The Teletransportation Accident

The malfunction

In the standard teletransportation scenario, you step into a scanner, your body is destroyed, your information is transmitted to Mars, and a perfect replica is constructed there. The replica has your memories, your personality, and your physical structure. Most defenders of psychological continuity say you survive: the replica is you.

Now suppose the machine malfunctions. Your body is scanned but not destroyed. The replica is constructed on Mars as planned. There are now two people, each with an equal psychological claim to be you. The replica on Mars has continuous memories up to the moment of scanning. You, standing in the scanner on Earth, are also continuous with your pre-scan self. Both of you believe you are the original. Both of you have the same claim.

Why the branching is a problem

Psychological continuity theory says personal identity consists in psychological continuity: the right kind of memory chains, personality persistence, and causal connection. In the accident case, both you and the replica have this. If psychological continuity were sufficient for identity, both of you would be you. But you and the replica are two distinct people. One thing cannot be identical to two distinct things. So at most one of you is the original you, and there is no principled basis for choosing which one.

The usual response is to say that psychological continuity is sufficient for identity only when it doesn't branch. When it does branch, as in this case, neither resulting person is strictly identical to the pre-branching person. The accident, on this view, killed you even though nobody died and two psychologically continuous people exist.

Parfit's conclusion

Parfit thought this was not a desperate result but an illuminating one. If what you care about in your own survival is psychological continuity and connectedness, the accident gives you everything you care about, twice over. What it fails to give you is identity in the strict sense, a one-to-one relation between the pre-accident person and exactly one post-accident person. But Parfit argued that strict identity is not what matters.

His view is that personal identity is not the deep fact we take it to be. Questions like "will I survive?" and "is that the same person?" do not track anything of ultimate importance. What matters is whether psychological continuity is preserved, and it can be preserved even when identity is not. The accident case is the clearest demonstration: survival in the sense that matters has occurred twice, but identity, in the strict logical sense, has not survived at all.