I. Introduction

Derek Parfit says in Reasons and Persons that he decided to study philosophy “almost entirely because [he] was enthralled” by the possibility of personal fission, that is, a person dividing in the manner of an amoeba. In 1971 he published an article, “Personal Identity” (hereafter '71), in which he sketched the themes that he later developed (with a few revisions) in Part III of Reasons and Persons (hereafter R&P). One of his themes is the so-called reductionist thesis that

A second general theme is that some of our commonsensical beliefs about rationality and morality need to be revised given the reductionist thesis (especially its rejection of a necessarily determinate entity at the center of our existence as persons). The possibility of fission plays a central role in Parfit's arguments for the practical ramifications of the reductionist thesis. For example, he argues that identity is not “what matters,” and his argument is based on his analysis of fission—in such a case, he argues, one would not survive the fission but nonetheless one would have what matters in survival (that is, one would have what one should care about insofar as one wishes to survive into the future).