How To Determine Which Is The True Theory Of Personal Identity
Swinburne (Richard)
Source: Gasser (Georg) & Stefan (Matthias), Eds. - Personal Identity: Complex or Simple?
Paper - Abstract

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Introduction (Full Text)

  1. The simple view1 of diachronic personal identity holds that personal identity is not constituted by continuities of mental or physical properties or of the physical stuff (that is, the bodily matter) of which they are made, but is a separate feature of the world from any of the former, although of course it is compatible with personal identity being caused by such continuities. On the simple view, as I shall understand it, a person P2 at t2 can be the same person as a person P1, at an earlier time t1, whatever the physical or mental properties and whatever the body possessed by each person. P2 may not at t2 remember2 anything done or experienced by P1 at t1 or earlier, and may have an entirely different character and a totally different body (including brain) from P1 at t1. The main arguments in favor of the simple view consist in adducing thought experiments in which persons undergo radical changes of mental life and bodily constitution, and in which – it is claimed – it is “possible” that they continue to exist; from which it follows that continuities of the kind mentioned are not necessary for personal identity.
  2. I begin with one example (of very many which have been put forward) to indicate the role of thought experiments in supporting the simple view. Suppose I have a severe brain disease affecting the right brain hemisphere. The only way to keep my body functioning is to replace this hemisphere. So the doctors remove my current right hemisphere and replace it by a right hemisphere taken from a clone3 of me. The new right hemisphere, let us suppose, contains the brain correlates of (that is the neurons, the states of which are the immediate causes of) similar but slightly different memories and character traits from mine. The resulting person would presumably to some extent behave like me and remember having done what I did and also to some extent behave like my clone4 and remember having done what my clone5 did (when what I did was different from what my clone6 did). Now suppose that the disease spreads to the left hemisphere, and that too – two years later – is replaced by a left hemisphere taken from a different clone7 of me, again containing the brain correlates of similar but slightly different memories and character from mine. Then my body would be directed by a brain made of totally different matter and sustaining rather different memories and character from those I had two years previously. Yet presumably to some extent, but to a lesser extent than after the first operation, the resulting person would still behave like me and remember having done what I did.
  3. But would the resulting person be me? That person would be a person largely continuous with the earlier me two years ago, apart from having had two large brain operations. One might think that the continuity of many mental and physical properties over this period has the consequence that the same person continues to exist. Yet the resulting person would have none of the brain matter and only some of the memories and character which were previously mine. I suggest that it is totally unobvious whether in this situation the resulting person would or would not be me. Yet the question “Would the resulting person be me?” is logically equivalent to the question “Would I survive the operations?” and so have the (pleasurable or unpleasant) experiences of the resulting person. And surely no one about to have a serious operation can think that the question of whether he will “survive” a brain operation is simply a question requiring an arbitrary decision about which of two senses we should give to the word “survive.” We (or at least most of us) seem to understand the alternatives as mutually incompatible factual alternatives – that I survive, or that I do not survive – in one clear and natural sense of “survive.” Yet it is totally unobvious what is the answer. If you think that – one way or other – the answer is obvious, it is easy to alter the thought experiment in such a way that the answer is no longer obvious. If you think it is obvious that the continuity of at least half the brain matter over each of the operations two years apart insures that I continue to exist, suppose the second operation to be performed after only one year or six months. If you think it obvious that when half my brain matter is removed in one operation I no longer exist, suppose a series of operations replacing only a tenth of the matter each time.
  4. In such a situation, which I call an ambiguous situation, it does seem possible that I have survived (i.e. continued to exist), and possible that I have not survived; and yet that we do not know (and have no further means of finding out) whether I have or have not survived. If what seems possible is indeed possible, my survival does not require any particular degree of strong physical and mental continuities8 which make it obvious that I do survive. It then follows that the difference between the situations of different degrees of continuity consists in the strength of the evidence that I continue to exist. Under normal conditions of very strong continuity of body (and in particular of the brain, the physical sustainer of mental life), memory (of what happened to a person with that body) and character, it is enormously probable that I continue to exist; it becomes less and less probable until we reach the ambiguous situation where it is as probable as not that I continue to exist. Why it is enormously probable that under those normal conditions I continue to exist is first because it is a fundamental epistemological principle that (apparent) memory beliefs are probably true (in the absence of counter-evidence), and my personal memories (that is memories “from the inside” about what I did and experienced) concern the actions and experiences of the person who had a brain strongly continuous with my present brain. Unless memories as such (in the absence of counter-evidence) are probably true (and so do not require to be rendered probable by evidence of some other kind in order to be probably true), we would know very little about the world. For we depend on memory for all the knowledge which we believe that we have acquired from others about history and geography, etc.; and while my memory of these things may coincide with yours, at any time I depend on my own memory of what others have told me for my belief that our memories do coincide, and so the personal memory of each of us must as such be probably true if we are to have virtually any knowledge at all. And the second reason why it is enormously probable that under those normal conditions I continue to exist is that the simplest, and so most probable, hypothesis supported by the strong continuity of memory and character sustained by the same brain is that these are mental properties belonging to the same person. It would be less simple, and so less probably true, to suppose that the memory and character strongly continuous with the previous memory and character sustained by a brain having strong continuity with the previous brain are the memory and character of a different person. So being the same person does not entail strong continuity of brain, character and memory; but the latter is good evidence of the former. This is the simple view.
  5. Some philosophers hold that personal identity, like the identity of artifacts9, can be a matter of degree. On this view a later person can be only partly identical to some earlier person, and so the result of such operations as I have described might be that the resultant person was only partly me. I do not myself think that it is logically possible that some person be partly me. But even if this were a possible result of the operations, it does not seem to be a necessary truth that the operations will have this result, because the history of all the physical bits and all the mental properties associated with them seems compatible with the subsequent person not being partly me. It still seems possible that, just as the resulting person is fully me if I have both heart and liver replaced, so after the half-brain transplants the resulting person is still fully me; and it is also possible that it is not me at all. Yet if we include the subsequent person being partly me as a possible result of the operations, we would now be ignorant about which out of three (rather than two) possible results of the operations had in fact occurred. If what “seems possible” is possible, that I survive the operations not merely in part but wholly (or alternatively not at all), partial survival is compatible with the simple view.
  6. The alternative to the simple view, the complex view, claims that personal identity is constituted (not merely evidenced) by a certain particular degree of continuity of physical and mental properties and of the matter which forms a person’s body. The main arguments in favor of this view are that the paradigm examples of personal identity are all ones in which there is continuity of these kinds, and that the simple view leads to contradictions. There are innumerable varieties of the complex view according to which degrees of continuity ensure the identity of a later person with an earlier person. One variety is the view that the concept of personal identity has no application outside normal situations of strong physical and mental continuities. Another variety of the complex view holds that necessarily (not merely possibly, as in the version of the simple view) personal identity is a matter of degree. The weaker the continuities, the lesser the degree to which the later person is the same as the earlier person. Again there is an issue about this variety of the complex view, as about the similar variety of the simple view, as to whether the notion of partial identity of persons makes any sense.


For the full text, see Swinburne - How To Determine Which Is The True Theory Of Personal Identity.

In-Page Footnotes

Footnote 2:
  • Ordinary language sometimes assumes that only true beliefs are correctly called “memories.” Thus it assumes that if I am correctly said to have a “memory” that I did so-and-so, I really did so-and-so.
  • I shall not follow that usage here, but shall understand by “memory” what on that usage would only be an apparent memory: it seeming to the subject that he “remembered” so-and-so.
Footnote 8:

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