Personal Identity (Fall 1999) / Notes / Physical and psychological fission

Physical and psychological fission

Brain splitting and the reduplication argument

The thrust of Williams's reduplication argument in "Personal identity and individuation" is to force us towards saying that personal identity must consist in something that can't (apparently) be reduplicated such as a physical object. The strong suggestion is that the human body is the relevant object.

But does that avoid the problem? Can't you split a body in two (such that each half continues to function)? If so, wouldn't that show that physical bodies are subject to the reduplication problem too? Then the reduplication argument would be a general problem about identity over time and not a specific problem faced by the psychological criterion of personal identity.

How could a body split in two (such that each half continues to function)? Amoebae do it all the time. Human beings can't, but, it's logically possible that they could. It's logically possible that we could be like amoebae: who knows, maybe twenty-third century medicine will make it really possible. What's more, there's reasonably good evidence that it may be actually possible (someday) to split the most important physical part of a human being: the brain.

Williams's answer

First, here's a more formal restatement of the problem. If a relationship between A and B could also hold between A and C at the same time, then the fact that relationship holds between A and B does not establish that A and B are identical. Can the relationship of physical continuity be like this?

Here's Williams's reasons for saying "no." If there are two contemporaneous physical bodies that have equally strong relations of physical continuity to an original physical body, then fission had to be an event in their history and both have to exist (or have existed). From the fact that one of the contemporaneous bodies exists, it has to be the case that the other does (or did) too. There can't just be one half of an orange.

But psychological continuity isn't like that. Two contemporaneous beings can have equally strong relations of psychological continuity with an original being and each would be related to the original in exactly the same way that it would have been related to it if the other did not exist.

Again, for physical bodies, the following two points cannot be true at the same time:

(1) two contemporaneous physical bodies have equally strong relations of physical continuity to an original body.

and

(2) each would have the very same relation of physical continuity to the original (i.e. the relation that it does, in fact, have) if the other did not exist.

So, the relationship of physical continuity that holds between A and B in the normal case (where there is no third body, C, that might have the same physical relationship to A as B does) is different from the relationship that holds between A and B when there is fission (and there is a third body, C, that might have the same physical relationship to A as B does).

So in the normal case, the relationship between A and B could not also hold between A and C at the same time. So, the fact that A is physically continuous with B is sufficient to show that A is the same physical body as B.

This sets physical continuity apart from the relationship between one set of psychological characteristics and another (or, one Quintonish soul and another, perhaps you prefer 'one mind and another'). There it can be true that:

(3) two contemporaneous sets of psychological characteristics have equally strong relations of psychological continuity to an original set.

and

(4) each would have the very same relation of psychological continuity to the original (i.e. the relation that it does, in fact have) if the other did not exist.

So, the relationship of psychological continuity that holds between A and B in the normal case (where there is no third thing, C, that has the same psychological relationship to A as B does) could also hold between A and C at the same time. So, the fact that A is psychologically continuous with B is not sufficient to show that A is the same psychological entity (mind, soul, set of psychological characteristics, etc.) as B.

The battle and the war

Suppose Williams is right: the bodily criterion isn't subject to the reduplication problem. What about Parfit's brain transplant cases, (or any kind of bodily fission you accept)?

Would I exist after such an operation? I have apparently contradictory opinions: "No one is identical with me; no one is the same person as me after the operation" and "I can survive such an operation".

What Parfit does is push the question of whether the possibility that a relation might be subject to reduplication really amounts to an objection to that relation's capturing what is most important about survival.

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