In philosophy, identity (from identitas, "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 (if x and y share all their properties, are they one and the same thing?), and questions about change and personal identity 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 and the social sciences. The philosophical concept concerns a relation, specifically, a relation that x and y stand in if, and only if 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, gender identity, national identity, online identity, and processes of identity formation). Lately, identity has been conceptualized considering humans’ position within the ecological web of life.
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 applet the same as applet+1?)
If an object's parts are entirely replaced over time, as in the Ship of Theseus example, in what way is it the same?
The law of identity originates from classical antiquity.
