||We often judge that a certain person at one time is the same person as a certain person at another. The person in the prison cell now is the same person as the person who stole the jewels. But what is it that makes for personal identity, that makes a certain person at one time the same person as a certain person at another?
Those dualists who hold that the mind is immaterial may hold that one person is identical to another just if he or she has the same immaterial soul. One objection to this account is Locke\'s. Locke claimed that it is possible for a person to transfer from one immaterial soul to another, so that sameness of immaterial soul is not necessary for personal identity. He also claimed that it is possible for one immaterial soul to be associated with one person at one time and then another later on, so, sameness of immaterial soul is not sufficient for personal identity.
Others have held that personal identity is a matter of bodily continuity. One can survive the loss of various limbs. Indeed one could survive a brain transplant operationâ€”I could move from one human body to another when my brain was transplanted from one to the other. So those who hold that personal identity is a matter of bodily continuity hold that personal identity is a matter of the continuity of the brain.
But this suggestion is vulnerable to an objection concerning the possibility of brain bisection. Suppose that the brain of person A is cut in half and that the result is two people, B and C, both of whom have half a brain each of which is continuous with A\'s brain. (Note that some people do survive with less than half of an ordinary brain.) Both B and C have brains that are continuous with A\'s brain, but they cannot both be identical to A. For if A is identical with B and B is identical with C, then B must be identical with C. (This is simply a consequence of the transitivity of identity, the logical truth that if x = y and y = z, then x = z.) But B and C, two people existing at the same time, can hardly be identical. So brain continuity is not sufficient for personal identity.
Locke argued that personal identity is a not a matter of sameness of immaterial soul, or of bodily continuity, but of memory. Person A is identical with an earlier person B just if A can remember performing B\'s action. This suggestion is also open to criticism. We need to distinguish between genuinely remembering performing an action and merely seeming to remember performing an action. Genuinely remembering performing an action is sufficient for being identical with the person who performed it. But as Butler pointed out, since one can genuinely remember performing an action only if one is the person who performed it, genuine memory presupposes and does not explain personal identity. And seeming to remember performing an action is hardly sufficient for being the person who performed it one can seem to remember doing things which were in fact done by other people, such as paying the bill. Further, neither genuinely nor seeming to remember performing an action is necessary for being the person who performed it, as the existence of amnesia makes clear. AJ
See also death; dualism; person; personhood.Further reading J. Foster, The Immaterial Self; , J.L. Mackie, Problems for Locke.