Phil 101: Fission Cases


Suppose David splits in two, like an amoeba, and the resulting halves of him grow into two autonomous persons. Call these two resulting people Lefty and Righty. They share David’s looks and personality, and they seem to remember events from David’s past life. We can call this process fission. What shall we say is the relation between David, Lefty, and Righty?

There are six choices:

  1. David is one and the same person as Lefty, but a different person from Righty.
  2. David is one and the same person as Righty, but a different person from Lefty.
  3. David is one and the same person as Lefty and he is also one and the same person as Righty. (David is wholly there when Lefty is, and also wholly there when Righty is.)
  4. David is not identical to either Lefty or Righty. David divided into two new persons, Lefty and Righty, and in the process David ceased to exist.
  5. There is some composite person, Lefty + Righty. Lefty is a part of this composite person and so is Righty. The composite person has four legs, four arms, etc. David is identical to this composite person. (Since Lefty is not identical to the composite person, David is not identical to Lefty. Ditto for Righty.)
  6. There only appeared to be a single David, in the first place. In fact, there were two people there all along, DavidL and DavidR. These two people shared a single body, a single brain, and a single course of experience, up until the time of the fission. (Think of how two roads might share a single stretch of pavement for a while.) After the fission, the two people then went their separate ways.

The problem with choices 1 and 2 is that Lefty and Righty each seem to have an equally good claim to be identical to David. Why should one of them be identical to David, but not the other?

The problem with choice 3 is that personal identity is a transitive relation. What it means for a relation to be transitive is that, if the relation holds between A and B, and it also holds between B and C, then it must also hold between A and C.

So in this case, if Lefty is one and the same person as David, and David is one and the same person as Righty, then, since personal identity is transitive, it follows that Lefty must also be one and the same person as Righty. But Lefty and Righty are clearly not one and the same person. They are two different people, with their own experiences, beliefs, and bank accounts. So it cannot be the case that David is identical to Lefty and also is identical to Righty.

Choices 5 and 6 are interesting, but complicated. For present purposes let us merely remark that Lefty and Righty do not appear to make up a composite person, any more than you and I make up a composite person. (Lefty and Righty do have a common set of shared memories, it is true, but they have gone their separate ways since the fission. They cannot “read each other’s mind,” or anything like that. They may disagree about many issues.) Let us also remark that it does not appear as if, before the fission, two people are occupying David’s body. So choices 5 and 6 both have some prima facie implausibility.

That leaves choice 4. When David undergoes fission, he ceases to exist. This is surprising. But it seems to be the most sensible, and least problematic, answer.

Here are some other possible fission cases:

  1. Suppose that we remove the two hemispheres of your brain from your body, and implant them into two new bodies. Each of the resulting persons survives, and seems to remember events from your past life. Neither seems to have a better claim to be identical to you than the other.

  2. Teletransportation

    sam brown, explodingdog

    Suppose we discover how to make a teletransporter that works like the one on Star Trek. It decomposes your body and your brain and records all the information about them. It then radios a signal to Mars, where another machine uses raw materials on Mars to create a person who perfectly duplicates you. This person walks and talks just like you. It thinks it’s you. It seems to remember your life. Would this person be you? Would you survive going through this teletransporter? Or would this person on Mars merely be a copy of you? Would you use such a teletransporter?

    What if you step into the teletransporter, and it decomposes your body and your brain and radios the signal to Mars. A perfect copy of you is created on Mars. At the same time, some hackers are “sniffing” the radio traffic to Mars, and they pick up your signal and create a perfect copy of you on Venus. Each of the copies looks like you and talks like you. They both seem to remember events from your past life. Neither seems to have a better claim to be identical to you than the other. This would be another case of fission.

The Problem with Proposal #5

Recall Proposal #5:

Proposal #5: Stage A* and stage B* are parts of the same person iff they are parts of a chain, all of whose later stages quasi-remember (or are able to quasi-remember) “from the inside” events experienced by the immediately preceding stages.

If the right thing to say about fission cases is that the original person is identical to neither of the resulting people, then Proposal #5 cannot be correct. For both Lefty and Righty are psychologically continuous with David. Hence, Proposal #5 would count them both as being identical to David. Yet we’ve just said that neither is identical to David.

(This is the objection Gretchen Weirob makes on pp. 32-33 of Perry’s dialogue. Gretchen states the point in a very compressed way. I have tried to spell it out in more detail.)