The main idea behind multiple occupancy
The main idea is to account for two things that we believe about persons that appear to be in conflict: (1) survival involves some kind of psychological continuity and (2) survival involves being identical with some future person. Parfit showed how these can come apart in various ways.
The solution, at least in the case of fission, is to say that what we believe is that two distinct persons shared at least one stage prior to fission. Thus the original person A really was B and C, in some sense.
The main disadvantage here is that it seems to involve "overpopulation:" A, prior to the split, is just one person, not two, we think. Lewis replies that there are sensible counting rules we could use that would not result in an overpopulated world. Lewis holds a four dimensional view: A is a shared temporal stage of the four dimensional persons B and C. Okey dokey. So how many people do we count when we point at A?
Answer: we should count persons not by using identity (how many persons are identical with A before fission) but what Lewis calls tensed identity. Before fission (time-f), B and C are identical-at-time-f: both have stages at time-f and all and only stages of B are stages of C at time-f.
So, B and C, the continuing, four dimensional persons, are not identical to one another: after fission, B will lead his life and C will lead his. But before fission, at time-f, they were "identical-at-a-time" because their stages were the same.
Is that cheating? Well, how many roads must I cross to get to Othersidetown? Route 5 and the Rutherford B. Hayes Parkway share a stretch of road that I must cross. I would count the roads that I must cross using "identity-along-a-path" rather than identity. The two roads are different, hence not identical, but it would be weird to say I have to cross two roads when I'm just going to cross one stretch of asphalt. Solution: count the roads using something other than identity.
The choice we're faced with is whether multiple occupancy is less palatable than the paradoxical claim that "I can survive even though no one will be me."
The second point, that survival seems to require continued identity, is just taken for granted. So is the overpopulation objection. Can we do better?
We've already done a lot of work to give a deep rationale for the view that survival requires continued identity: see our extensive discussions of Williams's reasons for thinking that questions of personal identity must always have determinate answers.
What about the overpopulation objection? I don't find this so clear. First, it doesn't obviously conflict with Williams's point. That was that for a person to survive into the future, it has to be the case that there is something that it will be like for that person in the future. I can't see any obvious conflict there. Nor is it obvious that it conflicts with any of what we know about the unity of consciousness. It seems right to say that various thoughts and experiences are unified in consciousness because they are had by the same person and belong to the same mind. But that's compatible, on its face, with multiple occupancy: there's one mind that is shared by two persons. Thus, all the thoughts and experiences in that mind could be united in the more or less normal way.
My best thought was that the problem might be approached like this. Take Locke's definition of a person: a person is a thinking, intelligent being with reason and reflection that can consider itself as itself. Suppose that's basically right: when you've got a unified series of thoughts, you've got a person.
Can we say there could be more than one person in this case while retaining this understanding of what a person is? Would multiple occupancy amount to a departure from this way of saying what a person is? After all, Locke gave us a way of identifying a single person; would his definition have made any sense if it was a definition of "one or more persons"? If multiple occupancy did clash with this definition, perhaps we might conclude that's too high a price to pay for resolving the conflict among our beliefs. (I see from my notes that this is what Eugene Mills seems to hold: see the next page. I'll have to take another look at that article.
Here's a response to that objection (pending a re-reading of Mills). What Locke defines is not a person but a mind. What our intuitions about personal identity (see (1) and (2) above) tell us is that a person is not a mind. Locke's definition of a mind is compatible with the multiple occupancy view of persons: there is only one mind in A, prior to fission, and it is shared by two person-stages of two people.
Other versions of multiple occupancy
There are other versions of multiple occupancy. I'll give a brief run-down of these on the next page.
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