Identity, killing, and the boundaries of our existence.

We all agree that it is generally wrong to kill persons. Although we may differ over exceptions, and over which beings qualify as persons, the judgment that it is wrong, other things equal, intentionally to kill paradigm persons represents common moral ground. At the same time, there is profound disagreement regarding the ethics of killing-or allowing to die-those beings whose moral status is less certain than that of paradigm persons: nonhuman animals, fetuses, infants, the severely retarded and the severely demented, individuals in permanent vegetative states (PVS), and others. Moreover, for those who assume that we human persons have moral status for as long as we exist, related controversies concern the boundaries of our existence: When did we come into existence, and when do we go out of existence or die? Let us refer to all these issues as the marginal cases. One commonly hears that the marginal cases represent areas of moral and/or ontological indeterminacy. According to this position, our clashing beliefs about, say, abortion or meat-eating reflect diverging assumptions about moral status that lie beyond the reach of rational adjudication. Many philosophers, however, are more optimistic. A common strategy for addressing the marginal cases is to appeal to a theory of moral status. A relatively novel approach within this broader strategy is to support a theory of moral status with a theory ofpersonal identity.