What justifies our holding a person morally responsible for some past action? Why am I justified in having a special prudential concern for some future persons and not others? Why do many of us think that maximizing the good within a single life is perfectly acceptable, but maximizing the good across lives is wrong? In these and other normative questions, it looks like any answer we come up with will have to make an essential reference to personal identity. So, for (...) instance, it seems we are justified in holding X responsible for some past action only if X is identical to the person who performed that action. Further, it seems I am justified in my special concern for some future person only if he will be me. Finally, many of us think that while maximization within a life affects only one person, a metaphysical unity, maximization across lives affects many different, metaphysically distinct, persons, and so the latter is wrong insofar as it ignores this fundamental separateness of persons. (shrink)