Locke’s position is that the identity of a person over time is independent of the identity of substances, whether material or immaterial. He argued for this position by challenging the alternatives. Specifically, he denied that the necessary and sufficient conditions for personal identity that the alternatives propose are correct.
His motivation for doing this was to show that materialism, the view that everything in the universe is made of physical matter, was compatible with resurrection after death. The reasons why this posed a problem are obvious enough: what if part of the matter that makes up my body also made up the bodies of other people?
Locke himself was unsure about whether the psychological activity in our minds is supported by material or immaterial substances (see § 13, p. 337). His claim was that it doesn’t matter. Personal identity over time consists in the ability to extend consciousness over thoughts and experiences at different times. Whatever the ultimate nature of the mind is, we can know that about persons, in his opinion. And that means that it wouldn’t matter if materialism turned out to be true. People could still be resurrected even if the matter of their bodies could not be reassembled.
Locke was willing to grant that psychological activity, that is, our thoughts and experiences, depends on either a material or immaterial substance. You can’t think without a material brain or an immaterial soul. (Again, he didn’t know which; neither do I, though I would bet pretty heavily on the brain.)
But he denied that you have to have the same material brain or immaterial soul in order to remain the same person.
The examples we discussed were meant to show this. That is, they were meant to show that it is possible for A to be the same person as B even if A and B have different brains, bodies, or immaterial souls.
Another way to put it is that Locke was willing to concede that a person needs to have some material brain or immaterial soul. He just denied that having the numerically identical material brain or immaterial soul was important. A person could survive with a suitable replacement.
Locke thought that the first case was uncontroversial. Why should people be any different than plants and animals (§§3–6, §12)? But once you’ve granted that, it’s hard to say why you shouldn’t say something similar about the other two.
He tried to make the point with examples. These examples were supposed to both refute the alternative and show that Locke’s view was correct. So he asked us to take seriously the story of the “very rational Man” who thought he had Socrates’s soul, that is, he asked us to imagine that the man really did have Socrates’s soul (§14, p. 339). Then he claimed that it’s obvious that the very rational Man could not be the same person as Socrates if he could not remember any of Socrates’s thoughts or experiences. If you accept that claim, it follows that having the same soul as Socrates is not a sufficient condition of being the same person as Socrates.
What about being the same living animal or “man,” to use Locke’s terminology? Is being the same man a necessary and sufficient condition of A’s being the same person as B? The example of the Prince and the Cobbler is supposed to refute those conditions. We are supposed to be persuaded that it is possible for the person of the Prince to start in the Prince’s body and move to the Cobbler’s body while remaining the same Prince person. That shows that having the same body is not a necessary condition of the Prince’s continuing to exist as the same person. It also implies that it is not a sufficient condition. If having the same body were a sufficient condition of being the same person, the person in the Cobbler’s body would always be the Cobbler. But we were persuaded that the person in the Cobbler’s body changed from the Cobbler person to the Prince person.
Danny wasn’t sure that these examples are genuinely possible. If so, Locke’s argument wouldn’t work for him, it seems to me. I did two things. First, I drew a distinction between what it technically possible and what is logically possible. For instance, air travel was not technically possible 500 years ago, but it was clearly logically possible for human beings to fly: we do it now, after all. The implication was supposed to be that “body-switching” might be logically possible even though, of course, we have no techniques for doing it now. Second, I argued that it didn’t take too much of a stretch to think that it might actually be technically possible to “copy” the information from one brain to another, such that the person with the original brain would have seemed to have switched bodies. Is that enough? Maybe.
What about amnesia, Lauri asked? If I were to lose my memories of, say, last week, would it follow that I did not exist last week? If so, what happened? Did the person with my name and body die to be replaced by me as I am now? You can multiply the problems if you think seriously about the limitations of memory.
Locke was willing to accept these problems. He thought our reluctance to punish people for what they cannot remember doing showed that we already treat persons this way. That is, he thought, we regard the person before as different than the one whose actions this person cannot remember, even though they share a body, name, and all the other normal marks of being the same person. We did not discuss this and I’ll leave it to you to think about whether that seems like the best explanation of our reluctance to punish people in the sorts of cases he mentions. See §§19–23.
Danny raised a very disturbing thought about the Prince and the Cobbler example. What if the person in the Prince body on that strange morning could remember the Prince’s thoughts and experiences just as well as the person in the Cobbler body on that morning could remember them? We would have a situation like this:
To see the problem, consider a simpler way of representing that:
We’ll discuss all three problems in the next several sessions.
One thing we did was to try to translate the opening sentence of §13. This proved extremely difficult, such that we managed to split on exactly what it meant! (It introduced a challenge to the proposition that having the same immaterial soul is a necessary condition for A’s being the same person as B. The first sentence of §14 does the same for the sufficient condition, for what it’s worth.)
If you’re having trouble with Locke’s prose, or Descartes’s or Hume’s, for that matter, there is an excellent resource available. Jonathan Bennett, a first rate philosopher, has “translated” some of these texts into contemporary prose. You can find these at his website: Early Modern Texts. You have to be careful with what you find on the internet but this is the real deal. I’m sorry that I didn’t mention it earlier.