Personal identity
the self · identity over time
What makes someone the same person over time, despite continuous change in body, memory and character — and what, if anything, that sameness consists in.
In practice
Every long-running organization faces the same question: after the staff, product and owners have all changed, the claim that it is the same company is doing work that needs an account.
The common mistake
Treating it as a question about a hidden fact. On several leading views there is no further fact beyond the continuity, so a case where the answer is unclear may have no answer rather than an unknown one.
Personal identity asks what makes the person reading this the same person who existed twenty years ago. Almost every physical component has been replaced, most memories are gone, and much of the character has changed.
The candidates
Body. The same organism persists. The difficulty is that the matter is replaced continuously, and the intuition that you would go with your brain in a transplant suggests the body is not what we are tracking.
Memory. Locke proposed that a person is the same person as whoever they can remember being. The problem, raised by Thomas Reid, is transitivity: an officer remembers being a boy, a general remembers being the officer and not the boy, so the general both is and is not the boy.
Psychological continuity. The modern repair replaces memory with an overlapping chain of memories, intentions and character. It handles Reid's objection and inherits a harder one.
Nothing. Hume looked inward and found only perceptions, never the self having them — bundle theory. Derek Parfit developed the consequence: identity is not what matters, and questions about whether a future person would be you can be empty rather than unanswered.
Why the fission case decides it
Suppose your brain is divided and each half sustains a person, both psychologically continuous with you. They cannot both be you, since they are two. Neither is more entitled than the other. And nothing about your relationship to either is worse than ordinary survival.
Parfit's conclusion is that what we care about is the continuity, not the identity, and that the further question of whether it is really you rests on a mistake about what a person is.
What it touches
Whether punishment reaches the person who committed the act, what a promise binds, and why prudence about your own future is different from concern for someone else. See substance and free will.
Concept web
Open the full webQuestions
What is personal identity in philosophy?
The question of what makes someone the same person over time despite change in body, memory and character, and what that sameness consists in.
What is Locke's memory theory?
That a person is the same person as whoever they can remember being. Reid objected that memory is not transitive, which the modern psychological-continuity view was developed to handle.
What is Parfit's fission argument?
If a brain is divided and both halves sustain psychologically continuous people, neither can be you though nothing is lost. Parfit concluded that continuity is what matters, not identity.