Personal Identity

Personal identity is the philosophical problem of what makes a person at one time the same person as a person at another time — and whether same is even the right concept to apply to persons across time.

The question has legal, moral, and psychological dimensions. If identity requires psychological continuity (memory, beliefs, personality), then a person with severe amnesia may not be identical to their pre-amnesia self — yet they remain legally and biologically continuous with them. If identity requires physical continuity, then the gradual replacement of the body's atoms over years poses no problem — yet a teleporter that destroys and recreates produces a discontinuity. These cases do not have clean answers because they reveal that we use multiple, sometimes incompatible, criteria for identity depending on the purpose at hand.

Derek Parfit's argument in Reasons and Persons (1984) remains the sharpest challenge: personal identity may not be what matters in survival. What matters is psychological connectedness and continuity, and these admit of degrees. Two people can share a branching causal history; asking which one is really me? may be asking a question with no fact of the matter.