Fission and the Failure of Self-interest 1

Fission cases, in which one person appears to divide into two persons, pose a serious challenge to many assumptions of commonsense ethics. Chief among these assumptions is the idea that self-interest is an important source of reasons. We normally think that something like the following is true: Self-Interest Thesis: For any person S, and any two possibilities p 1 and p 2 , S has an important kind of reason, namely self-interested reason, to prefer p 1 to p 2 whenever S should expect that S would fare better if p 1 were to obtain than if p 2 were to obtain. 2 However, given certain widely-held metaphysical assumptions, the Self-Interest Thesis has highly counterintuitive implications in cases involving fission. In particular, given the assumption that a person would not survive fission, the Self-Interest Thesis appears to imply that we have an important kind of reason to avoid fission, since fission would terminate our lives and hence reduce our total welfare. And yet it seems, intuitively, that we needn't have any important kind of reason to avoid fission. For it seems that, in many cases, fission could be just as good as ordinary survival with respect to everything we should care about. And so the Self-Interest Thesis appears to get things wrong. 3 Many philosophers have responded to this kind of argument by rejecting the metaphysical assumptions on which it rests. In particular, many philosophers have suggested that fission is a process that one would survive. 4 Some have suggested that one would survive fission as one or other of the two individuals who emerge from the operation, and some have suggested that one would survive fission as both these individuals. Still others have suggested that, when fission occurs, it is undergone by two distinct persons who are initially colocated and who part ways at the time of the operation. Adopting one of these alternative views, it has been argued, will allow us to hold on to our commonsense understanding