A Survival Guide to Fission

The fission of a person involves what common sense describes as a single person surviving as two distinct people. Thus, say most metaphysicians, this paradox shows us that common sense is inconsistent with the transitivity of identity. Lewis’s theory of overlapping persons, buttressed with tensed identity, gives us one way to reconcile the common sense claims. Lewis’s account, however, implausibly says that reference to a person about to undergo fission is ambiguous. A better way to reconcile the claims of common sense, one that avoids this ambiguity, is to recognize branching persons, persons who have multiple pasts or futures. Tomorrow Adam will undergo fission. His fission will be like that of an amoeba’s, with the two resulting people, call them Cain and Abel, both bodily and psychologically continuous with Adam. Since the fission will not take place until tomorrow, it seems that today Adam is still one person. Once the fission is complete, Cain and Abel will, it seems, be two different people. Both of these conclusions are supported by the concept of a person. If there is an embodied mind that is independent of other bodies and minds, then it constitutes a single person. If Adam is two people right now — or, more properly, if ‘Adam’ ambiguously refers to two people — how could we explain the fact that both are standing in the exact same spot having exactly the same thoughts? Wouldn’t this identity of mental and bodily properties constitute their being the same person? Similarly, if next week at this time Cain and Abel are identical, how could we explain the one’s being in Cairo wondering if he paid the child support for the month while at that same moment the other is in Dallas envying his neighbor’s new kayak? Doesn’t this difference constitute their being different people? Yet it also seems that Adam will not cease to exist once he undergoes fission tomorrow. The mental and physical changes that will lead from Adam to Cain are of a piece with the changes that we all undergo every day — except, of course, for the fact that some great mass of flesh will separate from him. Similarly, mutatis mutandis, for the changes that will lead from Adam to Abel. Intuitively, Adam does not die but survives. The puzzle is that what results from his survival are two people. Focusing on Adam and Cain and the series of changes between them, it seems they are the same person, yet similar reasoning suggests that Adam and Abel are also the same person. But how can this be, for, as we’ve said, surely Cain and Abel are not the same person? Common sense seems to have led us to deny the transitivity of identity.1 1 I speak of what ‘common sense’ says about such cases. The expression does not mean that what is being judged is common; rather, it means that the judgment is the unreflective opinion of the common person. It is not at all unusual to ask whether, according to common sense, it would be wrong to do act A, where A is something never encountered.