of three objects will be indiscernible. On the other hand, if the identity symbol is interpreted as \(I(A,x,y)\), this formula is false in any structure for \(L'\) with three or more elements.

If we do wish to view identity as a non-logical notion, then the phenomenon of language relativity suggests that it is best not to formalize identity using a single identity predicate “=”. Instead, we have the following picture: We begin with a language \(L\) and define an L-theory with identity to be a theory whose logical axioms are those of FOL and which is such that \(L\) contains a two-place predicate \(E_L\) satisfying the non-logical axiom \(\textrm{Ref}\,'\) and the non-logical axiom schema \(\textrm{LL}'\)

The pure \(L\)-theory with identity is the \(L\)-theory whose sole non-logical axiom is \(\textrm{Ref}\,'\) and whose sole non-logical axiom schema is \(\textrm{LL}'\).

Now the phenomenon of language relativity can be described more accurately as follows. Let \(L_1\) be a sublanguage of \(L_2\) and assume that \(T_1\) and \(T_2\) are, respectively, the pure \(L_1\)-theory with identity and the pure \(L_2\)-theory with identity. Let \(M_1\) and \(M_2\) be models of \(T_1\) and \(T_2\), respectively, having the same domain. Assume that \(a\) and \(b\) are individual constants having the same interpretation in \(M_1\) and \(M_2\). Let \(E_1\) and \(E_2\) be the identity symbols of \(L_1\) and \(L_2\). It can happen that \(E_1 (a,b)\) is true in \(M_1\) but \(E_2 (a,b)\) is false in \(M_2\). We can then say, with Geach (1967; see §4) and others, that the self-same objects indiscernible according to one theory may be discernible according to another.

There are two further philosophically significant features of the standard account of identity. First, identity is a necessary relation: If \(a\) and \(b\) are rigid terms (terms whose reference does not vary with respect to parameters such as time or possible world) then

Assuming certain modal principles, the necessity of distinctness (ND) follows from NI.

Note that the necessary truth of \(a = b\) does not imply the necessary existence of objects \(a\) or \(b\). We may assume that what a rigid term \(a\) denotes at a possible world (or moment of time) \(w\) need not exist in \(w\). Secondly, we do not ordinarily say things of the form “\(x\) is the same as \(y\)”. Instead, we say “\(x\) and \(y\) are the same person” or “\(x\) and \(y\) are the same book”. The standard view is that the identity component of such statements is just “\(x\) is the same as \(y\)”. For example, according to the standard view, “\(x\) and \(y\) are the same person” reduces to “\(x\) and \(y\) are persons and \(x\) is the same as \(y\)”, where the second conjunct may be formalized as in FOL = .

2. Paradoxes of Identity

The concept of identity, simple and settled though it may seem (as characterized by the standard account), gives rise to a great deal of philosophical perplexity. A few (by no means all) of the salient problems are outlined below. These are presented in the form of paradoxes—arguments from apparently undeniable premises to obviously unacceptable conclusions. The aim here is to make clear just what options are available to one who would stick close to the standard account. Often (but not always) little or no defense or critique of any particular option is offered. In the next section, we shall see what the relative identity alternative offers by comparison.

The most fundamental puzzle about identity is the problem of change. Suppose we have two photographs of a dog, Oscar. In one, \(A\), Oscar is a puppy, in the other, \(B\), he is old and gray-muzzled. Yet we hold that he is the same dog, in, it appears, direct violation of LL. More explicitly, \(B\) is a photograph of an old dog with a gray muzzle; \(A\) is a photograph of a young dog without a gray muzzle. \(A\) and \(B\) are photographs of the same dog. But according to LL, if the dog in \(B\) has a property (e.g., having a gray muzzle) that the dog in \(A\) lacks, then \(A\) and \(B\) are not photographs of the same dog. Contradiction.

Various solutions have been proposed. The most popular are the following two: (1) Simple properties such as having or lacking a gray muzzle are actually relations to times. Oscar has the property of lacking a gray muzzle at time t and the property of having a gray muzzle at (a later) \(t'\); but there is no incompatibility, since being thus and so related to time \(t\) and not being thus and so related to time \(t'\) are compatible conditions, and hence change involves no violation of LL. (2) Oscar is an object that is extended in time as well as space. The puppy Oscar and old gray-muzzled Oscar are distinct temporal parts or stages of the whole temporally extended Oscar. Both photographs are therefore not photographs of the whole Oscar at all because there cannot be still photographs of Oscar.

These proposals may seem plausible, and indeed most philosophers subscribe to one or other of them. The most common objections—that on the temporal parts account, objects are not “wholly present” at any given time, and that on the relations-to-times account, seemingly simply properties of objects, such as Oscar having a gray muzzle, are complicated relations—do little more than affirm what their targets deny. Yet the objections are an attempt to give voice to a strong intuition concerning our experience as creatures existing in time. Both (1) and (2) treat time and change from a “God’s eye” point of view. (1) presupposes time laid out “all at once”, so to speak, and similarly for (2). But we experience no such thing. Instead, while we are prepared to wait to see the whole of a baseball game we are watching, we are not prepared to wait to see the whole of a painting we are viewing.

The following paradox—a variation of the paradox of change—raises some new questions. It is due to the Stoic philosopher Chrysippus (c. 280 BCE – c. 206 BCE) and has been resurrected by Michael Burke (1994). In Chrysippus’s example, “Dion” is the name of a human and “Theon” is the name of the proper part of Dion that includes everything but his [Dion’s] left foot. Dion then loses his foot and Chrysippus wishes to know which of those objects—Dion or Theon—survives after Dion loses the foot. Burke’s example is to suppose that, at some point \(t'\) in the future, poor Oscar will lose his tail. He then considers the proper part of Oscar, as he is now (at \(t)\), consisting of the whole of Oscar minus his tail. Call this object “Oscar − ”. According to the standard account of identity, Oscar and Oscar − are distinct at \(t\) and hence, by ND, they are distinct at \(t'\). (Intuitively, Oscar and Oscar − are distinct at \(t'\) since Oscar has a property at \(t'\) that Oscar − lacks, namely, the property of having had a tail at \(t\). Notice that this argument involves a tacit appeal to ND—or NI, depending on how you look at it.) Hence, if both survive, we have a case of two distinct physical objects occupying exactly the same space at the same time. Assuming that is impossible, and assuming, as commonsense demands, that Oscar survives the loss of his tail, it follows that Oscar − does not survive. This conclusion is paradoxical because it appears that nothing happens to Oscar − in the interval between \(t\) and \(t'\) that would cause it to perish.

The first group of solutions does not take into consideration the amputation event that occurred at \(t'\) when they account for what exists—you may dub them untemporalized solutions. One extreme option in this category is to deny that there are such things as Oscar − . Undetached proper parts of objects don’t exist (van Inwagen 1981). A less extreme version allows for commonsense undetached parts, like tails or feet—of course Oscar − does not belong to this category, so it does not exist (Carmichael 2020). On the other hand, a more extreme option is to deny the existence of both Oscar and Oscar − —this may follow from some reductionist view like the one that only elementary particles exist. A third, less extreme, option is to insist that objects of different kinds, e.g., a clay statue and the piece of clay it is composed of, can occupy the same space at the same time, but objects of the same kind, e.g., two statues, cannot (Wiggins 1968; for refinements, see Oderberg 1996). So Oscar and Oscar − are different despite the fact that from \(t'\) onwards they occupy the same space. Guillon (2021) notes that there are three varieties of this colocationist view. First, one may maintain that they are different despite sharing all parts: every proper part of Oscar is a proper part of Oscar − and vice versa . The second colocationist variety allows for two different material objects to be sums of the same set of material objects—so Oscar and Oscar − may be different although they are ultimately composed of the same particles. Finally, one of them, most probably Oscar − , may be considered a proper part of the other. Obviously, these three options are meaningful only if one does not assume the full strength of general existentional mereology . The last global option is to claim that Oscar and Oscar − are two distinct temporally extended objects—a dog part, Oscar − , and a dog, Oscar, which overlaps at \(t'\). Temporal parts of distinct objects can occupy the same space at the same time.

The solutions in the second group, temporalized solutions, recognize the world’s ontological inventory has changed at \(t'\). For instance, Chrysippus, according to Philo of Alexandria, claimed that “it is necessary that Dion remains while Theon has perished” (Long and Sedley 1987, p. 172), so Dion exists throughout the whole period, but Theon dies at \(t'\). Accepting some kind of mereological essentialism (Chisholm 1973), one may hold an opposite view: since all parts are essential to Oscar, the amputation event killed Oscar while Oscar − either was created by the event or enjoyed existence throughout the whole period in question. Then you may maintain that the event at \(t'\) either (i) brought Oscar − into existence or (ii) did not disrupt Oscar − ’s prior existence (i.e., Oscar − existed at all times from \(t\) to \(t'\)). Finally, a proponent of contingent identity may claim that at t Oscar is different from Oscar − , but at \(t'\) they become identical.

(This paradox is also known as the paradox of 1001 cats; Geach 1980, Lewis 1993.) Focus on Oscar and Oscar − at \(t\)—before Oscar loses his tail. Is Oscar − a dog? When Oscar loses his tail the resulting creature is certainly a dog. Why then should we deny that Oscar − is a dog? We saw above that one possible response to Chrysippus’ paradox was to claim that Oscar − does not exist from \(t'\) on. But even if we adopt this view, how does it follow that Oscar − , existing as it does at \(t\), is not a dog? Yet if Oscar − is a dog, then, given the standard account of identity, there are two dogs where we would normally count only one. In fact, for each of Oscar’s hairs, of which there are at least 101, there is a proper part of Oscar—Oscar minus a hair—which is just as much a dog as Oscar − . There are then at least 101 dogs (and in fact many more) where we would count only one. Some claim that things such as dogs are “maximal”. No proper part of a dog is a dog (Burke 1993). One might conclude as much simply to avoid multiplying the number of dogs populating the space reserved for Oscar alone. But the maximality principle may seem to be independently justified as well. When Oscar barks, do all these different dogs bark in unison? If a thing is a dog, shouldn’t it be capable of independent action? Yet Oscar − cannot act independently of Oscar. Nevertheless, David Lewis (1993) has suggested a reason for counting Oscar − and all the 101 dog parts that differ (in various different ways) from one another and Oscar by a hair, as dogs, and in fact as Dalmatians (Oscar is a Dalmatian). Lewis invokes Unger’s (1980) “problem of the many”. Oscar sheds continuously but gradually. His hairs loosen and then dislodge, some such remaining still in place. Hence, within Oscar’s compass at any given time there are congeries of Dalmatian parts sooner or later to become definitely Dalmatians; some in a day, some in a second, or a split second. It seems arbitrary to proclaim a Dalmatian part that is a split second away from becoming definitely a Dalmatian, a Dalmatian, while denying that one a day away is a Dalmatian. As Lewis puts it, we must either deny that the “many” are Dalmatians, or we must deny that the Dalmatians are many. Lewis endorses proposals of both types but seems to favor one of the latter type according to which the Dalmatians are not many but rather “almost one”. In any case, the standard account of identity seems unable on its own to handle the paradox of 101 Dalmatians. It requires that we either deny that Oscar minus a hair is a dog—and a Dalmatian—or else that we must affirm that there is a multiplicity of Dalmatians, all but one of which is incapable of independent action and all of which bark in unison no more loudly than Oscar barks alone.

Suppose that on day 1 Jones purchases a piece of clay \(c\) and fashions it into a statue \(s_1.\) On day 2, Jones destroys \(s_1\), but not \(c\), by squeezing \(s_1\) into a ball and fashions a new statue \(s_2\) out of \(c\). On day 3, Jones removes a part of \(s_2\), discards it, and replaces it using a new piece of clay, thereby destroying \(c\) and replacing it by a new piece of clay, \(c'\). Presumably, \(s_2\) survives this change. Now what is the relationship between the pieces of clay and the statues they “constitute”? A natural answer is: identity. On day \(1, c\) is identical to \(s_1\) and on day \(2, c\) is identical to \(s_2\). On day \(3, s_2\) is identical to \(c'\). But this conclusion directly contradicts NI. If, on day \(1, c\) is (identical to) \(s_1\), then it follows, given NI, that on day \(2, s_1\) is \(s_2\) (since \(c\) is identical to \(s_2\) on day 2) and hence that \(s_1\) exists on day 2, which it does not. By a similar argument, on day \(3, c\) is \(c'\) (since \(s_2\) is identical to both) and so \(c\) exists on day 3, which it does not. We might conclude, then, that either constitution is not identity or that NI is false. Neither conclusion is wholly welcome. Once we adopt the standard account less NI, the latter principle follows directly from the assumption that individual variables and constants in quantified modal logic are to be handled exactly as they are in first-order logic. And if constitution is not identity, and yet statues, as well as pieces of clay, are physical objects (and what else would they be?), then we are again forced to affirm that distinct physical objects may occupy (exactly) the same space at the same time. The statue \(s_1\) and the piece of clay \(c\) occupy the same space on day 1. Even if this is deemed possible (Wiggins 1980), it is unparsimonious. The standard account is thus prima facie incompatible with the natural idea that constitution is identity.

Philosophers have not argued by direct appeal to NI or ND. Typically (e.g., Gibbard 1975, Noonan 1993, Johnston 1992), arguments that \(c\) and \(s_1\) are not identical run as follows: \(c\) exists prior to the existence of \(s_1\) and hence the two are not identical. Again, \(s_1\) possesses the property of being such that it will be destroyed by being squeezed into a ball, but \(c\) does not possess this property \((c\) will be squeezed into a ball but it will not thereby be destroyed). So again the two are not identical. Further, whatever the future in fact brings, c might have been squeezed into a ball and not destroyed. Since that is not true of \(s_1\), the two are not identical. On a careful analysis, however, each of these arguments can be seen to rely on NI or ND, provided one adopts the standard account of modal/temporal predicates. This last proviso suggests an interesting way out for one who adheres to the standard account of identity but who also holds that constitution is identity (see below).

Some philosophers find it important or at least expedient to frame the issue in terms of the case of a statue \(s\) and piece of clay \(c\) that coincide throughout their entire existence. We bring both \(c\) and \(s\) into existence by joining two other pieces of clay together, or we do something else that guarantees total coincidence. It seems that total coincidence is supposed to lend plausibility to the claim that, in such a case at least, constitution is identity (and hence NI is false—Gibbard 1975). It may do so, psychologically, but not logically. The same sorts of arguments as against the thesis that constitution is identity apply in such a case. For example, \(s\) may be admired for its aesthetic traits, even long after it ceases to exist, but this need not be true of \(c\). And \(s\) has the property, which \(c\) lacks, of being destroyed if squeezed into a ball. Those who defend the thesis that constitution is identity need to defend it in the general case of partial coincidence; and those who attack the thesis do so with arguments that work equal well against both total and partial coincidence. The assumption that \(s\) and \(c\) are totally coincident is therefore inessential.

The doctrine of temporal parts offers only limited help. The statement that \(c\) is identical to \(s_1\) on day 1 but identical to \(s_2\) on day 2 can be construed to mean that \(c\) is a temporally extended object whose day 1 stage is identical to \(s_1\) and whose day 2 stage is identical to \(s_2\). Since the two stages are not identical, NI does not apply. Similarly, we can regard \(s_2\) as a temporally extended object that overlaps \(c\) on day 2 and \(c'\) on day 3. But unless temporal parts theorists are prepared to defend a doctrine of modally extended objects—objects extended through possible worlds analogous to objects extended in time—there remains a problem. \(s_2\) might have been made of a different piece of clay, as is in fact the case on day 3. That is, it is logically possible for \(s_2\) to fail to coincide with the day 2 stage of \(c\). But it is not logically possible for the day 2 stage of \(c\) to fail to coincide with itself.

Lewis recognizes this difficulty and proposes to deal with it by appealing to his counterpart theory (Lewis 1971, 1986, and 1993). Different concepts, e.g., statue and piece of clay , are associated with different counterpart relations and hence with different criteria of trans-world identity. This has the effect of rendering modal predicates “Abelardian” (Noonan 1991, 1993). The property determined by a modal predicate may be affected by the subject term of a sentence containing the predicate. The subject term denotes an object belonging to this or that kind or sort. But different kinds or sorts may determine different properties (or different counterpart relations). In particular, the properties determined by the predicate “might not have coincided with \(c_2\)” (where \(c_2\) names the day 2 stage of \(c)\) in the following sentences,

• \(s_2\) might not have coincided with \(c_2\),
• \(c_2\) might not have coincided with \(c_2\),

are different , and hence (a) and (b) are compatible, even assuming that \(s_2\) and \(c_2\) are identical. (It should be emphasized that counterpart theory is not the only means of obtaining Abelardian predicates. See Noonan 1991.)

The upshot seems to be that the advocate of the standard account of identity must maintain either that constitution is not identity or that modal predicates are Abelardian. The latter option may be the fruitful one, since for one thing it seems to have applications that go beyond the issue of constitution.

Imagine a wooden ship restored by replacing all its planks and beams (and other parts) by new ones. Plutarch reports that such a ship was

… a model for the philosophers with respect to the disputed arguments … some of them saying it remained the same, some of them saying it did not remain the same. (Plutarch, Life of Theseus , 23; translated by Michael Rea in Rea 1995, p. 531).

Hobbes added the catch that the old parts are reassembled to create another ship exactly like the original. Both the restored ship and the reassembled one appear to qualify equally to be the original. In the one case, the original is “remodeled”, in the other, it is reassembled. Yet the two resulting ships are clearly not the same ship.

Some have proposed that in a case like this our ordinary “criteria of identity” fail us. The process of dismantling and reassembling usually preserves identity, as does the process of part replacement (otherwise no soldier could be issued just one rifle and body shops would function as manufacturers). But in this case the two processes produce conflicting results: We get two ships, one of which is the same ship as the original, by one set of criteria, and the other is the original ship, by another set of criteria. There is a similar conflict of criteria in the case of personal identity: Brain duplication scenarios (Wiggins 1967, Parfit 1984) suggest that it is logically possible for one person to split into two competitors, each with equal claim to be the original person. We take it for granted that brain duplication will preserve the psychological properties normally relevant to reidentifying persons and we also take it for granted that the original brain continues to embody these properties even after it is duplicated. In this sense there is a conflict of criteria. Such a case of “fission” gives us two distinct embodiments of these properties.

Perhaps we should conclude that identity is not what matters. Instead, what matters is some other relation, but one that accounts as readily as identity for such facts as that the owner of the original ship would be entitled to both the restored version and the reassembled one. For the case of personal identity, Parfit (1984) develops such a response in detail. A related reaction would be to claim that if both competitors have equal claim to be the original, then neither is the original. If, however, one competitor is inferior, then the other wins the day and counts as the original. It seems that on this view certain contingencies can establish or falsify identity claims. That conflicts with NI. Suppose that \(w\) is a possible world in which no ship is assembled from the discarded parts of the remodeled ship. In this world, then, the remodeled ship is the original. By NI, the restored ship and the original are identical in the actual world, contrary to the claim of the “best candidate” doctrine (which says that neither the remodeled nor the reassembled ship is the original). There are, however, more sophisticated “best candidate” theories that are not vulnerable to this objection (Nozick 1982).

Some are convinced that the remodeled ship has the best claim to be the original, since it exhibits a greater degree of spatio-temporal continuity with the original (Wiggins 1967). But it is unclear why the intuition that identity is preserved by spatio-temporal continuity should take precedence over the intuition that identity is preserved in the process of dismantlement and reassembly. Furthermore, certain versions of the ship of Theseus problem do not involve the feature that one of the ships competing to be the original possesses a greater degree of spatio-temporal continuity with the original than does the other (see below). Others are equally convinced that identity is not preserved by total part replacement. This view is often suggested blindly, as a stab in the dark, but there is in fact an interesting argument in its favor. Kripke (1980) argues that a table made out of a particular hunk of wood could not have been made out of a (totally) different hunk of wood. His reasoning is this: Suppose that in the actual world a table \(T\) is made out of a hunk of wood \(H\); and suppose that there is a possible world \(w\) in which this very table, \(T\), is made out of a different hunk of wood, \(H'\). Then assuming that \(H\) and \(H'\) are completely unrelated (for example, they do not overlap), so that making a table out of the one is not somehow dependent upon making a table out of the other, there is another possible world \(w'\) in which \(T\), as in the actual world, is made out of \(H\), and another table \(T'\), exactly similar to \(T\), is made out of \(H'\). Since \(T\) and \(T'\) are not identical in \(w'\), it follows by ND that the table made out of \(H'\) in \(w\) is not \(T\). Note, however, that the argument assumes that the table made out of \(H'\) in \(w'\) is the same table as the table made out of \(H'\) in \(w\).

Kripke’s reasoning can be applied to the present case (Kripke and others might dispute this claim; see below). Let \(w\) be a possible world just like the actual world in that \(O\), the original ship, is manufactured exactly as it is in the actual world. In \(w\), however, another ship, \(S'\), exactly similar to \(O\), is simultaneously built out of precisely the same parts that \(S\), the remodeled ship, is built out of in the actual world. Since \(S'\) and \(O\) are clearly different ships in \(w\), it follows by ND that \(O\) and \(S\) are not the same ship in the actual world. Note again that the argument assumes that \(S\) and \(S'\) are the same ship, but it seems quite a stretch to deny that. Nevertheless, some have done so. Carter (1987) claims (in effect) that \(S\) and \(S'\) are not identical, but his argument simply assumes that \(O\) and \(S\) are the same ship. Alternatively, one might view the (Kripkean) argument as showing only that while \(S\) is the same ship as \(O\) in the actual world, \(S\) (that is, \(S')\) is not the same ship as \(O\) in \(w\). But this is not an option for one who adheres to the standard account and hence adheres to ND. In defending this view, however, Gallois (1986, 1988) suggests a weakened notion of rigid designation and a corresponding weakened formulation of ND. (See Carter 1987 for criticism of Gallois’ proposals. See also Chandler 1975 for a precursor of Gallois’ argument.)

If we grant that \(O\) and \(S\) cannot be the same ship, we seem to have a solution to the ship of Theseus paradox. By the Kripkean argument, only the reassembled ship has any claim to being the original ship, \(O\). But this success is short-lived. For we are left with the following additional paradox: Suppose that \(S\) eventuates from \(O\) by replacing one part of \(O\) one day at a time. There seems to be widespread agreement that replacing just one part of a thing by a new exactly similar part preserves the identity of the thing. If so, then, by the transitivity of identity, \(O\) and \(S\) must be the same ship. It follows that either the Kripkean argument is incorrect, or replacement of even a single part (or small portion) does not preserve identity (a view known as “mereological essentialism”; Chisholm 1973).

As indicated, Kripke denies that his argument (for the necessity of origin) applies to the case of change over time: “The question whether the table could have changed into ice is irrelevant here” (Kripke 1972, p. 351). So the question whether \(O\) could change into \(S\) is supposedly “irrelevant”. But Kripke does not give a reason for this claim, and if cases of trans-temporal identity and trans-world identity differ markedly in relevant respects—respects relevant to Kripke’s argument for the necessity of origin—it is not obvious what they are. (But see Forbes 1985, and Lewis 1986, for discussion.) The argument above was simply that \(O\) and \(S\) cannot be the same ship since there is a possible world in which they differ. If this argument is incorrect it is no doubt because there are conclusive reasons showing that \(S\) and \(S'\) differ. Even so, such reasons are clearly not “irrelevant”. One may suspect that, if applied to the trans-temporal case, Kripke’s reasoning will yield an argument for mereological essentialism. Indeed, a trans-world counterpart of such an argument has been tried (Chandler 1976, though Chandler views his argument somewhat differently). In its effect, this argument does not differ essentially from the “paradox” sketched in the previous paragraph (which may well be viewed as an argument for mereological essentialism). Subsequent commentators, e.g., Salmon (1979) and Chandler (1975, 1976), do not seem to take Kripke’s admonition of irrelevance seriously.

In any case, there \(is\) a close connection between the two issues (the ship of Theseus problem and the question of the necessity of origin). This can be seen (though it may already be clear) by considering a modified version of the ship of Theseus problem. Suppose that when \(O\) is built, another ship \(O'\), exactly like \(O\), is also built. Suppose that \(O'\) never sets sail, but instead is used as a kind of graphic repair manual and parts repository for \(O\). Over time, planks are removed from \(O'\) and used to replace corresponding planks of \(O\). The result is a ship \(S\) made wholly of planks from \(O'\) and standing (in the end), we may suppose, in exactly the place \(O'\) has always stood. Now do \(O\) and \(O'\) have equal claim to be \(S\)? And can we then declare that neither is \(S\)? Not according to the Kripkean line of thought. It looks for all the world as though the process of “remodeling” \(O\) is really just an elaborate means of dismantling and reassembling \(O'\). And if \(O'\) and \(S\) are the same ship, then since \(O\) and \(O'\) are distinct, \(O\) and \(S\) cannot be the same ship.

This argument is vulnerable to the following two important criticisms: First, it conflicts with the common sense principle that (1) the material of an object can be totally replenished or replaced without affecting its identity (Salmon 1979); and secondly, as mentioned, it conflicts with the additional common sense principle that (2) replacement by a single part or small portion preserves identity. These objections may seem to provide sufficient grounds for rejecting the Kripkean argument and perhaps restricting the application of Kripke’s original argument for the necessity of origin (Noonan 1983). There is, however, a rather striking problem with (2), and it is unclear whether the conflict between (1) and the Kripkean argument should be resolved in favor of the former.

The problem with (2) is this. Pick a simple sort of objects, say, shoes, or better, sandals. Suppose \(A\) and \(B\) are two exactly similar sandals, one of which \((A)\) is brand new and the other \((B)\) is worn out. Each consists of a top strap and a sole, nothing more. If \(B\)’s worn strap is replaced by \(A\)’s new one, (2) dictates that the resulting sandal is \(B\) “refurbished”. In fact, if the parts of \(A\) and \(B\) are simply exchanged, (2) dictates that the sandal with the new parts, \(A'\), is \(B\) and the sandal with the old parts, \(B'\), is \(A\). It follows by ND that \(A\) and \(A'\) and \(B\) and \(B'\) are distinct. This is surely the wrong result. The intuition that \(A\) and \(A'\) are the same sandal is very strong; and the process of exchanging the parts of \(A\) and \(B\) seems to amount to nothing more than the dismantling and reassembling of each. This example is no different in principle than the more elaborate trans-world cases discussed by Chisholm (1967), Chandler (1976), Salmon (1979), or Gupta (1980). (One who claims that \(A\) and \(A'\) differ in that \(A'\) comes into existence after \(A\), does not have much to go on. \(A\) cannot be supposed to persist after \(A'\) comes into existence. We do not end up with two new sandals and one old one. Why then couldn’t it be \(A\) itself that reappears at the later time?)

The following paradox—perhaps the ultimate paradox of identity—derives from an argument of Church (1982). Suppose Pierre thinks that London and Londres are different cities, but of course doesn’t think that London is different from London, or that Londres is different from Londres. Assuming that proper names lack Fregean senses, we can apply LL to get the result that London and Londres are distinct. We have here an argument that, given the standard account of identity, merely thinking that \(x\) and \(y\) are distinct is enough to make them so. There are, of course, a number of ways around this conclusion without abandoning the standard account of identity. Church himself saw the argument (his version of it) as demonstrating the inadequacy of Russellian intensional logic—in which variables and constants operate as they do in extensional logic, i.e., unequipped with senses. (For another reaction, see Salmon 1986.) But there are strong arguments against the view that names (or variables) have senses (Kripke 1980). In light of these arguments, Church’s argument may be viewed as posing yet another paradox of identity.

The general form of Church’s argument has been exploited by others to reach further puzzling conclusions. For example, it has been used to show that there can be no such thing as vague or “indeterminate” identity (Evans 1978; and for discussion, Parsons 2000). For \(x\) is not vaguely identical to \(x\); hence, if \(x\) is assumed to be vaguely identical to \(y\), then by LL, \(x\) and \(y\) are (absolutely) distinct. As it stands, Evans’ argument shows at best that vaguely identical objects must be absolutely distinct, not that there is no such thing as vague identity. But some have tried to amend the argument to get Evans’ conclusion (Parsons 2000; and see the entry on vagueness). In any case, it is useful to see the connection between Evans’ argument and Church’s. If, for example, “vaguely identical” is taken to mean “thought to be identical”, then the two arguments collapse into one another. Church’s line of argument would seem to lead ultimately to the extreme antirealist position that any perceived difference among objects is a real difference. If one resolves not to attempt to escape the clutches of LL by some clever dodge—by disallowing straightforward quantifying-in, for example, as with the doctrine of Abelardian predicates—one comes quickly to the absurd conclusion that no statement of the form \(x = y\), where the terms are different, or are just different tokens of the same type, can be true. Yet it might just be that the fault lies not in ourselves, but in LL.

The Christian doctrine of Trinity is sometimes construed as a paradox that involves incongruent identity statements. For example, the Athanasian Creed puts it like this: “[…] we worship one God in Trinity. […] the Father is God; the Son is God; and the Holy Spirit is God. And yet they are not three Gods; but one God”. One may conceptually unbundle these statements into the the following set of claims:

• The Father is a God.
• The Son is a God.
• The Holy Spirit is a God.
• The Father is not identical with the Son.
• The Father is not identical with the Holy Spirit.
• The Son is not identical with the Holy Spirit.
• There is at most one God, i.e., if \(x\) is a God and \(y\) is a God, then \(x\) is identical with \(y\).

It is easy to see that this set is logically inconsistent and consequently one may claim that the doctrine itself is conceptually incongruent.

The fundamental claim of relative identity—the claim the various versions of the idea have in common—is that, as it seems in the passenger/person case, it can and does happen that \(x\) and \(y\) are the same \(F\) and (yet) \(x\) and \(y\) are not the same \(G\). Now it is usually supposed that if \(x\) and \(y\) are the same \(F\) (\(G\) etc.), then that implies that \(x\) and \(y\) are \(F\)s \((G\)s, etc.) If so, then the above schema is trivially satisfied by the case in which \(x\) and \(y\) are the same person but \(x\) (\(y\)) is not a passenger at all. But let us resolve to use the phrase “\(x\) and \(y\) are different \(G\)s” to mean “\(x\) and \(y\) are \(G\)s and \(x\) and \(y\) are not the same \(G\)”. Then the nontrivial core claim about relative identity is that the following may well be true:

RI is a very interesting thesis. It seems to yield dramatically simple solutions to (at least some of) the puzzles about identity. We appear to be in a position to assert that young Oscar and old Oscar are the same dog but nonetheless distinct “temporary” objects; that Oscar and Oscar − are the same dog but different dog parts; that the same piece of clay can be now (identical to) one statue and now another; that London and Londres are the same city but different “objects of thought”, and so forth. Doubts develop quickly, however. Either the same dog relation satisfies LL or it does not. If it does not, it is unclear why it should be taken to be a relation of identity . But if it satisfies LL, then it follows, given that Oscar and Oscar − are different dog parts, that Oscar − is not the same dog part as Oscar − . Furthermore, assuming that the same dog part relation is reflexive, it follows from the assumption that Oscar − and Oscar − are the same dog (and that LL is in force), that Oscar and Oscar − are indeed the same dog part, which in fact they are not.

It may seem, then, that RI is simply incoherent. These arguments, however, are a bit too quick. On analysis, they show only that the following three conditions form an inconsistent triad:

• RI is true (for some fixed predicates \(F\) and \(G)\).
• Identity relations are equivalence relations.
• The relation x and y are the same F figuring in (1) satisfies LL.

For suppose that the relation x and y are the same G , figuring in (1), is reflexive and that \(x\) is a \(G\). Then \(x\) is the same \(G\) as \(x\). But according to (1), \(x\) and \(y\) are not the same \(G\)s; hence, according to (3), it is not the case that \(x\) and \(y\) are the same \(F\); yet (1) asserts otherwise. Now, most relative identity theorists maintain that while identity relations are equivalence relations, they do not in general satisfy LL. However, according to at least one analysis of the passenger/person case (and others), the same person relation satisfies LL but the same passenger relation is not straightforwardly an equivalence relation (Gupta 1980). It should be clear though that this view is incompatible with the principle of the identity of indiscernibles : If \(x\) and \(y\) are different passengers, there must be, by the latter principle, some property \(x\) possesses that \(y\) does not. Hence if the same person relation satisfies LL, it follows that \(x\) and \(y\) are not the same person. For the remainder we will assume that identity relations are equivalence relations. Given this assumption (and assuming that the underlying propositional logic is classical—cf. Parsons 2000), RI and LL are incompatible in the sense that within the framework of a single fixed language for which LL is defined, RI and LL are incompatible.

Yet the advocate of relative identity cannot simply reject any form of LL. There are true and indispensable instances of LL: If \(x\) and \(y\) are the same dog, then, surely, if \(x\) is a Dalmatian, so is \(y\). The problem is that of formulating and motivating restricted forms of LL that are nonetheless strong enough to bear the burden of identity claims. There has been little systematic work done in this direction, crucial though it is to the relative identity project. (See Deutsch 1997 for discussion of this issue.) There are, however, equivalence relations that do satisfy restricted forms of LL. These are sometimes called “congruence relations” and they turn up frequently in mathematics. For example, say that integers \(n\) and \(m\) are congruent if their difference \(n - m\) is a multiple of 3. This relation preserves multiplication and addition, but not every property. The numbers 2 and 11 are thus congruent but 2 is even and 11 is not. There are also non-mathematical congruencies. For example, the relation x and y are traveling at the same speed preserves certain properties and not others. If objects \(x\) and \(y\) are traveling at the same speed and \(x\) is traveling faster than \(z\), the same is true of \(y\). Such similarity relations satisfy restricted forms of LL. In fact, any equivalence relation satisfies a certain minimal form of LL (see below).

There are strong and weak versions of RI. The weak version says that RI has some (in fact, many) true instances but also that there are predicates \(F\) such that if \(x\) and \(y\) are the same \(F\), then, for any equivalence relation, \(E\), whatsoever (whether or not an identity relation), \(E(x,y)\). This last condition implies that the relation x and y are the same F satisfies LL. The relation \(P\) defined so that \(P(x,y)\) if and only if \(H(x)\) and \(H(y)\), where \(H\) is some predicate, is an equivalence relation. Hence, if \(H\) holds of \(x\) but not of \(y\), there is an equivalence relation (namely, \(P(x,y))\) that fails to hold of \(x\) and \(y\). If we add that in this instance “\(x\) and \(y\) are the same \(F\)” is to be interpreted in terms of the relation \(I(A,x,y)\), then the weak version of RI says that there is such a thing as relative identity and such a thing as absolute identity as well. The strong version, by contrast, says that there are (many) true instances of RI but there is no such thing as absolute identity. It is difficult to know what to make of the latter claim. Taken literally, it is false. The notion of unrestricted identity (in the sense of “unrestricted” explained in §1) is demonstrably coherent. We return to this matter in §5.

The puzzles about identity outlined in §2 (and there are many others, as well as many variants of these) put considerable pressure on the standard account. A theory of identity that allows for instances of RI is an attractive alternative (see below §4). But there is a certain kind of example of RI, frequently discussed in the literature, that has given relative identity something of a bad name. The passenger/person example is a case in point. The noun “passenger” is derived from the corresponding relational expression “passenger in (on) …”. A passenger is someone who is a passenger in some vehicle (on some flight, etc.). Similarly, a father is a man who fathers someone or who is the father of someone. This way of defining a kind of things from a relation between things is perfectly legitimate and altogether open-ended. Given any relation \(R\), we can define “an \(R\)” to apply to anything \(x\) that stands in \(R\) to something \(y\). For example, we can define a “schmapple” to be an apple in a barrel. All this is fine. But we can’t infer from such a definition that the same apple might be two different schmapples. From the fact that someone is the father of two different children, we don’t judge that he is two different fathers. The fact that airlines choose to count passengers as they do, rather than track persons, is their business, not logic’s.

However, when \(R\) is an equivalence relation, we are entitled to such an inference. Consider the notorious case of “surmen” (Geach 1967). A pair of men are the “same surman” if they have the same surname; and a surman is a man who bears this relation to someone. So now it appears that two different men can be the same surman, since two different men can have the same surname. As Geach (1967) insists (also Geach 1973), surmen are defined to be men, so they are not merely classes of men. Hence we seem to have an instance of RI, and obviously any similarity relation (e.g., \(x\) and \(y\) have the same shape) will give rise to a similar case. Yet such instances of RI are not very interesting. It is granted all around that when “\(F\)” is adjectival, different \(G\)s may be the same \(F\). Different men may have the same surname, different objects the same color, etc. Turning an adjectival similarity relation into a substantival one having the form of an identity statement yields an identity statement in name only.

A word about the point of view of those who subscribe to the weak version of RI. The view (call it the “weak view”) is that ordinary identity relations concerning (largely) the world of contingency and change are equivalence relations answering to restricted forms of LL. The exact nature of the restriction depends on the equivalence relation itself, though there is an element of generality. The kinds of properties preserved by the same dog relation are intuitively the same kinds of properties as are preserved by the same cat relation. From a logical point of view the best that can be said is that any identity relation, like any equivalence relation, preserves a certain minimal set of properties. For suppose \(E\) is some equivalence relation. Let \(S\) be the set containing all formulas of the form \(E(x,y)\), and closed under the formation of negations, conjunctions, and quantification. Then \(E\) preserves any property expressed by a formula in \(S\). Furthermore, on this view, although absolutely distinct objects may be the same \(F\), absolutely identical objects cannot differ at all. Any instance of RI implies that \(x\) and \(y\) are absolutely distinct.

4. The Paradoxes Reconsidered

Let us look back at the paradoxes of identity outlined in §2 from the perspective of the weak view regarding relative identity. That view allows that absolutely distinct objects may be the same \(F\), but denies that absolutely identical objects can be different \(G\)’s. This implies that if \(x\) and \(y\) are relatively different objects, then \(x\) and \(y\) are absolutely distinct, and hence only pairs of absolutely distinct objects can satisfy RI. If \(x\) and \(y\) are absolutely distinct, we shall say that \(x\) and \(y\) are distinct “logical objects”; and similarly, if \(x\) and \(y\) are absolutely identical objects, then \(x\) and \(y\) are identical logical objects. The term “logical object” does not stand for some new and special kind of thing. Absolutely distinct apples, for example, are distinct logical objects.

The following is the barest sketch of relativist solutions to the paradoxes of identity discussed in §2. No attempt is made to fully justify any proposed solution, though a modicum of justification emerges in the course of §6. It should be kept in mind that some of the strength of the relativist solutions derives from the weaknesses of the absolutist alternatives, some of which are discussed in §2.

Young Oscar and old Oscar are the same dog but absolutely different things, i.e., different logical objects. The material conditions rendering young Oscar and old Oscar the same dog (and the same Dalmatian) are precisely the same as the material conditions under which young Oscar and old Oscar would qualify as temporal parts of the same dog. The only difference is logical . The identity relation between young Oscar and old Oscar can be formalized in an extensional logic (Deutsch 1997), but a theory of temporal parts requires a modal/temporal apparatus. Young Oscar is wholly present during his youth and possesses the simple, non-relational, property of not having a gray muzzle.

Oscar and Oscar − both survive Oscar’s loss of a tail. At both \(t\) and \(t'\) Oscar and Oscar − are the same dog, but at \(t\), Oscar and Oscar − are distinct logical objects. This implies (by ND) that Oscar and Oscar − are distinct logical objects even at \(t'\) Hence, we must allow that distinct logical objects may occupy the same space at the same time. This is not a problem, however. For although Oscar and Oscar − are distinct logical objects at \(t'\), they are physically coincident.

The relativist denies that dogs are “maximal”. It is not true that no proper part of a dog is dog. All the 101 (and more) proper parts of Oscar differing from him and from one another by a hair are dogs. In fact, many (though of course not all) identity preserving changes Oscar might undergo correspond directly to proper parts of (an unchanged) Oscar. But there is no problem about barking in unison, and no problem about acting independently. All 101 are the same dog, despite their differences, just as young Oscar and old Oscar are the same dog. The relativist denies that the dogs are many rather than deny that the many are dogs (Lewis 1993).

Constitution is identity, absolute identity. The relation between the piece of clay \(c\) and the statue \(s_1\) on day 1 is one of absolute identity. So we have that \(c = s_1\) on day 1, and for the same reason, \(c = s_2\) on day 2. Furthermore, since \(s_1\) and \(s_2\) are different statues, it follows (on the weak view) that \(s_1\ne s_2\). In addition, the piece of clay \(c\) constituting \(s_1\) on day 1 is (relatively) the same piece of clay as the piece of clay constituting \(s_2\) on day 2. (The identity is relative because we have distinct objects—the two statues—that are the same piece of clay.) It follows that no name of the piece of clay c can be a rigid designator in the standard sense . That is, no name of \(c\) denotes absolutely the same thing on day 1 and on day 2. For on day 1, a name of the piece of clay \(c\) would denote \(s_1\) and on day 2, it would denote \(s_2\), and \(s_1\) and \(s_2\) are absolutely distinct. Nevertheless, a name of the piece of clay may be relatively rigid : it may denote at each time the same piece of clay . Although no name of the piece of clay \(c\) is absolutely rigid, that does not prevent the introduction of a name of \(c\) that denotes \(c\) at any time (or possible world). (Kraut 1980 discusses a related notion of relative rigidity.)

There is, however, a certain ambiguity in the notion of a name of the piece of clay, inasmuch as the piece of clay may be any number of absolutely distinct objects. The notion of relative rigidity presupposes that a name for the piece of clay refers, with respect to some parameter \(p\), to whatever object counts as the piece of clay relative to that parameter. This may be sufficient in the case of the piece of clay, but in other cases it is not. With respect to a fixed parameter \(p\) there may be no unique object to serve as the referent of the name. For example, if any number of dog parts count, at a fixed time, as the same dog, then which of these objects serves as the referent of “Oscar”? We shall leave this question open for the time being but suggest that it may be worthwhile to view names such as “Oscar” as instantial terms—terms introduced into discourse by means of existential instantiation. The name “Oscar” might be taken as denoting a representative member of the equivalence class of distinct objects qualifying as the same dog as Oscar. It would follow, then, that most ordinary names are instantial terms. (An alternative is that of Geach 1980, who draws a distinction between a name of and a name for an object; see Noonan 1997 for discussion of Geach’s distinction.)

In this case, the relativist, as so far understood, may seem to enjoy no advantage over the absolutist. The problem is not clearly one of reconciling LL with ordinary judgments of identity, and the advantage afforded by RI does not seem applicable. Griffin (1977), for example, relying on RI, claims that the original and remodeled ship are the same ship but not the same collection of planks, whereas the reassembled ship is the same collection of planks as the original but not the same ship. This simply doesn’t resolve the problem. The problem is that the reassembled and remodeled ships have, prima facie , equal claim to be the original and so the bald claims that the reassembled ship is not—and the remodeled ship is—the original are unsupported. The problem is that of reconciling the intuition that certain small changes (replacement of a single part or small portion) preserve identity, with the problem illustrated by the sandals example of §2.5. It turns out, nevertheless, that the problem \(is\) one of dealing with the excesses of LL. To resolve the problem, we need an additional level of relativity. To motivate this development, consider the following abstract counterpart of the sandals example:

On the left there is an object \(P\) composed of three parts, \(P_1, P_2\), and \(P_3\). On the right is an exactly similar but non-identical object, \(Q\), composed of exactly similar parts, \(Q_1, Q_2\), and \(Q_3\), in exactly the same arrangement. For the sake of illustration, we adopt the rule that only replacement of (at most) a single part by an exactly similar part preserves identity. Suppose we now interchange the parts of \(P\) and \(Q\). We begin by replacing \(P_1\) by \(Q_1\) in \(P\) and replacing \(Q_1\) by \(P_1\) in \(Q\), to obtain objects \(P^1\) and \(Q^1\). So \(P^1\) is composed of parts \(Q_1,\) \(P_2,\) and \(P_3\), and Q\(^1\) is composed of parts \(P_1,\) \(Q_2,\) and \(Q_3\). We then replace \(P_2\) in \(P^1\) by \(Q_2\), to obtain \(P^2\), and so on. Given our sample criterion of identity, and assuming the transitivity of identity, \(P\) and \(P^3\) are counted the same, as are \(Q\) and \(Q^3\). But this appears to be entirely the wrong result. Intuitively, \(P\) and \(Q^3\) are the same, as are \(Q\) and \(P^3\). For \(P\) and \(Q^3\) are composed of exactly the same parts put together in exactly the same way, and similarly for \(Q\) and \(P^3\). Furthermore, \(Q_3 (P_3)\) can be viewed as simply the result of taking \(P (Q)\) apart and putting it back together in a slightly different location. And this last difference can be eliminated by switching the locations of \(P^3\) and \(Q^3\) as a last step in the process.

Suppose, however, that we replace our criterion of identity by the following more complicated rule: \(x\) and \(y\) are the same relative to z , if both \(x\) and \(y\) differ from \(z\) at most by a single part. (This relation is transitive, and is in fact an equivalence relation.) For example, relative to \(P\), \(P, P^1, Q^2\), and \(Q^3\) are the same, but \(Q, Q^1, P^2\) and \(P^3\), are not. Of course, replacement by a single part is an artificial criterion of identity. In actual cases, it will be a matter of the degree or kind of deviation from the original (represented by the third parameter, \(z)\). The basic idea is that identity through change is not a matter of identity through successive, accumulated changes—that notion conflicts with both intuition (e.g., the sandals example) and the Kripkean argument: Through successive changes objects can evolve into other objects. The three-place relation of identity does not satisfy LL and is consistent with the outlook of the relativist. Gupta (1980) develops a somewhat similar idea in detail. Williamson (1990) suggests a rather different approach, but one that, like the above, treats identity through change as an equivalence relation that does not satisfy LL.

Church’s argument implies that if Pierre’s doxastic position is as described (in §2.6), then London and Londres are distinct objects. Assuming the standard account of identity, the result is that either Pierre’s doxastic position cannot be as described or else London and Londres are different cities (or else we must punt). Since London and Londres are not different cities, the standard account entails that Pierre’s doxastic position cannot be as described (or else we must punt). This was Church’s own position as regards certain puzzles about synonymy, such as Mates’ puzzle (Mates 1952). Church held that one who believes that lawyers are lawyers, must indeed believe that lawyers are attorneys, despite any refusal to assent to (or desire to dissent from) “Lawyers are attorneys” (Church 1954). Kripke later argued (Kripke 1979) that assent and failure to assent must be taken at face value (at least in the case of Pierre) and Pierre’s doxastic position is as described. Kripke chose to punt—concluding that the problem is a problem for any “logic” of belief. The relativist concludes instead that (a) Pierre’s doxastic position is as described, (b) if so, London and Londres are distinct objects, and (c) London and Londres are nonetheless the same city. Whether this resolution of Church’s paradox can be exploited to yield solutions to Frege’s puzzle (Salmon 1986) or Kripke’s puzzle (1979) remains to be seen. Crimmins (1998) has suggested that the analysis of propositional attitudes requires a notion of “semantic pretense”. In reporting Pierre’s doxastic position we engage in a pretense to the effect that London and Londres are different cities associated with different Fregean senses. Crimmins’ goal is to reconcile (a), (c) and the following, (d): that the pure semantics of proper names (“London”, “Londres”) is Millian or directly referential (Kripke 1979). The relativist proposes just such a reconciliation but suggests that the pretense can be dropped.

The solution provided by RI is straightforward: we need to state the doctrine not in terms of absolute identity, but in terms of relative identity. So instead of one predicate of absolute identity the trinitarian creed may need two predicates of relative identity, e.g., being the same person and being the same being (cf. van Inwagen 1988, 2003), or, as suggested by (Anscombe and Geach 1961), being the same person and being the same God. Then we may reformulate the creed as follows:

• The Father is not the same person as the Son.
• The Father is not the same person as the Holy Spirit.
• The Son is not the same person as the Holy Spirit.
• There is at most one God, i.e., if \(x\) is a God and \(y\) is a God, then \(x\) is the same being as \(y\).

Van Inwagen produced a model (in the sense of formal semantics) where all these statements are satisfied, so under this interpretation the doctrine of Trinity comes out consistent. Incidentally let me note that he also gave an analogous interpretation for the doctrine of Incarnation (see van Inwagen 1994). For an extended discussion of the coherence of this solution see the section on relative identity theories in the entry on Trinity . Note that Branson (2019) provides a taxonomy of all possible solutions to the paradox, including the ones that resort to relative identity.

The philosopher P.T. Geach first broached the subject of relative identity and introduced the phrase “relative identity”. Over the years, Geach suggested specific instances of RI (a variant of the case of Oscar and his tail is due to Geach 1980) and in this way he contributed to the development of the weak view concerning relative identity, i.e., the view that while ordinary identity relations are often relative, some are not. But Geach maintains that absolute identity does not exist. What is his argument?

That is hard to say. Geach sets up two strawman candidates for absolute identity, one at the beginning of his discussion and one at the end, and he easily disposes of both. In between he develops an interesting and influential argument to the effect that identity, even as formalized in the system FOL = , is relative identity. However, Geach takes himself to have shown, by this argument, that absolute identity does not exist. At the end of his initial presentation of the argument in his 1967 paper, Geach remarks:

We thought we had a criterion for a predicable’s expressing strict identity [i.e., as Geach says, “strict, absolute, unqualified identity”]; but the thing has come apart in our hands; and no alternative rigorous criterion that could replace this one has thus far been suggested. (Geach 1967, p. 6 [1972, p. 241])

It turns out, as we’ll see, that all that comes apart is the false notion that in FOL = the identity symbol defines the relation \(I(A,x,y)\). Let us examine Geach’s line of reasoning in detail, focusing on the presentation in his 1967 article, the locus classicus of the notion of relative identity.

Geach begins by urging that a plain identity statement “\(x\) and \(y\) are the same” is in need of a completing predicate: “\(x\) and \(y\) are the same \(F\)”. Frege had argued that statements of number such as “this is one” require a completing predicate: “this is one \(F\)”, and so it is, Geach claims, with identity statements. This is a natural view for one who subscribes to RI. The latter cannot even be stated without the completing predicates. Nevertheless, both the claim itself and the analogy with Frege have been questioned. Some argue that the analogy with Frege is incorrect. For example, Carrara and Sacchi (2007) maintain that Frege’s position that any ascription of a number to something is always relative to a concept boils down to the claim that concepts are essential in cardinality statements because without them there is no specification of what is to be counted. Once this is settled, Frege would count by means of absolute identity. Others argued that while the analogy is correct, both Frege and Geach are wrong (Perry 1978 and Alston and Bennett 1984). These matters do not bear directly on the question of the coherence and truth of RI or the question of absolute identity. One who adopts the weak view would not want to follow Geach on this score. And one could maintain the “completing thesis” without being committed to RI. Furthermore, the completing thesis occupies a puzzling role in Geach’s dialectic. Immediately following his statement of the thesis, Geach formalizes FOL = on the basis of the single formula:

(The “W” is for Hao Wang, who first suggested it. The reader is invited to prove Ref and LL from W.) But we hear no complaint about the syntax of W despite its involving a seemingly unrelativized identity symbol. It turns out, however, that Geach apparently thinks of the completing predicate as being given by the whole descriptive apparatus of \(L\) or a fragment thereof.

Geach now observes

… if we consider a moment, we see that an I-predicable in a given theory T need not express strict, absolute, unqualified identity; it need mean no more than that two objects are indiscernible by the predicables that form the descriptive resources of the theory—the ideology of the theory …. (1967, p. 5 [1972, p. 240])

Here an “I-predicable” is a binary relation symbol “=” satisfying (W). Geach’s focus at this point is on the need to relativize an I-predicable to a theory \(T\). Geach then immediately saddles the friend of absolute identity with the view that for “real identity” we need not bring in the ideology of a definite theory. This is Geach’s first strawman. When logicians, in discussing FOL = , speak of “real identity”—and they often do (see Enderton 2000 or Silver 1994, for example)—they do not mean a relation of universal identity , since the universal set does not exist. Nor do they intend, in formulating LL, to use “true of” in a completely unrestrained way which gives rise to semantic paradox. It is no argument against those who wish to distinguish mere indiscernibility from real identity to say that they “will soon fall into contradictions”, e.g., Grelling’s or Russell’s. The relation \(I(A,x,y)\) is sufficiently relativized. (It is relativized to a set A .)

We come next to the main point:

Objects that are indiscernible when we are confined to the ideology of \(T\) may perfectly well be discernible in the ideology of a theory \(T^1\) of which \(T\) is a fragment. (Geach 1967, p. 5 [1972, p. 240])

The warrant for this claim can only be the language relativity of identity when treated as a non-logical notion (see §1). That this is what Geach has in mind is clear from some approving remarks he makes in his 1973 article about Quine’s (1970) proposal to treat identity as a non-logical notion. But how does it follow that absolute identity does not exist? Geach seems to think that the defender of absolute identity will look to Ref and LL (or W)—and not beyond—for a full account of “strict, absolute, unqualified” identity. That is not so. The fact that these formulas in themselves define only indiscernibility relations is a logical commonplace. So this is Geach’s second strawman.

Is Geach’s argument at least an argument that identity is relative? Does language relativity support the conclusion that RI is true even of identity as formalized in FOL = ? The general idea appears to be that language relativity suggests that we take identity to be indiscernibility, and conclude that objects identical relative to one ideology \(F\) may be different relative to another ideology \(G\), and that this confirms RI. Notice first of all that this argument relies on the identity of indiscernibles: that indiscernibility implies identity. This principle is not valid in FOL = even when the latter is treated as a proper theory. Language relativity does not imply that the distinctness of distinct objects cannot go unnoticed.

Secondly, the interesting cases of RI do not involve a shift from an impoverished point of view to an improved one—whether this is seen in epistemic terms (which Geach disputes—Geach 1973) or in purely logical terms. We do not affirm that old Oscar and young Oscar, for example, are the same dog on the grounds that there is an ideology with respect to which old Oscar and young Oscar are indistinguishable. Such an ideology would be incapable of describing any change in Oscar. It is true that the same dog relation determines a set of predicates that do not discriminate between the members of certain pairs of dogs—the dogs in the photographs mentioned earlier, for example. And it is true that these predicates determine a sublanguage in which the same dog relation is a congruence, i.e., no predicate of the sublanguage distinguishes \(x\) from \(y\), if \(x\) and \(y\) are the same dog. But the very sense of such statements as that old Oscar and young Oscar are the same dog requires a language in which a change in Oscar is expressible. We are talking, after all, about old Oscar and young Oscar. If we take seriously the idea that change involves the application of incompatible predicates, then the sublanguage cannot express the contrast between old Oscar and young Oscar.

Third, the phenomenon of language relativity (in the technical sense discussed in §1) has led many philosophers, including Geach, to the view that ideology creates ontology. There is no antecedently given domain of objects, already individuated, and waiting to be described. Instead, theories carve up the world in various ways, rendering some things noticeably distinct and others indiscernible, depending on a theory’s descriptive resources. The very notion of object is theory-bound (Kraut 1980). This sort of anti-realism may seem to go hand in hand with relative identity. Model theory, however, is realist to its core and language relativity is a model-theoretic phenomenon. It is a matter of definability (in a structure). Referring back to §1, in order to make sense of language relativity we have to start with a pair of distinct objects, \(a\) and \(b\) (distinct from the standpoint of the metalanguage), and hence a pair of objects we assume are already individuated. These objects, however, are indistinguishable in \(M\), since no formula of \(L'\) defines a subset of \(M\) containing the one object and not the other. When we move to \(M'\), we find that there is a formula of the enriched language that defines such a subset in \(M'\). Thus, language relativity is not really any sort of relativity of identity at all. We must assume that the objects \(a\) and \(b\) are distinct in order to describe the phenomenon. If we are living in \(M\), and suspect that Martians living in \(M'\) can distinguish \(a\) from \(b\), our suspicion is not merely to the effect that Martians carve things up differently than we do. Our own model theory tells us that there is more to it than that. Our suspicion must be to the effect that \(a\) and \(b\) are absolutely distinct. If we are blind to the difference between \(a\) and \(b\), but the Martians are not, then there must be a difference; and even if we are living in \(M\), we know there’s a difference, or at least we can suspect there is, since model theory tells us that such suspicion is well founded.

Let us go back to Geach’s remark that we “ need not ” interpret identity absolutely. While this is true, we need not interpret it as indiscernibility either. There are always the quotient structures (Quine 1963). Instead of taking our “reality” to be \(M\), and our “identity” to be indiscernibility in \(M\), we can move to the quotient structure, \(\textit{QM}\), whose elements are the equivalence classes, \([x]\), for \(x\) in \(M\). If \(x\) and \(y\) are indiscernible in \(M\), then in \(\textit{QM}\), \([x]\) and [\(y\)] are absolutely identical. We can do this even if we wish to treat FOL = as a proper theory. For example, suppose \(L'\) is a language in which people having the same income are indiscernible. The domain of \(M\) now consists of people. \(\textit{QM}\), however, consists of income groups, equivalence classes of people having the same income, and identity in \(\textit{QM}\) is absolute. Geach objects to such reinterpretation in terms of the quotient structures on the grounds that it increases the ontological commitments not only of \(L'\) but of any language of which \(L'\) is a sublanguage.

Let’s focus on \(L'\) first. From a purely model-theoretic point of view the question is moot. We cannot deny that \(\textit{QM}\) is a structure for \(L'\). Thus, \(L'\) is committed to people vis à vis one structure and to income groups vis à vis the corresponding quotient structure. But let’s pretend that the structures are “representations of reality”, and so the question now becomes: Which representation is preferable? Is there then any reason to prefer the ontology of \(M\) to that of \(\textit{QM}\)? \(M\) contains people but no sets of people, whereas \(\textit{QM}\) contains sets of people but no people. By Quine’s criterion of ontological commitment—that to be is to be the value of a variable—commitment to a set of objects does not carry a commitment to its elements. That is one of the odd consequences of Quine’s criterion. Unless there is some ontological reason to prefer people to sets of people (perhaps because sets are never to be preferred), the ontologies of \(M\) and \(\textit{QM}\) seem pretty much on a par. Both commit \(L'\) to one kind of thing.

Geach makes the additional claim that the ontological commitments of a sublanguage \(L'\) of a language \(L\) are inherited by \(L\) (Geach 1973). Suppose then that \(L\) is a language containing expressions for several equivalence relations defined on people: say, same income , same surname , and same job . Geach argues that \(L\) need only be committed to the existence of people. Things such as income groups, job groups (equivalence classes of people with the same job), and surmen can all be counted using the equivalencies, without bringing surmen, job groups, and income groups into the picture. Consider any sublanguage for which any one of these equivalence relations is a congruence, i.e., for which \(\textrm{LL}'\) holds. Pick the language, \(L_1\), for example, in which people having the same job are indiscernible. More precisely, we assume that \(T_1\) is the pure theory with identity whose ideology is confined to the language \(L_1\). Let \(M_1\) be a model of \(T_1\). We may imagine the domain of \(M_1\) to consist of people, and we can interpret indiscernibility in \(M_1\) to be the relation x and y have the same job . Geach would argue that if \(L_1\) is committed to the elements of \(\textit{QM}_1\)—the job groups—then so is \(L\). But that is not true. If \(T\) is a theory of the three distinct equivalence relations formulated in \(L\), the most \(T\) (or \(L)\) would be committed to are the partitions determined by the equivalence relations; and in any case, it would be perfectly consistent to insist that, whatever the ontological commitments of \(L_1\), reality, as described by \(L\), consists of people.

The foregoing considerations are rather abstract. To see more clearly what is at stake, let us focus on a specific example. Geach (1967) mentions that rational numbers are defined set-theoretically to be equivalence classes of integers determined by a certain equivalence relation defined on “fractions”, i.e., ordered pairs of integers (1/2 is \(\langle 1,2\rangle\), 2/4 is \(\langle 2,4\rangle\), etc.). He suggests that we can instead construe our theory of rational numbers to be about the fractions themselves, taking the I-predicable of our theory to be the following equivalence relation, \(E\):

This approach, Geach says, would have “the advantage of lightening a theory’s set-theoretical burdens. (In our present example, we need not bring in infinite sets of ordered pairs of integers into the theory of rationals.)” (Geach 1972, p. 249)

The first thing to notice about this example is that E cannot be the I-predicable of such a theory, since \(E\) is defined in terms of identity (look at the right side of R). It is “=” that must serve as the I-predicable, and it renders distinct ordered pairs of integers discernible. The moral is that not all equivalence relations can be drafted to do the job of identity, even given a limited ideology. There is, indeed, a plausible argument that any equivalence relation presupposes identity—not necessarily in the direct way illustrated by (R), but indirectly, nonetheless (see §6). Moreover, from the standpoint of general mathematics, once we have (R), we have the (infinite) equivalence classes it determines and the partition it induces. These are inescapable. Even from a more limited viewpoint, it seems that once we have enough set theory to give us ordered pairs of integers and the ability to define (R), we get the partition it induces as well.

Geach perceives an ontological advantage in relative identity; but his argument is unconvincing. Shifting to the quotient structures, as Quine suggested, does not induce a “baroque, Meinongian ontology” (Geach 1967, p. 10 [1972, p. 245]). In particular, the “home language” \((L)\) does not inherit the commitment of the fragment \((L_1)\), and the ontology of an arbitrary model of the pure theory of identity based on the latter language is at least no more various than that of the corresponding quotient model. There are, however, a number of ways in which relative identity does succeed in avoiding commitment to certain entities required by its absolute rival. These are discussed in the replies to objections 4 and 5 in the next section.

The following constitute a “start-up” set of objections and replies concerning relative identity and/or aspects of the foregoing account of relative identity and its rival. Time and space constraints prevent a more extended initial discussion. In addition, there is no presumption that the objections discussed below are the most important or that the initial replies to them are without fault. It is hoped that the present discussion will evolve into a more full-blown one, involving contributions by the author and readers alike. Should the discussion become lengthy, old or unchallenged objections and/or replies can be placed in the archives.

Objection 1 : “Relativist theories of identity, all of which are inconsistent with Leibniz’s principle [LL], currently enjoy little support. The doubts about them are (a) whether they really are theories of numerical identity, (b) whether they can be made internally consistent, and (c) whether they are sufficiently motivated” (Burke 1994, p. 133).

Reply : In reverse order: (c) The issues discussed in §2 and §4 surely provide sufficient motivation. (b) No proof of inconsistency has ever been forthcoming from opponents of relative identity, and in fact the weak view is consistent inasmuch as it has a model in the theory of similarity relations. The arguments outlined in the second paragraph of §3 are frequently cited as showing that relative identity is incoherent; but they show only that RI is incompatible with (unrestricted) LL. (a) See the replies to objections 2 and 3 below.

Objection 2 : If an identity relation obeys only a restricted form of LL—if it preserves only some properties and not all —then how do we tell which properties serve to individuate a pair of distinct objects?

Reply : Similarity relations satisfy only restricted forms of LL. How then do we tell which properties are preserved by the same shape relation and which are not? It is no objection to the thesis that identity relations in general preserve some properties and not others to demand to know which are which. At best the objection points to a problem we must face anyway (for the case of similarity). In general, a property is preserved by an equivalence relation if it “spreads” in an equivalence class determined by the relation: If one member of the class has the property, then every member does. Every property spreads in a singleton, as absolute identity demands.

Objection 3 : If identity statements are mere equivalencies, what distinguishes identity from mere similarity?

Reply : The distinction between identity and similarity statements (or sentences) is usually drawn in terms of the distinction between substantival and adjectival common nouns. If \(F\) is a common noun standing for a kind of things , e.g., “horse”, then “\(x\) and \(y\) are the same \(F\)” is a statement of identity, whereas if \(F\) is an a common noun standing for a kind of properties of things, then “\(x\) and \(y\) are the same \(F\)” is a statement of similarity. (It’s interesting to note that when the noun is proper, i.e., a proper name, the result is a statement of similarity, not identity—as in “He”s not the same Bill we knew before’.) This distinction rests ultimately on the metaphysical distinction between substance and attribute, object and property. While the distinction no doubt presupposes the concept of individuation (the bundle theory, for example, presupposes that we have the means to individuate properties), there is no obvious reason to suppose that it entails the denial of RI, i.e., the claim that no instance of RI is true. For a beginner’s review—from an historical perspective—of the issues concerning substance and attribute, see O’Connor (1967); and for more recent and advanced discussion and bibliography, see the entry on properties .

Objection 4 : Consider the following alleged instance of RI:

“If ‘\(A\)’ and ‘\(B\)’ refer to the same objects throughout (1), the first conjunct of (1) is not an identity statement, and the counterexample (to the thesis that no instance of RI is true) fails. If both conjuncts are identity statements in the required sense, ‘\(A\)’ and ‘\(B\)’ must refer to word types in the first conjunct and word tokens in the second, and the counterexample fails” (Perry 1970, p. 189).

Reply : First, if “in the required sense” means “satisfies LL”, then the objection buys correctness only at the price of begging the question. Advocates of relative identity will maintain that the relation A is the same word type as \(B\) is an identity relation, defined on tokens, that does not satisfy LL.

Secondly, even if one insists that in this case intuition dictates that if \(A\) and \(B\) refer to tokens in both conjuncts of (1), then “\(A\) is the same word type as \(B\)” expresses only the similarity relation: A and B are tokens of the same type , there are other cases where, intuitively, both conjuncts of RI involve identity relations and yet the relevant terms all refer to the same kind of things; for example,

as said of young Oscar and old Oscar. Here there is no temptation to suppose that the relation A and B are the same dog is not an identity relation. One may invoke a theory—a theory of temporal parts, for example—that construes the relation as a certain kind of similarity, but that is theory, not pretheoretical intuition. It is no objection to the relativist’s theory , which holds in part that “\(A\) and \(B\) are the same dog” expresses a relation of primitive identity, that there is an alternative theory according to which it expresses a similarity relation obtaining between two temporal parts of the same object. Furthermore, in the case of (2), \(A\) and \(B\) refer, again intuitively, to the same things in both conjuncts.

Third, there are cases in which the relative identity view does possess an ontological advantage. Consider

Suppose \(A\) and \(B\) are understood to refer to one sort of thing—pieces of clay—in the first conjunct and another—statues—in the second conjunct. Assume that the piece of clay \(c\) denoted by \(A\) in the first conjunct constitutes, at time \(t\), the statue \(s.\) Then assuming that statues are physical objects, there are two distinct physical objects belonging to different kinds occupying the same space at \(t\). Some, notably Wiggins (1980), hold that this is entirely possible: Distinct physical objects may occupy the same space at the same time, provided they belong to different kinds. The temporal parts doctrine supports and encourages this view. A statue may be a temporal part of a temporally extended piece of clay. But one statue, it seems, cannot be a temporal part of another. Even so, however, the duality of constituter and thing constituted is unparsimonious (cf. Lewis 1993), and the relativist is not committed to it.

Again, consider

One can say that in the first conjunct, \(A\) and \(B\) refer to books (absolutely the same book), whereas in the second conjunct, \(A\) and \(B\) refer to (absolutely distinct) copies. But the alleged duality of books and copies of books is unparsimonious and the relativist is not committed to it. There is no reason to concede to the philosopher that we do not actually purchase or read books ; instead we purchase and read only copies of books. Any copy of a book is just as much the “book itself” as is any other copy. Any copy of a book is the same book as any other copy. Nelson Goodman once remarked that “Any accurate copy of the text of a poem or novel is as much the original work as any other” (Goodman 1968, p. 114). Goodman was not suggesting that the distinction between poem and copy collapses. If it does collapse, however, we have an explanation of why any accurate copy is as much the original work as any other: any such copy is the same work as any other.

Objection 5 : Geach remarks that “[a]s for our recognizing relative identity predicables: any equivalence relation … can be used to specify a criterion of relative identity” (Geach 1972, p. 249). But §3 above contains a counterexample. Some equivalence relations are defined in terms of the I-predicable of a theory and hence cannot serve as such. (Any pair of I-predicables for a fixed theory are equivalent.) In fact it seems that any equivalence relation presupposes identity (cf. McGinn 2000). For example, the relation x and y are the same color presupposes identity of colors, since it means that there are colors \(C\) and \(C'\) such that \(x\) has \(C\) and \(y\) has \(C'\), and \(C = C'\). Identity, therefore, is logically prior to equivalence.

Reply: This is a good objection. It does seem to show, as the objector says, that identity is logically prior to ordinary similarity relations. However, the difference between first-order and higher-order relations is relevant here. Traditionally, similarity relations such as x and y are the same color have been represented, in the way indicated in the objection, as higher-order relations involving identities between higher-order objects (properties). Yet this treatment may not be inevitable. In Deutsch (1997), an attempt is made to treat similarity relations of the form “\(x\) and \(y\) are the same \(F\)” (where \(F\) is adjectival) as primitive, first-order, purely logical relations (see also Williamson 1988). If successful, a first-order treatment of similarity would show that the impression that identity is prior to equivalence is merely a misimpression—due to the assumption that the usual higher-order account of similarity relations is the only option.

Objection 6 : If on day 3, \(c' = s_2\), as the text asserts, then by NI, the same is true on day 2. But the text also asserts that on day 2, \(c = s_2\); yet \(c \ne c'\). This is incoherent.

Reply: The term \(s_2\) is not an absolutely rigid designator and so NI does not apply.

Objection 7 : The notion of relative identity is incoherent: “If a cat and one of its proper parts are one and the same cat, what is the mass of that one cat?” (Burke 1994, p. 138).

Reply : Young Oscar and Old Oscar are the same dog, but it makes no sense to ask: “What is the mass of that one dog”? Given the possibility of change, identical objects may differ in mass. On the relative identity account, that means that distinct logical objects that are the same \(F\) may differ in mass—and may differ with respect to a host of other properties as well. Oscar and Oscar − are distinct physical objects, and therefore distinct logical objects. Distinct physical objects may differ in mass.

Objection 8 : We can solve the paradox of 101 Dalmatians by appeal to a notion of “almost-identity” (Lewis 1993). We can admit, in light of the “problem of the many” (Unger 1980), that the 101 dog parts are dogs, but we can also affirm that the 101 dogs are not many; for they are “almost one”. Almost-identity is not a relation of indiscernibility, since it is not transitive, and so it differs from relative identity. It is a matter of negligible difference. A series of negligible differences can add up to one that is not negligible.

Reply : The difference between Oscar and Oscar − is not negligible and the two are not almost-identical. Lewis concedes this point but proposes to combine almost-identity with supervaluations to give a mixed solution to the paradox. The supervaluation solution starts from the assumption that one and only one of the dog parts is a dog (and a Dalmatian, and Oscar), but it doesn’t matter which. It doesn’t matter which because we haven’t decided as much, and we aren’t going to. Since it is true that any such decision renders one and only one dog part a dog, it is plain-true, i.e., supertrue, that there is one and only one dog in the picture. But it is not clear that this approach enjoys any advantage over that of relative identity; in fact, it seems to produce instances of RI. Compare: Fred’s bicycle has a basket attached to it. Ordinarily, our discourse slides over the difference between Fred’s bicycle with its basket attached and Fred’s bicycle minus the basket. (In this respect, the case of Fred’s bicycle differs somewhat from that of Oscar and Oscar − . We tend not to ignore that difference.) In particular, we don’t say that Fred has two bicycles even if we allow that Fred’s bicycle-minus is a bicycle. Both relative identity and supervaluations validate this intuition. However, both relative identity and supervaluations also affirm that Fred’s bicycle and Fred’s bicycle-minus are absolutely distinct objects. That is, the statement that Fred’s bicycle and Fred’s bicycle-minus are distinct is supertrue. So the supervaluation technique affirms both that Fred’s bicycle and Fred’s bicycle-minus are distinct objects and that there is one and only one (relevant) bicycle. That is RI, or close enough. The supervaluation approach is not so much an alternative to relative identity as a form of it.

Objection 9 : One may argue that the plausibility of RI rests to a large degree on certain linguistic phenomena and that we do not need it after all because these phenomena are in fact apparent and the notion of relative identity can be explained away. For example, Moltmann (2013) has recently argued that some apparent statements of sortal-relative identity can be analysed on linguistic grounds as statements of absolute identity. Consider an example of such statements: “This is the same lump of clay but not the same statue as that”, which is uttered in front of, say, two photographs: one presenting the clay and the other showing the statue made out of the clay. Moltmann starts her analysis focussing on the two pronouns at stake: this and that , the so-called bare demonstratives. The analysis reveals two different functions thereof: referential and presentational. Due to the referential function a bare demonstrative picks up a unique feature, or a trope as Moltmann claims. Secondly, this feature (trope) is used to recognise its bearer (in any possible world that is accessible from the world where the pronoun is uttered). Now in the example above this refers to a complex trope, say the complex composed of this particular brownness and this roundness, and “with the help of” this trope maps to a lump of clay as a bearer of this trope. Similarly, that refers to another complex trope, say this particular brownness and this angularity and maps to a bearer of this trope. The crucial assumption here is that the tropes in question may have multiple bearers, e.g., the brownness and roundness is a trope borne both by the clay and the statue. Moltmann’s semantics of the (apparent) statements of sortal-relative identity of the form “This is the same … as that” has it that such statements are true if this refers to a trope such that at least one, but not all, of its bearers is identical (in the sense of absolute identity) to a bearer of the trope to which that refers. So our sentence “This is the same lump of clay but not the same statue as that” is true when (i) one bearer of the this trope is the same as a bearer of the that trope and (ii) another bearer of that first trope is not the same as a bearer of the other trope.

Reply : The semantics outlined by Moltmann has its ontological cost because its proponent needs to acknowledge that a single trope may have multiple bearers. But even if you are persuaded by her argumentation to this controversial position, you may still accept RI. Moltmann’s theory concerns a specific class of relative identity statements and does not directly affect the applications of RI described above, which need not be rendered as identity statements involving bare demonstratives. For instance, accepting this theory you are at liberty to say that the original and remodeled ship are the same ship but not the same collection of planks, whereas the reassembled ship is the same collection of planks as the original but not the same ship. So even if the whole body of linguistic evidence for RI can be explained away, we may use relative identity as a theoretical explanatory tool, e.g., to provide resolutions to the paradoxes described above.

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Christian theology, philosophy and | identity | identity: of indiscernibles | identity: transworld | logic: ancient | logic: classical | personal identity | properties | trinity | vagueness

Acknowledgments

The SEP editors would like to thank Christopher von Bülow for reporting a list of typographical and other issues in an earlier version of this entry.

Copyright © 2022 by Harry Deutsch Pawel Garbacz < garbacz @ kul . lublin . pl >

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