Look at a photograph of yourself as a small child. Almost every cell in that body has since been replaced, your beliefs have changed, and you remember little of that time. In what sense is that child you? This is the problem of personal identity over time.

Two kinds of identity

Philosophers distinguish two senses of “same.” Numerical identity means one and the same thing; qualitative identity means being exactly alike. Two new coins from the same mint may be qualitatively identical but are numerically two. The question about the child in the photo is about numerical identity: you are clearly not exactly alike, but are you one and the same person?

The ship of Theseus

The ancient writer Plutarch reported a puzzle about the ship of the hero Theseus, preserved in Athens for centuries. As planks rotted, they were replaced one by one, until no original plank remained. The puzzle asks whether a thing stays the same when all its parts are replaced. Thomas Hobbes, in the seventeenth century, added a twist: suppose someone gathered the old planks and rebuilt them into a second ship. Which one is the ship of Theseus? Our bodies, constantly renewing their material, raise a similar question.

Locke's memory theory

John Locke added a chapter on identity to the second edition of his Essay Concerning Human Understanding (1694). He defined a person as “a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places.” His key claim was that being the same person depends on remembering past experiences as one's own, from the inside. Sameness of body, or even of soul, is not what makes a person the same.

Locke offered a striking example. Suppose the consciousness of a prince, with all his memories, entered the body of a cobbler. Everyone would see the cobbler's body, but Locke said the person in the cobbler's body is the prince.

Objections

Thomas Reid (1785) offered the brave officer objection. A boy is flogged for stealing apples. As a young officer he remembers the flogging. As an old general he remembers capturing an enemy flag as an officer, but has forgotten the flogging. By Locke's theory, the general is the officer and the officer is the boy, yet the general is not the boy. Reid's point is that the memory theory breaks the rule that identity is transitive: if A is B and B is C, then A must be C. Joseph Butler (1736) added that memory presupposes identity rather than creating it. Later defenders replied that what matters is overlapping chains of memory and other psychological connections, called psychological continuity.

Teleporters and Parfit

Modern philosophers test these ideas with thought experiments. Imagine a teleporter that scans your body, destroys it, and builds an exact copy on Mars, with all your memories. Did you travel, or did you die and get replaced?

Derek Parfit, in Reasons and Persons (1984), pushed such cases further. Suppose each half of your brain were transplanted into a different body and both survivors had your memories. Both cannot be you, yet neither has a better claim. Parfit concluded that personal identity is not what matters; psychological continuity is, and it can come in degrees. He found this view liberating, because it made him less concerned about his own death; the boundary between himself and others seemed less like a wall. Critics, including those who hold that we are essentially human animals, reply that our identity is simply the continued life of a single organism.

Worked example: the teleporter booth

Ayo works on Mars and visits home on Earth by teleporter. The machine scans every cell of her body, destroys the original, and builds an exact duplicate on Mars a few minutes later. The person who steps out has Ayo's memories, personality and plans, and is sure she is Ayo.

The body view says Ayo died in the booth: her body was destroyed, and a new body was made. The Martian is a perfect copy, not Ayo.

The memory view, in Locke's spirit, says Ayo survived: the person on Mars remembers Ayo's life from the inside, just as the prince did in the cobbler's body.

Now change the case. One day the machine malfunctions. It builds the duplicate on Mars but fails to destroy the original on Earth. Two people now remember being Ayo. They cannot both be numerically identical to her, since they are clearly two people, and each has an equal claim. This is just the kind of branching case Parfit used.

Parfit's answer is to stop asking which one is “really” Ayo. What matters, he says, is psychological continuity, and both survivors have it. Ordinary teleportation is about as good as ordinary survival. The branching case is strange, but not as bad as death.

Many people resist this. They feel that if they stepped into the booth, they would simply end, whatever the copy felt. Test yourself: would you use the machine? Your answer reveals which theory of personal identity you are already relying on.