## Fission Cases

### Fission

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.

• 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.

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.

• Contrast that to the following case (see the brief Parfit selection "What We Believe Ourselves To Be," in the coursepack). You step into a teletransporter, and it "scans" your body and your brain without destroying them; then it radios the signal to Mars, where it creates a perfect copy of you. Shortly after this process, your body on Earth dies. This does not seem to be a case of fission. This is more like the case of Lisa, above. Here, the person who remains on Earth has a much better claim to be identical to you than the copy on Mars. After all, the person who remains on Earth continues to occupy your body. It's just that the person created on Mars is fortunate enough to live further into the future than the person who remains on Earth. (Hence, it sure would be nice to be the person created on Mars, rather than the person who remains on Earth, and is soon to die. But wishing won't make it so.)

What if the teletransporter destroyed your brain and body as it scanned them. Would it then be you who shows up on Mars?

Let's think about some other kind of cases for a few moments.

Remember the scene in The Matrix, when the character Cypher was choosing to live in the illusory world of the Matrix. Cypher would have his memories of the outside world erased, so that he forgets that there is an outside world, and forget that his experiences are all an illusion. As far as he's concerned, it doesn't matter if he's a slave to the machines, so long as he doesn't know about it. What he doesn't know won't hurt him.

Most people would choose (i). The lesson is: things that happen can sometimes be bad or undesirable even if, were they to happen, you wouldn't know that they had happened.

Would it be OK if I killed you? Painlessly and without warning? You would never know you were about to die, and then after you're dead you wouldn't know anything had happened, either. Do you have any objection to that? Suppose I give you a choice between (i) nothing, and (ii) I pay you \$10, but then I'll kill you without warning next week. You'll never know about this. (I arrange to have your memory of why I paid you \$10 erased.) What would you choose?

What about if I killed you, painlessly and without warning, but I hypnotize everyone so that no one else notices you're gone. So now your family and friends don't undergo any suffering or grief. No one will notice your absence. Do you have any objection to that?

Now, suppose that once again, I kill you, painlessly and without warning, but this time, instead of hypnotizing everyone, I introduce a perfect duplicate of you into your life. So no one knows you're gone, because they all falsely believe that the duplicate is you. Even the duplicate thinks he is you, because we gave him a bunch of false memories of your childhood. (Like the replicants in Blade Runner.) Do you have any objection to that?

Some philosophers think that's exactly what it would be like, if you went through a teletransporter that decomposed your brain and body as it scanned them. On their view, teletransportation isn't a way to TRAVEL. It's a way to get yourself killed, and to have a perfect duplicate of you made at the other end. It might not make much of a difference to your family and friends, whether they're dealing with the original or the duplicate. But since you're not the duplicate, you're you, and you'd like to still be around next week, it will make a big difference to you.

Here's an argument for this view of teletransportation. Recall the episode in Parfit's "What We Believe Ourselves To Be." There you stepped into the teletransporter, and it recorded all the information about your body without destroying you. It created a perfect duplicate of you on Mars. In that story, it seems like you'd still be hanging around back on Earth. You can talk to the duplicate by videophone. This reinforces our sense that the duplicate isn't really you, it's just a copy of you. Then after an hour or so, we'll kill the person left on Earth. (It would be inconvenient to have two versions of you running around.)

In this case, when there's a delay between the time when the machine records the information about you and the time when your body on Earth is destroyed, it does seem that the person who comes out of the teletransporter on Mars is just a copy of you. So why should things be any different if your body is destroyed immediately after the teletransporter records the information about you? Or if it's destroyed simultaneously? Why should those matters of timing make a difference to whether the person who steps out of the teletransporter on Mars is really you or just a copy of you?

What do you think is the right way to think about teletransportation? Is it a way to travel, or a way to commit suicide while creating a copy of yourself?

### 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.)

### Proposal #6

Dave Cohen responds to Gretchen's criticism as follows. Take a person stage A*. If at a later time there is only one person stage B* psychologically continuous with A*, then B* and A* are parts of the same person. But this is only true when there is one candidate B*. If there are several candidates B*, C*,...--all of which are psychologically continuous with A*--then some or all of them may fail to be identical to A*. Personal identity only consists in psychological continuity when there are no competitors.

Proposal #6: If A* and B* are parts of a chain of psychologically continuous person stages, and if the later stages in this chain do not co-exist with any other "competitor" stages, not part of the chain, which are also psychologically continuous with A*, then A* and B* are parts of the same person (p. 33).

One of my former students, Tom Kelly, offered the following analogy.

Take a piece of chalk, Chalky. Now suppose that as I write on the board with Chalky, it gradually gets worn down to a stub Stubby, half the size of the original piece of chalk. If I started the class holding Chalky in my hand, and end the class holding Stubby in my hand, it seems right to say that I've had the same piece of chalk in my hand for the course of the class. That is, it seems right to say that Chalky and Stubby are the same piece of chalk. (It's just that this piece of chalk has gotten worn down as the class progressed.) Contrast a second case, where I hold Chalky up at the start of class, and break it perfectly in half. In my right hand I have a stub S1, and in my left hand I have a stub S2. We can imagine that S1 is exactly the same size and shape as Stubby was, in the first case. We can even imagine that S1 and Stubby are made of the same molecules. However, although in the first case it seemed right to say that Stubby is the same piece of chalk as Chalky, here it does not seem right to say that S1 is the same piece of chalk as Chalky. S1 has a competitor, S2. S1 and S2 seem to have equally good claims to be identical to Chalky; hence, we're forced to say that neither is identical to Chalky. In the first case, though, where the stub in my right hand had no competitors, it did seem right to count that stub as the same piece of chalk as Chalky. So the presence or absence of competitors makes a difference to whether we count the stub as being the same piece of chalk as Chalky.

The defenders of proposals like Proposal #5 and Proposal #6 are often referred to as "psychological continuity theorists." There are some other refinements of Proposal #5 we could consider instead. I'll label them as follows:

Proposal #7: If A* and B* are parts of a chain of psychologically continuous person stages, and if the later stages in this chain do not co-exist with any other "at least as close competitor" stages, not part of the chain, which are also psychologically continuous with A*, then A* and B* are parts of the same person.
That view is known as the "closest competitor theory".

Proposal #8: If A* and B* are parts of a chain of psychologically continuous person stages, and where that chain also includes most or all of the same persisting physical body, then A* and B* are parts of the same person.

Proposal #9: If A* and B* are parts of a chain of psychologically continuous person stages, and where that chain also includes most or all of the same persisting physical brain, then A* and B* are parts of the same person.
Proposal #9 is discussed later in the Perry dialogue. Proposals #7 and #8 aren't discussed, though they are natural variations on the views that are discussed. I will postpone discussion of these alternatives to Proposal #6 until the lecture notes for Perry's Third Night.

### Objections to Proposal #6

Proposal #6 is the most sophisticated of the accounts we've seen so far. But that does not make it immune to criticism. On pp. 34-36, Gretchen advances several objections to this proposal.

We will note only the first of these objections here. This is that Proposal #6 makes the answer to the question "Does the chain connecting A* and B* compose a single person?" depend on matters extrinsic to that chain. Whether A* and B* count as parts of a single person will depend on whether B* has any competitors--that is, on whether there are any other person stages elsewhere in the universe which are also psychologically continuous with A*. But this seems wrong-headed. The facts about whether A* and B* are parts of the same person shouldn't turn on what other chains of person stages exist elsewhere in the universe. Personal identity should be an intrinsic matter. It should only depend or supervene on what the chain connecting A* and B* is like, intrinsically. Facts about other chains should not be relevant. (Think about whether this objection also applies to Proposals #7, #8, or #9.)