What makes you, you · Quiz question 20/35
The Branch-Line Case (Fission and Duplication)
The teleporter glitches: the original survives and a copy appears too. Which one is you? Parfit's Branch-Line case and the fission problem.
Back to the teleporter. This time it glitches: your original body in Seoul isn't destroyed. And an identical person also appears in New York.
In this case, the person in New York isn't me — it's a copy.
This is Derek Parfit's "Branch-Line Case" from Reasons and Persons (1984). A new scanner records your information without destroying your body, so after a replica is made on Mars, the original on Earth is still there — and, thanks to a side effect, will die in a few days. Earlier, in his 1971 paper, Parfit discussed "fission," where one person splits into two.
The moment the original survives, most people feel "the one on Mars is a copy." If you agreed, so do you. The trouble is for psychological continuity theories: the Mars person is just as psychologically continuous with the original, so if that's the only criterion, both should be you — but two different people can't both be identical to you (identity is one-to-one).
Bernard Williams used this "duplication problem" in the 1950s to attack the memory criterion. Psychological theorists usually add a clause: continuity makes identity only when there's no competitor. Then the first teleport (original destroyed) is survival and this case isn't. Parfit found that odd: your relationship to the Martian is the same in both cases, yet whether you live or die depends on an outside fact — whether an original remains on Earth. So Parfit concluded that psychological connection itself, not identity, is what matters.
The original is you. The Mars person, however similar, is someone else.
Key names: Eric Olson, Bernard WilliamsWhen the branch splits, neither has strict identity — or the original takes priority.
Key names: Robert Nozick (closest continuer theory, Robert Nozick)"Which one is really me?" isn't the important question. Your psychological link to the Martian is as good as ordinary survival.
Key names: Derek Parfit