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Is psychological continuity sufficient for personal identity?
We have a philosophical problem of specifying the criteria for when to say that a person persists through time - that is, keeps the same identity. The most intuitive criterion of personal identity is the bodily criterion - that a person is the same person as one in the past if they have the same body. This is not to say that it retains all the same matter, because living things are involved in a constant exchange of matter with their environment, and in fact human beings change all their molecules over a course of about seven years. What the criterion consists in is that the matter constituting the person now has resulted from that constituting the person in the past by a series of more or less gradual replacements in such a way that it is correct to say that the body of the person now is identical with the body of the person in the past, in the same way as you might specify for a ship or an oak tree. This criterion is the most intuitive because it tends to work in ordinary life. We reidentify people by their bodies. An improvement on the bodily criterion is the brain criterion, owing to the fact that the brain seems to have special importance in determining the psychology of the person whose brain it is, in a way that their knee or their liver doesn't. If Brown's brain is transplanted into Robinson's body, then we are inclined to say that the one who has Robinson's body is Brown, because he has all of Brown's memories, characteristics, desires and intentions. However, not even the entire brain is necessary here. The brain has two very similar hemispheres. People who have half their brain removed can survive intact and the remaining half of their brain can learn to perform the tasks that used to be performed by the other hemisphere. Therefore a further improvement is that what is necessary for personal identity is not identity of the whole brain, but identity of enough of the brain to be the brain of a living person. This is called the physical criterion. However even the physical criterion does not seem to be necessary for someone to continue being the same person. This thought-experiment is given by Williams 1970: imagine that someone needs brain repairs and so they have the information in their brain removed and put into some storage device (i.e. the device is put into some state information-theoretically equivalent to the total state of the brain) and part of the repair process actually involves giving him a new brain, after which the information is put back into the new brain, and he recovers all his dispositions, with regard to memory and so forth. We would certainly want to say that this was the same man. It seems that psychological identity is secured without identity of brain or part-brain - and it turns out to be the psychological features that make us want to say that they are the same person. This is the kind of line of thinking that has led many philosophers to adopt psychological continuity as the criterion for personal identity. Psychological continuity consists in there being overlapping chains of direct psychological connections such as memory, beliefs, desires and intentions. It is not necessary to go into details about the psychological continuity criterion, however, because there is an argument which undermines any 'empirical' criterion for personal identity.
Imagine a case of 'fission' - where one person divides into two. My brain is divided in half, and each half is housed in a new body. Both resulting people have my character and apparent memories of my life. Now the question is, what happens to me? Either I survive as both people, one of them, or none. It is impossible to say that I survive as both people while being identical to both of them, because identity is not a one-many relation. It is also bizarre to say that I survive as only one of them, since there are no criteria for deciding which one. It is also problematic to say that I do not survive at all (I die), because we would be perfectly happy to say that I would survive if my brain was only transplanted into one body - but we certainly do not think that my survival in that cases is dependent on the non-existence of the rival candidate, which is surely a completely irrelevant factor. This thought-experiment shows any empirical criterion for personal identity to be inadequate for deciding the identity question in this case, unless the question is allowed to turn on a seemingly totally insignificant factor. If we want our criteria for personal identity to be decisive in all possible cases, then it would seem that no empirical criterion, physical or psychological, could be up to the job, since it will always be possible to come up with some reduplication scenario where the question of identity seems to have no plausible answer. One reaction to the inadequacies of empirical criteria for personal identity is to take what is called the Simple View. On this view, personal identity doesn't consist in any of these empirical things. Personal identity is a simple and unanalysable fact, and the empirical criteria are only evidence for personal identity. According to this view, there are always answers to questions of personal identity; it's just that in some cases personal identity can be underdescribed by the empirical data. An alternative to the simple view, which is the view that I would take, is that personal identity doesn't consist in any of these empirical facts because it does not truly consist in anything - there is no right answer to these questions except by convention. It is easy to think of cases of non-human things where questions of identity are tricky, such as in cases of things like bridges and ships where parts are replaced and repaired to such an extent that it is not made of any of the same stuff any more. The question of identity needs to be decided for reasons of convenience (such as whether to keep the same name), but anyone would agree that to go on arguing about the matter would be pointless - at some point the decision needs to be made fairly arbitrarily. One could argue that with persons it is exactly the same. The problem cases just go to show that our use of the language of identity is conventional, and that our conventions do not cover certain unusual cases. The identity of persons is not unique from the identity of things. This is in line with the argument that Parfit gives. According to Parfit, the fission argument and other problem cases undermine any empirical criterion as a criterion for identity, but he believes that psychological continuity is a successful criterion for survival, which does not require identity after all. This is Parfit's argument as summarised by Lewis: (a) Identity is a relation with a certain formal character. It is one-one and does not admit of degree. (b) A relation of mental continuity and connectedness need not have that formal character; we can imagine problem cases in which any such relation is one-
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