- The philosophical problem of personal identity can seem a matter of life and death. For what matters1 to us in our personal survival seems to involve, partly because in our experience it always has involved, the preservation of at least our bodily identity over time.
- The supposition that it does, however, the supposition that what matters2 is some form of identity, is susceptible to certain intractable logical difficulties arising from some recent results in neurophysiology. The Sperry experiments on epileptics whose Corpora Collosa have been cut present fairly persuasive evidence that the human brain exists as a pair of very similar hemispheres, each one of which could in principle exist and (with a little tampering) function fully independently of the other.
- Only technological (and perhaps some moral) difficulties prevent a brain being divided into two, one hemisphere being transplanted3 to one new skull, the other to another. In such a case, our usual criteria of personal identity — bodily or psychological continuity4 — would break down. For they would present us with two (over time) equally eligible, but (at a given time) bodily and psychologically quite distinct candidates for the continued identity of the original person. And that would involve a violation of the transitivity of that relation: the original person could not be identical with both of the resulting persons without both of the resulting persons being, as they fairly clearly are not, identical with each other.
- On the other hand, whatever it is that matters to us in our personal survival would seem to be preserved between the original and each of the resulting persons. There is no reason to suppose there to be at any time even a break in the (diverging) streams of consciousness. Such duplication5 couldn't be as bad as death: it might arguably be preferable to the usual uniqueness.
- Rather, then, than have what matters6 to us in survival be contravened by logical law, it seems more reasonable to suppose, as Derek Parfit7 has recently done us the service of supposing, that what matters8 to us is not identity over time:
The relation of the original person to each of the resulting ones contains all that interests us — all that matters — in any ordinary case of survival.... Identity is a one-one relation. [This] case serves to show that what matters9 in survival need not be one-one. Indeed, not only may what matters10 be one-many, as in the envisaged case of fission, but, considering fissions of two brains followed by fusions of the odd halves, what matters11 may also be many-one.
- This would, to be sure, involve a travesty of ordinary talk. A person, on this account, may survive yet not continue to exist, since she may survive as two different persons. For example: a candidate for fission would survive but not continue to exist as (would not be identical with) either of the resulting persons. Where it will be important to avoid equivocation between this and ordinary use, I shall speak instead of a relation "S" which preserves what matters12 in x's survival. Similarly, I shall use 'death' to refer not to the end point of a single personal existence, but, rather, to the furthest temporal point(s) of the longest stretch of any given S-related chain. It may well have been the obvious awkwardness of this manner of speaking that obscured for so long this way of dealing with Sperry's results.
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