  There are two plausible replies, both familiar by now. On the one  hand, we can go four-dimensionalist (as does Sider 2001a), and hold that  Y and Z are indeed two people who were,  essentially, present all along, overlapping in all their spatial  parts during the temporal stage of the life when they were both known as “X.” This allows us to preserve the slogan that one person cannot be responsible for the actions of another  – in other words, it is indeed numerical identity that is necessary for responsibility – while also preserving the  thought that it is the robust psychological relation that obtains  between the various parties that grounds our belief that X  has not gotten away with his crime just by undergoing fission.