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See identity (disambiguation) for other usages of this term.

In philosophy, identity is the quality of being "the same as". It is of particular interest to logicians and metaphsicians.


In logic, the identity relation is normally, (by definition), the transitive, symmetrical, and reflexive relation that holds only between a thing and itself. That is, identiy is the two-place predicate, _=_, such that for all x, y, "x=y" is true iff x is y.

More usefully, it can be expressed formally in second-order logic or in set theory: For all objects x, y, if for all properties F, Fx iff Fy, then x=y

It is an axiom of most normal modal logics that for all x, if x=x then necessarily x=x.

(These definitions are of course inapplicable in some area of quantified logic, such as fuzzy logic and fuzzy set theory, and with respect to vague objects.)


Metaphysicians, and sometimes philosophers of language and mind, ask other questions: A traditional view is that of Gottfried Leibniz, who held that x is the same as y if and only if every predicate true of x is true of y as well.

Leibniz's ideas have taken root in the philosophy of mathematics, where they have influenced the development of the predicate calculus as Leibniz's law. Mathematicians sometimes distinguish identity from equality. More mundanely, an identity in mathematics may be an equation that holds true for all values of a variable.

More recent metaphysicians have discussed trans-world identity -- the notion that there can be the same object in different possible worlds.

Two objects can be called identical, meaning that they have the same shape, size and other properties. Thus, when we interchange the two objects, we do not see any difference. However, in terms of a stricter sense of identity, the initial and final situation are different. By observing not just the initial and final situation but the move itself, we can know this.

In cognition, identity is discussed in terms of whether or not an individual is self-reflective (i.e., whether it is aware of its own identity). For example, in 2002, some papers indicated that dolphins possess the ability to identify themselves in mirrors.

The psychological idea of identity in humans is tied up in self-image, one's view or model of oneself. Psychologists and counsellors interest themselves in psychological identity: an individual person's sense of self.

In sociological and political terms, identity is individuals' labelling of themselves as members of particular groups -- such as Nation, Social class, Subculture, Ethnicity, Gender, Employment, and so forth. It is in this sense which sociologists and historians speak of a national identity of a particular country, and feminist and queer theorists speak of gender identity.

Many people feel pride in their Identity groups, which furthers a sense of Community and Belonging. Often they will attempt to add to their identity by behaving in certain ways that have only a superficial connection, often the behaviour wasn't even established within the group, but through the Stereotypes of Oppressors. Though, it should not be mistaken that all people who identify a certain way attempt to add more to it. Identity has been a central element of pride movements such as gay pride or black consciousness, which seek to strengthen Politically oppressed groups by improving members' sense of identity. However, many consider a national or ethnic identity as a cultural background for demagogy, ethnic and religious conflicts, and the like.

To designers of secure computer systems, identity is a core concept of authentication. Identity theft is said to occur when one person gains control of credentials (such as credit card numbers or passwords) which belong to another, thus becoming able to masquerade as the "stolen" identity.
Identity might rely on: 1 This is individual only to you, unless an administrative mistake is made, or someone gets hold of this number by reading it from paper in your trash.
2 This is individual to you, your identical twin, your clone, or anyone who can obtain a sample of your DNA and amplify it
3 This is individual only to you, or anyone wearing a suitable contact lens with the pattern of your iris on, or the 1% of the population who will be recognised as you as a false positive.
4 This is individual only to you, unless someone copies your fingerprint off any clean surface using a digital camera and then prints it out and transfers it to a relief pattern on a piece of gel.
5 This is individual only to you. Be prepared to have a dental X-ray every time you need to identify yourself.
6 This could be individual only to you: watch out. See ka-tzetnik.

Note: all of the other identifiers listed are unlikely to be unique to you, and allow confusion between yourself and the previous or subsequent occupants of your address and users of your phone number.

See also Recognition of human individuals, identity and change.

In object oriented programming identity refers to object identity, which is a mechanism for distinguishing different objects from each other. This is based on the philosophical concept of identity, but applied to object oriented design and analysis.

In music George Perle provides the following example using "family" for "identity":
"C-E, D-F#, Eb-G, are different instances of the same interval...the other kind of identity...has to do with axes of symmetry. C-E belongs to a family of symmetrically related dyads as follows:"

D D# E F F# G G#
D C# C B A# A G#

Thus in addition to being part of the interval-4 family, C-E is also a part of the sum-2 family.

This article needs splitting into multiple articles and making into a disambiguation page.