Last lecture we talked about John Locke’s memory theory of personal identity, which is a version of a psychological approach to solving the problem of personal identity (POPI). The main intuition, or idea, that motivates a psychological approach to POPI is called the transplant intuition. The transplant intuition is just this: Imagine that Johnny Depp’s mind (brain, soul, whatever) is taken out of his own body and is transplanted into the body of another person. Many philosophers think that the proper response to a scenario like this is to say that the body that has Johnny Depp’s brain is Johnny Depp. Parfit thinks that instances of fission are problematic for psychological approaches such as John Locke's.
My version of the Parfit fission example: Brian’s Brain. Imagine Brian’s brain has been cut into two parts. By some fete of magic or neuroscientific wonder, the two halves of the brain are exactly the same; each one functions as Brian’s brain. One half is given to Jay and the other to Bob. Both Jay and Bob can remember Brian’s life in an experiential way. In other words, they have the same autobiographical memory. We call this a fission scenario (we can contrast this with a fusion scenario, where two minds are joined into one body).
Fission cases are a problem for a Lockean theory of personal identity. In a fission scenario, both Jay and Bob have the same quality of autobiographical memory. According to Locke’s theory of personal identity, Jay and Bob are both identical to Brian. By the law of transitivity (if A=B and B=C, then A=C), Jay and Bob are identical. But this seems obviously false! Jay and Bob are two distinct people! Parfit thinks there are three ways to react to this kind of problem for Locke caused by the transitivity of identity.
Solution 1: Neither Jay nor Bob is identical to Brian. Brian’s identity is lost. Response: But each of those halves of the brain is really Brian’s brain! Just one of those halves is enough to be Brian! Whatever happens to the other half is irrelevant.
Solution 2: Brian is either Bob or Brian is Jay. Response: It is silly to say that it is either one or the other because there is no way to discriminate, or tell the difference, between the two beforehand. To choose one or the other as the bearer of identity would be silly since they are both equally good candidates beforehand.
Solution 3: Brian is both Bob and Jay. Response: But this consequence is ridiculous. By transitivity of identity, Bob and Jay are both identical to Brian and each other, but we know that Bob and Jay are actually distinct, so we must deny transitivity of identity, which is a silly law to reject.
Parfit thinks that this third option is the most feasible, but he wants to draw a distinction between survival and identity. Hey says that Brian is not identical to Bob or Jay. However, Brian does survive as both Bob and Jay. Parfit thinks that much of the confusion in the discussion about POPI comes from mixing up the words “survival” and “identity”. Parfit says that only academic philosophers worry about identity. Most people only worry about survival.