A lot of discussion of memory theories of personal identity invokes science-fictional thought experiments, such as when memories are swapped between two brains.
One of the classic papers is Shoemaker’s “Persons and their Pasts”. There, Shoemaker accounts for personal identity across time, at least in the absence of branching, in terms of appropriate causal connections between apparent memories, not just any causal connections.
This matters. Imagine that Alice and Bob both get total memory wipes, so on the memory theory they cease to exist. But the person inhabiting the Alice body then reads Bob’s vividly written diary, which induces in her apparent memories of Bob’s life. I think most memory theorists will want to deny that after the reading of the diary, Bob comes back to life in Alice’s body. Not only would this be a highly counterintuitive consequence, but it would violate the plausible principle that whether someone is dead does not depend on future events, absent something like time travel. For suppose this sequence:
Monday: Memory wipe
Tuesday: Person inhabiting Alice’s body lives a confused life
Wednesday: Person inhabiting Alice’s body reads Bob’s diary, comes to think she’s Bob, and gains all sorts of “correct” apparent memories of Bob’s life.
On Wednesday, the person inhabiting Alice’s body has memories of the person inhabiting Alice’s body on Tuesday, so by the memory theory they are the same person. But if on Wednesday, it is Bob who inhabits Alice’s body, then Bob also already existed on Tuesday by transitivity of identity. On the other hand, if Alice hadn’t read the diary on Wednesday, Bob would not have existed either on Wednesday or on Tuesday. So whether Bob is alive on Tuesday depends on future events, despite the absence of anything like time travel, which is absurd.
To get around diary cases, memory theorists really do need to have an appropriateness condition on the causal connections. Shoemaker’s own appropriateness condition appears inadequate: he thinks that what is needed is the kind of connection that makes a later apparent memory and an earlier apparent memory be both of the same experience. But Alice’s induced apparent memories are of the experiences that Bob so vividly described in his diary, which are the same experiences that Bob set down his memories of.
What the memory theorist should insist on are causal chains that are of the right kind for the transmission of memories, modulo any sameness-of-person condition. But now it is far from clear that the science-fictional scenarios in the literature satisfy this condition. Certainly, the scanning of memories in a brain and the imposition of the same patterns on a brain isn’t the normal way for memories to be causally transmitted over time. That it’s not the normal way does not mean that it’s not an appropriate way, but at least it’s far from clear that it is an appropriate way.
It would be interesting what one should say about a memory theory on which the appropriate causal chain condition is sufficiently strict that the only way to transfer memories from one head to another would be by physically moving the brain. (Could one move a chunk of the brain instead? Maybe, but only if it turns out that memories can be localized. And even so it’s not clear whether coming along with a mere chunk of the brain is the appropriate way to transmit memories; the appropriate way may require full cerebral context.) Such a version of the memory theory would not do justice to “memory swapping” intuitions about the memories from one brain being transferred to another. And I take it that such memory swapping intuitions are important to the case for the memory theory.
Here’s another implausible consequence of this kind of memory theory. Suppose aliens are capturing people, and recording their brain data using a method that destroys the memories. However, being somewhat nice, the aliens then use the recording to restore the memories, and then return the person to earth. On the memory theory, anybody coming back to earth is a new individual. That doesn’t seem quite right.
A challenge for the memory theorist, thus, is to have an account of the appropriate causal chain condition that is sufficiently lax to allow for the memory swap intuitions that often motivate the theory but is strict enough to rule out diary cases. This is hard.
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