Philosophy & Public Affairs 31.4 (2003) 413-442
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 of personal identity.
Theories of personal identity generally offer a definition of the term person and address the issue of personal identity:"In what does a person's continuing existence over time consist?" Since the publication of Parfit's Reasons and Persons, personal identity theorists have also generally addressed the issue of what matters in survival: "What in our continuing existence (e.g., identity itself, psychological continuity) primarily matters from a prudential or self-interested standpoint?" Since the 1990s, the issue of our essence has also become prominent: "What are we human persons, most fundamentally: persons, human animals, or something else?"
Can philosophical argumentation help us to settle some or all of the marginal cases? Two recent, outstanding works—Jeff McMahan's The Ethics of Killing and David Boonin's A Defense of Abortion—answer confidently in the affirmative.
McMahan defends and appeals to a theory of personal identity in developing a theory of moral status and the ethics of killing in addressing the full range of marginal cases. His book is an enormously rich contribution to personal identity theory, ethical theory, and applied ethics. Let me briefly describe the five hefty chapters, each of which could be a short book of scholarly significance.
Chapter 1, which will receive extensive consideration in the next section, addresses personal identity theory. McMahan argues that we are essentially neither souls, human animals, nor persons (defined as self-conscious beings), but rather embodied "minds" (embodied beings with the capacity for consciousness); our identity is a function of the continuation of this capacity. As for what matters in survival, McMahan contends that the degree to which one should be egoistically concerned about some event in one's future varies with the degree of psychological unity between oneself now and oneself at the later time. Chapter 2 presents the most probing investigation of the harm of death of which I am aware. Its thesis is that we should understand this harm not in terms of the (prudential) value of one's life as a whole, but in terms of one's time-relative interest in continuing to live; as explained in the next section, this thesis draws from his view of what matters. Chapter 3 defends a two-tier account of the ethics of killing (as discussed more fully in this article's section on abortion). The ethics of killing "minded" nonpersons—in particular sentient animals and severely cognitively impaired humans—is to be governed by the time-relative interest account, which suggests that death harms nonpersons less than persons. Meanwhile, the ethics of killing persons is to be governed by a requirement of equal respect, which overrides consideration of persons' time-relative interests in continuing to live. The highly original Chapter 4 tackles abortion, employing both the...