Very few people seriously question the continuity of one’s personal identity over time. We all tend to assume that it is rational for a person to think of him-/herself as a conscious subject that continues to exist over time, through all of the changes in his/her body.
Let us suppose that some future technological breakthrough makes possible the instant cloning of a grown-up human body, as well as the splitting and successful re-combining of brain hemispheres and even whole bodies. There is no inherent impossibility involved in producing such technology, and thus this can form a basis for thought-experiments. Let us consider just one such experiment:
The body B1 of the person P1 is put to the cloning machine, and a clone body, B2 (presumably producing a similar person P2), is produced. After this, both of them are deep-frozen and split in half, so that we have the body halves B1a, B1b, B2a and B2b. Then the halves B1a and B2b are combined, and similarly those of B1b and B2a. Next, the bodies are unfrozen and put back to life. Now, some puzzling questions will arise: Does P1 still exist? If P1 has ceased to exist, at what point did this happen?
Many different answers can be proposed to such questions, but every answer that assumes materialism seems to face insurmountable difficulties. For example, one might argue that uniqueness is a necessary condition for personal identity (and thus, P1 ceased to exist when a copy of the body B1 was produced – even before the operation of splitting and recombining). This could seem somewhat plausible in light of the fact that the clone body B2 would be exactly identical, and would presumably produce a person that is in every observable and inferable respect identical to P1. However, a moments reflection reveals that this is absurd. For this would mean that one way to kill a person is to produce a perfect copy of the person’s body! Another option would be to say that P1 ceased to exist after the splitting-and-recombining operation, and now a different person exists (the same fate would then presumably fall on P2). But if that is so, we can further ask what will happen if the splitting-and-recombining operation is now repeated so that B1a and B1b (as well as B2a and B2b) are put back together. Will P1 exist again? If so, then does it matter whether this is done immediately or after, say, 2 years’ delay? Or how about a similar operation that is conducted incrementally in a step-by-step fashion over a period of 2 years? It is important to realize that what we are asking is not whether the person that gains consciousness after the operation is in every observable way identical to the previous person in his mental life. Rather, the question is whether the very same conscious subject of thoughts and experiences that existed as P1 will continue to have the future thoughts and experiences after the split-and-recombine operation.
What thought experiments such as the one that we have just examined seem to imply is that when the issue is approached from a materialist perspective, the question of personal identity is either an arbitrary matter with no clear “right” answers (in the sense of a correspondence to actual states of affairs), or it must be answered in the negative (in other words, the answer must be that personal identity does not continue – rather, there exists a series of persons over the period of time that the human being continues to exist). But if it is an arbitrary matter in such extreme cases as the one that we have examined, it is hard to avoid the conclusion that it is arbitrary in the normal cases as well. (If there is a non-arbitrary, actual continuity of personal identity at all, how can the answers to questions pertaining to it become “arbitrary” in any logically possible circumstances?) And if the materialist takes the view that there in fact is no continuity of personal identity, this too will have implications that are very hard to accept. (Certainly, there is a sense in which it is more warranted for a prisoner in a death row to fear his own execution than that of some other person whom he has never met.) In summary, I, at least, am much more confident of the fact that my personal identity does continue over time, than I could ever be of the truth of materialism. Therefore, if the materialistic reduction requires me to deny the continuity of my personal identity, this becomes a good reason to reject the materialistic reduction.