Personal identity is the unique numerical identity of a person over time.[1][2] 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 can be said to be the same person, persisting through time.

In philosophy, the problem of personal identity[3] is concerned with how one is able to identify a single person over a time interval, dealing with such questions as, "What makes it true that a person at one time is the same thing as a person at another time?" or "What kinds of things are we persons?"

In contemporary metaphysics, the matter of personal identity is referred to as the diachronic problem of personal identity.[lower-alpha 1][4] The synchronic problem concerns the question of what features and traits characterize a person at a given time. Analytic philosophy and continental philosophy both inquire about the nature of identity. Continental philosophy deals with conceptually maintaining identity when confronted by different philosophic propositions, postulates, and presuppositions about the world and its nature.[5][6]