We explain what the theory of identity is and the philosophers who dealt with it. In addition, we explore the principles of identity in ontology and logic.

Identity is the set of traits that make a person unique and different from others.

What is identity?

Identity is a philosophical notion used to express the relation of sameness that a thing bears to itself. In philosophy, to say that a thing is the same as itself means to say that it is identical to itself, whereby "thing" is understood both subjects and objects insofar as they are "in themselves".

Just as it happens with the concept of contradiction, the concept of identity can be examined from various perspectives. It is generally used to formulate different principles. In ontology or metaphysics, the concept of identity is used to establish the ontological principle of identity. In logic, on the other hand, it is used to establish the law of identity.

The ontological principle of identity as well as the law of identity in logic seem to have originated in the theory of identity that emerged in Ancient Greece. This theory is related both to the idea of permanence of a subject who is the same over time, and to the search for one's own interiority (identity whose foundation can be traced in logical and ontological principles). Thus it is often difficult to establish which originated first, whether the ontological and logical principles or the theory of identity.

Beyond these principles, identity can be broadly reagrded, as Aristotle points out, in various senses: real identity, rational or formal identity, numerical identity, specific identity, generic identity, intrinsic or extrinsic identity, causal identity, primary or secondary identity, etc. However, it should be noticed that all forms of identity may be reduced to two: logical or formal identity and ontological or real identity.


  • In philosophy, identity is the relation that a thing bears to itself.
  • The principles of identity are: the ontological (metaphysical) principle, which states that every thing is identical with itself, and the logical (real) principle, which states that for every entity, the real and the material thing are different aspects of a single reality.
  • According to Plato, identity should be understood as the idea of the unchanging being. Identity is given by the entity itself and its essence, which does not change, unlike sensible and temporal things.

Etymology of the term "identity"

The term "identity" was expressed in Ancient Greek through the adjective autó (αὐτό), meaning "same" or "the same," and the pronoun to autó (το αυτό), which is translated as "this". Both are equivalent in Latin to ipse, "he himself," or idem, "this". The use of the demonstrative indicates that a subject or object is one and the same.

Thus, an object or subject:

  • Is there and is remembered as in its previous presence.
  • Is in itself, being itself or by itself, as Plato states when he says that there exists "man himself" as unchanging idea and self.

This etymological conception of the Platonic origin uses "authenticity" as a form of identity to indicate that the identity of an object is given by the entity itself, who grants its authenticity and essence, without its essence changing, unlike sensible and temporal things.

As for its Latin etymology, identity as ipse or idem is an adjective or demonstrative pronoun that stresses the fact that something is a particular thing distinct from something else. "Identity" is a late Latin form of identitas, "sameness": idem entitas.

Identity theory

The identity theory is used to refer to subjects and objects that are "in themselves". To speak of the identity of the subject means that the subject as subjicere ("to lie" or "to place under") is what remains underneath beyond its accidents (which are those attributes that can change), and which remains always the same.

The latter implies that what was before, is what is now and what will be later, beyond its accidental attributes. That there is "authenticity," as Plato holds, or identity means to say that a subject is and is permanently in its substance or essence.

These first approximations to the concept of identity originated in Ancient Greece in the legal sphere in relation to cause. Identity arises from being responsible, from being accountable for what is done and said.

This presupposes admitting that the subject is permanent, is unchanging and is the cause of the action for which he is judged and for which he must be held accountable. If the subject were not the same and, if the defendant in a trial, for example, were not the same as yesterday, he could not be held responsible for the acts committed and could therefore not be sentenced.

The act of being held accountable for an act of which one is the cause originates in the law of identity, as formulated by Parmenides and later reaffirmed by Aristotle: "whatever is, is". This means that being must be understood as being the same over time and not being its opposite (from which the principle of non-contradiction follows).

Thus, identity, as personal or object identity, is founded on the identity of being, even if in Ancient Greece the idea of "personal" identity had not yet appeared.

Principles of identity

Identity as a philosophical notion can be thought of from various perspectives, though the distinction most customarily drawn is between the ontological and the logical perspectives. From them derive the ontological principle of identity and the law of identity in logic.

  • Ontological principle of identity. It states that every thing is identical with itself, in Latin: ens est ens.
  • Law of identity. In logic, it states that any given entity is the same as itself: x is x.

Setting both principles apart is not a simple task. In the history of philosophy, both terms have often been used interchangeably and their senses have even been confused. Some authors assert that the foundation of the law of identity is in the ontological principle or that both are aspects of the same notion that holds that what is real is the identical, since the material thing, as an object, can be distinguished from "another" material thing: this tree is not the same as that one.

Ontological principle of identity

The ontological principle of identity states that every thing is equal to itself, in Latin ens est ens. Some authors argue that this occurs when applying the logical principle over time. To say that an object is identical to itself is not to affirm that a proposition is equal to itself, but rather to make a statement regarding the nature of a real object. Thus, for something to be equal to itself over time implies that it remains the same at all times.

For Kant, identity is a transcendental postulate of reason. In his Critique of Pure Reason, Kant argues that identity is a subject’s activity that allows to identify the manifold representations manifesting themselves, as being given, of a unity that is always the same (an idea similar to the notion of the "I").

Schelling and Hegel also shared similar views. Even Heidegger saw identity as referring to an entity considered equal to itself, which is in unity with itself.

Law of identity

The law of identity states that every entity is identical with itself: x is x. This is presented as a logical tautology which states "if p, then p"; and also: "p if and only if p". Both statements contain a constant ("if... then ..." and ''... if and only if....") and a propositional variable "p".

The law of identity is based not on propositional terms but on sets of members. For instance, instead of saying "if the moon, then the moon", where the term "moon" stands for the propositional variable p and refers to a self-identification, the logic of identity can state "the moon is the Earth's satellite", establishing an identification or equivalence between "the moon" and being "the Earth's satellite".

This is so because the logic of identity operates with different laws, such as the law of substitutivity of identity (which states that two entities are identical if what is asserted of one is true of the other) and the law of transitivity (which states that if two entities are equal to a third, they are equal to each other).

Identity of indiscernibles

A particular case of the laws of identity is that formulated by German philosopher Gottfried Wilhelm Leibniz (1646-1716), the identity of indiscernibles. Also known as Leibniz's law, the identity of indiscernibles is one of the major metaphysical postulates of the German philosopher.

Roughly speaking, this principle denies that two things can have equal properties and at the same time be numerically distinct. Thus, two things that are absolutely equal and indiscernible are identical and are the same thing.

The foundation of the identity of indiscernibles lies in the following ontological principles:

  • If two objects share all their properties, then they are identical and, consequently, they are the same object.
  • If two objects share all their qualitative properties, then they are identical.
  • If two objects share all their qualitative non-relational properties, then they are identical.

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ESPÍNOLA, Juan Pablo Segundo. "Identity".
Encyclopedia of Humanities. 19 January, 2024,

About the author

Author: Juan Pablo Segundo Espínola

Bachelor of Arts in philosophy (University of Buenos Aires)

Translated by: Marilina Gary

Degree in English Language Teaching (Juan XXIII Institute of Higher Education, Bahía Blanca, Argentina).

Updated on: 19 January, 2024
Posted on: 28 September, 2023

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