The continuity of identity, despite ongoing change in the person and the world, has challenged thinkers since ancient times. Identity has its roots in the Latin word for same. According to the Oxford English Dictionary (on-line), identity involves “the sameness of a person or thing at all times or in all circumstances” (OED 2.a) and, more specifically, personal identity involves “the condition or fact of remaining the same person throughout the various phases of existence.” Locke (1690), perhaps the first to propose a modern sense of personal identity, wrote that “The Identity of the same Man consists … in nothing but a participation of the same continued Life, by constantly fleeting Particles of Matter, in succession vitally united to the same organized Body” (ii. xxvii. Sect. 6). Yet the roots of Locke's statement go at least as far back as Plato's symposium (Plato, 390, 207.d).

Psychologists in our era have attempted to solve the riddle of identity by proposing the construction of a self structure (Marcia, 1980), concept, or theory (Schlenker and Weigold, 1989), built out of cognitive and social constituents. Whether viewed as a schema of the self, a theory, a set of traits or dispositions, or a hierarchy of defenses and goals, identity is understood by conventional theories as a stable structure built up over development. For such an identity, continuity over time is not difficult to explain. Building-block structures maintain their sameness by virtue of an invariant relation among their parts and an invariant set of functions or transactions with the world.