Phil 89: 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 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.) We assume for this option that there is just one David, and as we’ll discuss below, we assume also we’re not willing to say that Lefty and Righty are the same persion.
  2. David is one and the same person as Lefty, but a different person from Righty.
  3. David is one and the same person as Righty, but a different person from Lefty.
  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 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.
  6. 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.)

The problem with choices 2 and 3 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? (Someone who believes we have souls and that they are essential to personal identity may be in a better position to answer this than other philosophers. Note that we’re only here discussing what the logical options are, what the facts might be regardless of whether anyone is in a position to know them. The fact that, if some answer to these questions is true, we’re not able to know it’s true, may or may not be a problem. That would need further discussion.)

The problem with choice 1 is that personal identity is a transitive and symmetric 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. What it means for a relation to be symmetric is that if it holds between A and B, it must also hold between B and A.

So in this case, if Lefty is one and the same person as David, and Righty is also one and the same person as David, then, since personal identity is transitive and symmetric, 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, locations in space, and (eventually) their own bank accounts. So, since Lefty is not the same person as Righty, 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 it does not appear as if, before the fission, two people are occupying David’s body. Let us also 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 end up disagreeing about many issues.) 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.

The important fact about fission cases, which forces us to say that the original person ceases to exist, is that the two people at the end of the story have equally good claims to be identical to the original person. Consider a different sort of story. Suppose Lisa donates some cells to a science lab. Then she goes home and goes to sleep. The science lab grows Lisa’s cells into a clone of Lisa. Call the clone Cleo. Back home, the person in Lisa’s bed wakes up. Call this person Betty. Now who is the same person as Lisa: Cleo or Betty? Clearly, Betty is the same person as Lisa. In this case, Betty has a much better claim to be identical to Lisa than Cleo has. After all, Betty occupies the same body that Lisa occupied the night before. Cleo was merely grown from a few discarded cells. So this is not a fission case. In a fission case, we need to have two people who have equally good claims to be identical to an original person.

We can call the Lisa/Cleo case an offshoot case, and reserve the label fission case for cases like the splitting-in-half-like-an-amoeba.

Here’s another possible fission case. If we destroyed one of the hemispheres of your brain, the person who came out of the operation would be psychologically continuous with you. (This is in fact used as a drastic but acceptable treatment for otherwise-inoperable brain tumors.) Let’s suppose that instead of destroying a hemisphere, we removed both hemispheres of your brain, and implanted each of them into its own new body. 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. Maybe they remember different events, but each remembers as much as the other. Or maybe your memory system has redundancies built into it, so that many memories are had by each of the resulting people.

sam brown, explodingdog

Suppose a teletransporter radios a signal to Mars, where another receiver machine uses raw materials to create a person who perfectly duplicates you. They walk and talk just like you. They think that they’re you. They seem to remember your life. Would it be you? Would the teletansporter have transported you to Mars? Or would it have destroyed you and merely made a copy of you?

We can think about three different variations of this.

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, whose later stages are able to quasi-remember “from the inside” (enough details of enough) events experienced by the stages that come before them. We might also include more constraints about what has to stay psychologically the same.

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 in our fission cases, both Lefty and Righty are psychologically continuous with the original person David, in the sense described by Proposal #5. 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.)

Additionally, in our offshoot cases, we have two resulting people, one of whom is and one of whom isn’t identical to the original person. Yet both of the resulting people would be psychologically continuous with the original. So Proposal #5 seems to imply they’d both be identical to the original.

What’s more, since identity is transitive and symmetrical, Proposal #5 seems also to imply that the people at the end of the stories would have to be identical to each other, too. But we said that doesn’t seem to be right.