What Makes You the Same Person

Locke made a move in 1694 that the argument has never got out from under. He separated three things that had been treated as one: the substance, the man and the person. The substance is whatever you are made of. The man is the living human animal. The person is the thinking thing that can consider itself as itself, and (this is the crucial addition) can be held to account for what it did. Person, for Locke, is a forensic term.

From that, the memory criterion follows almost directly. To be accountable for a deed, you must be able to own it from the inside; so personal identity extends exactly as far as consciousness can be extended backwards. He accepts the strange consequences without flinching. If the consciousness of a prince were carried into the body of a cobbler, the resulting man would be the prince as far as anything that matters is concerned, and would be the right person to punish for the prince's crimes.

Thomas Reid demolished the letter of it with a case anyone can follow. A boy is flogged for robbing an orchard. Years later a young officer takes a standard in his first campaign, and remembers being the flogged boy. Years after that the officer, now a general, remembers taking the standard and remembers nothing whatever of the orchard. By Locke's criterion the officer is the boy, the general is the officer, and the general is not the boy. But identity is transitive (that is not a substantive claim about persons, it is part of what identity means) so the criterion cannot be right as stated.

The repair is to replace direct memory with overlapping chains. Today you are strongly connected to yesterday, yesterday to the day before, and the chain runs unbroken from boy to general even though its ends do not touch. This is psychological continuity, it preserves transitivity, and it is now the standard view. It also inherits a problem the original did not have, because chains can branch and identity cannot.

Consider fission. The two hemispheres of a brain are separated and each is placed in a body, and both resulting people wake with your memories, your intentions, your projects and your sense of humour. They are not each other, so at most one can be you. Nothing distinguishes their claims. Saying that neither is you is worse than arbitrary: it means you were destroyed by a procedure that left two people carrying everything you cared about, which looks less like death than like the best outcome anyone has ever had.

Derek Parfit's response was to stop defending identity. What matters in survival, he argued, is psychological continuity itself. Identity is a further question, and it is empty. We think it must have an answer because our language forces a yes or no, not because anything in the world does. If you divide, nothing you care about is lost; you simply have twice as much of it. Parfit found this thought consoling and wrote that it made him care less about his own death. Most readers find it impossible to accept and cannot say which premise they would reject.

The argument stops being academic in a hospital. A man signed an advance directive twenty years ago refusing treatment in precisely the condition he is now in. He has advanced dementia, appears content, and does not recognise the document or the man who wrote it. On a strict continuity view the psychological connections may be so attenuated that the author and the patient are arguably not the same person, and one person's instructions do not govern another's care. On a bodily or narrative view the identity holds and the directive stands. Clinical ethics committees meet on this, and they divide, which is the best available evidence that the question is not a game.

What nobody has managed is an account that handles the ordinary cases, survives fission, respects transitivity and still explains why your own future matters to you in a way nobody else's does. The last clause is the one that keeps the subject alive. Every theory here can be made to work if you are willing to be told that the thing you feared losing was never there.