philosophy Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. ...
, identity (from , "sameness") is the relation each thing bears only to itself. The notion of identity gives rise to many philosophical problems, including the
identity of indiscernibles The identity of indiscernibles is an ontological principle that states that there cannot be separate objects or entities that have all their properties in common. That is, entities ''x'' and ''y'' are identical if every predicate possessed by ' ...
(if ''x'' and ''y'' share all their properties, are they one and the same thing?), and questions about change and
personal identity Personal identity is the unique numerical identity of a person over time. Discussions regarding personal identity typically aim to determine the necessary and sufficient conditions under which a person at one time and a person at another time ca ...
over time (what has to be the case for a person ''x'' at one time and a person ''y'' at a later time to be one and the same person?). It is important to distinguish between ''qualitative identity'' and ''numerical identity''. For example, consider two children with identical bicycles engaged in a race while their mother is watching. The two children have the ''same'' bicycle in one sense (''qualitative identity'') and the ''same'' mother in another sense (''numerical identity''). This article is mainly concerned with ''numerical identity'', which is the stricter notion. The philosophical concept of identity is distinct from the better-known notion of identity in use in
psychology Psychology is the scientific study of mind and behavior. Psychology includes the study of conscious and unconscious phenomena, including feelings and thoughts. It is an academic discipline of immense scope, crossing the boundaries be ...
and the
social science Social science is one of the branches of science, devoted to the study of societies and the relationships among individuals within those societies. The term was formerly used to refer to the field of sociology, the original "science of so ...
s. The philosophical concept concerns a ''relation'', specifically, a relation that ''x'' and ''y'' stand in
if, and only if In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false. The connective is bicon ...
they are one and the same thing, or ''identical to'' each other (i.e. if, and only if ''x'' = ''y''). The sociological notion of identity, by contrast, has to do with a person's self-conception, social presentation, and more generally, the aspects of a person that make them unique, or qualitatively different from others (e.g.
cultural identity Cultural identity is a part of a person's identity, or their self-conception and self-perception, and is related to nationality, ethnicity, religion, social class, generation, locality or any kind of social group that has its own distinct c ...
gender identity Gender identity is the personal sense of one's own gender. Gender identity can correlate with a person's assigned sex or can differ from it. In most individuals, the various biological determinants of sex are congruent, and consistent with the ...
national identity National identity is a person's identity or sense of belonging to one or more states or to one or more nations. It is the sense of "a nation as a cohesive whole, as represented by distinctive traditions, culture, and language". National identity ...
online identity Internet identity (IID), also online identity or internet persona, is a social identity that an Internet user establishes in online communities and websites. It may also be an actively constructed presentation of oneself. Although some people cho ...
, and processes of
identity formation Identity formation, also called identity development or identity construction, is a complex process in which humans develop a clear and unique view of themselves and of their identity. Self-concept, personality development, and values are all c ...
). Lately, identity has been conceptualized considering humans’ position within the ecological web of life.

Metaphysics of identity

Metaphysicians and philosophers of language and mind ask other questions: * What does it mean for an object to be the same as itself? * If x and y are identical (are the same thing), must they always be identical? Are they ''necessarily'' identical? * What does it mean for an object to be the same, if it changes over time? (Is apple''t'' the same as apple''t''+1?) * If an object's parts are entirely replaced over time, as in the
Ship of Theseus The Ship of Theseus is a thought experiment about whether an object that has had all of its original components replaced remains the same object. According to legend, Theseus, the mythical Greek founder-king of Athens, had rescued the children o ...
example, in what way is it the same? The law of identity originates from
classical antiquity Classical antiquity (also the classical era, classical period or classical age) is the period of cultural history between the 8th century BC and the 5th century AD centred on the Mediterranean Sea, comprising the interlocking civilizations o ...
. The modern formulation of identity is that of
Gottfried Leibniz Gottfried Wilhelm (von) Leibniz . ( – 14 November 1716) was a German polymath active as a mathematician, philosopher, scientist and diplomat. He is one of the most prominent figures in both the history of philosophy and the history of mat ...
, who held that ''x'' is the same as ''y''
if and only if In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false. The connective is bico ...
every predicate true of ''x'' is true of ''y'' as well. Leibniz's ideas have taken root in the
philosophy of mathematics The philosophy of mathematics is the branch of philosophy that studies the assumptions, foundations, and implications of mathematics. It aims to understand the nature and methods of mathematics, and find out the place of mathematics in people' ...
, where they have influenced the development of the
predicate calculus Predicate or predication may refer to: * Predicate (grammar), in linguistics * Predication (philosophy) * several closely related uses in mathematics and formal logic: **Predicate (mathematical logic) **Propositional function **Finitary relation, o ...
as Leibniz's law. Mathematicians sometimes distinguish identity from equality. More mundanely, an ''identity'' in
mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
may be an ''
equation In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in ...
'' that holds true for all values of a variable.
Hegel Georg Wilhelm Friedrich Hegel (; ; 27 August 1770 – 14 November 1831) was a German philosopher. He is one of the most important figures in German idealism and one of the founding figures of modern Western philosophy. His influence extends ...
argued that things are inherently self-contradictory and that the notion of something being self-identical only made sense if it were not also not-identical or different from itself and did not also imply the latter. In
Hegel Georg Wilhelm Friedrich Hegel (; ; 27 August 1770 – 14 November 1831) was a German philosopher. He is one of the most important figures in German idealism and one of the founding figures of modern Western philosophy. His influence extends ...
's words, "Identity is the identity of identity and non-identity." More recent metaphysicians have discussed trans-world identity—the notion that there can be the same object in different possible worlds. An alternative to trans-world identity is the counterpart relation in Counterpart theory. It is a similarity relation that rejects trans-world individuals and instead defends an objects counterpart – the most similar object. Some philosophers have denied that there is such a relation as identity. Thus
Ludwig Wittgenstein Ludwig Josef Johann Wittgenstein ( ; ; 26 April 1889 – 29 April 1951) was an Austrian- British philosopher who worked primarily in logic, the philosophy of mathematics, the philosophy of mind, and the philosophy of language. He is consid ...
writes ('' Tractatus'' 5.5301): "That identity is not a relation between objects is obvious." At 5.5303 he elaborates: "Roughly speaking: to say of two things that they are identical is nonsense, and to say of one thing that it is identical with itself is to say nothing."
Bertrand Russell Bertrand Arthur William Russell, 3rd Earl Russell, (18 May 1872 – 2 February 1970) was a British mathematician, philosopher, logician, and public intellectual. He had a considerable influence on mathematics, logic, set theory, linguistics, a ...
had earlier voiced a worry that seems to be motivating Wittgenstein's point ('' The Principles of Mathematics'' §64): " entity, an objector may urge, cannot be anything at all: two terms plainly are not identical, and one term cannot be, for what is it identical with?" Even before Russell,
Gottlob Frege Friedrich Ludwig Gottlob Frege (; ; 8 November 1848 – 26 July 1925) was a German philosopher, logician, and mathematician. He was a mathematics professor at the University of Jena, and is understood by many to be the father of analytic phi ...
, at the beginning of "
On Sense and Reference In the philosophy of language, the distinction between sense and reference was an idea of the German philosopher and mathematician Gottlob Frege in 1892 (in his paper "On Sense and Reference"; German: "Über Sinn und Bedeutung"), reflecting the ...
," expressed a worry with regard to identity as a relation: "Equality gives rise to challenging questions which are not altogether easy to answer. Is it a relation?" More recently, C. J. F. Williams has suggested that identity should be viewed as a second-order relation, rather than a relation between objects, and Kai Wehmeier has argued that appealing to a binary relation that every object bears to itself, and to no others, is both logically unnecessary and metaphysically suspect.

Identity statements

Kind-terms, or sortalsTheodore Sider
"Recent work on identity over time"
''Philosophical Books'' 41 (2000): 81–89.
give a criterion of identity and non-identity among items of their kind.

See also

* Counterpart theory *
Difference (philosophy) Difference is a key concept of philosophy, denoting the process or set of properties by which one entity is distinguished from another within a relational field or a given conceptual system. In the Western philosophical system, difference is tra ...
* Exact similarity and identity *
Four-dimensionalism In philosophy, four-dimensionalism (also known as the doctrine of temporal parts) is the ontological position that an object's persistence through time is like its extension through space. Thus, an object that exists in time has temporal parts ...
perdurantism Perdurantism or perdurance theory is a philosophical theory of persistence and identity.Temporal parts
* Teleportation paradox



* 1998: ''Occasions of identity''. Oxford: Oxford University Press.
Google books
* 1984: ''Reasons and persons''. Oxford: Oxford University Press.
Google books
* 1985: Can amoebae divide without multiplying? ''Australasian journal of philosophy'', 63(3): 299–319. * 2000: eview of (1998) ''Philosophical review'', 109(3): 469–471
* 2001: eview of (1998) ''British journal for the philosophy science'', 52(2): 401–405.

External links

*Stanford Encyclopedia of Philosophy
First published Wed 15 Dec 2004; substantive revision Sun 1 Oct 2006. *Stanford Encyclopedia of Philosophy
Identity over time
First published Fri 18 March 2005. *Stanford Encyclopedia of Philosophy
Personal identity
First published Tue 20 Aug 2002; substantive revision Tue 20 Feb 2007. *Stanford Encyclopedia of Philosophy
Relative identity
First published Mon 22 April 2002. * Fernando Andacht, Mariela Michel,
A Semiotic Reflection on Selfinterpretation and Identity
'. {{Authority control Concepts in epistemology Concepts in logic Concepts in metaphilosophy Concepts in metaphysics Concepts in the philosophy of mind Concepts in the philosophy of science Metaphilosophy Metaphysics of mind Ontology Philosophical logic Philosophical problems Philosophical theories Philosophy of logic Philosophy of psychology