|1. Psychological Continuity|
A quick overview of Shoemaker's defense of the psychological continuity criterion.
Locke's view: memory criterion: A at t1 is the same person as B at t2 iff B at t2 remembers (at least some of) the experiences of A at t1.
[We might want to refine in one way or another. I prefer a 4D version: A-at-t1 is part of the same person as B-at-t2 iff B-at-t2 remembers (at least some of) the experiences of A-at-t1.]
Problem 1: Reid: contradiction. Suppose C at t3 remembers some of the experiences of B at t2, and B at t2 remembers some of the experiences of A at t1, but C at t3 remembers none of the experiences of A at t1. Then on Locke's account, A = B and B = C, but A is not identical with C. But identity is transitive, so if A = B and B = c then (by transitivity) A = C. Contradiction!
Solution 1: don't use memory, use the ancestral of memory. A at t1 is the same person as B at t2 iff there is a chain if intermediates I1, I2, . . . In such that B remembers experiences of In, who remembers experiences of . . . who remember experiences of I1, who remembers experiences of A.
Problem 2: Butler: circularity. Not enough for B at t2 to remember things that happened to A at t1, since you can remember things that happened to people other than you. We could say: B at t2 remembers things that happened to A at t1 from the inside, so to speak. But that seems to mean that B at t2 remembers things that happened to A at t1 and B = A, which would make the account circular. [There are various circularity worries in this general ballpark.]
Solution 2: use quasi-memory rather than memory. To quasi-remember an experience is basically to seem to remember it as having been one's own, but leaves open the possibility that it might not have occurred or, more interestingly, that it might not have been one's own. (If someone uses the matter duplicator on me, my duplicate will quasi-remember my past experiences as though they were his own, but maybe they aren't.)
Problem 3: Couldn't I survive an experience that gives me total amnesia?
Solution 3: Expand the account from continuity of memory to psychological continuity, where this includes memories, but also beliefs, desires, character traits, intentions, etc.
|2. The Fission Problem|
Fission seems to cause major problems for any kind of continuity view (bodily as well as psychological).
Let A step into the duplicator, and B and C step out. (Or: split up A's brain, but half in B and half in C. Or: suppose A has the possibility of splitting, like an amoeba, into two. Etc. etc.)
It seems clear that B is not C. (They may be exactly alike, but they are not numerically the same person, because there are two of them!)
Can we say A = B and A = C? No; by symmetry and transitivity, this implies that B = C, leading to a contradiction.
Can we say A = B but not A = C? There's no logical contradiction there, but whatever reasons there are for saying A = B apply equally to C, so there's no reasonable ground for choosing A = B instead of A = C.
Can we say A = C but not A = B? Same problem as the previous possibility.
Can we say A is not B and A is also not C? No logical
contradiction, but there may still be a major problem: If C had not come into
existence, some would say that A = B. Similarly, if B had not come into existence,
some would say that A = C. But, as Parfit famously asked, "How can a double
success be a failure?"
|3. Two Approaches|
Moving perhaps a bit too quickly, it seems that an account in terms of continuity of any kind will face the fission problem. (As well as a fusion problem, but set that aside for now, as the examples are even more farfetched than the fission ones.) Identity is 1-1, but no sort of continuity is necessarily 1-1.
So what should we say? There are two things we could say.
1. The "complex view" or the "Bundle view": identity is a kind of myth. There really isn't any interesting relation that has the properties we think of personal identity as having. We should basically toss the idea of personal identity in favor of a relation or collection of relations that can be one-many or many-one and that are matters of degree rather than yes/no.
2. The "simple view" or the "Ego view" (which amounts to adopting Cartesian dualism): B at t2 is the same person as A at t1 iff B and A have the same soul. Continuity of experiences etc. may be evidence for having the same soul, but is not conclusive. This is the view that Swinburne supports.