  What justifies our holding one person over another morally responsible for a past action? Why am I justified in having a special prudential concern for one particular future person over all 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? For these and other  normative questions, it looks like any answer we come up with will  have to make 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.