No serious national debate about abortion preceded the Supreme Court’s 1973 decision in Roe v. Wade. Instead, most of the cultural and philosophical battles have come afterwards. Since then, two general strategies have been developed by those who believe that abortion is at least sometimes morally acceptable. First, some argue that not all human beings are persons. While it is uncontroversial that what is present from the moment of conception is a human being, some argue that until certain characteristics necessary for personhood are present, we do not have a person and so we do not have anything that possesses serious moral status. This has been the strategy of philosophers Peter Singer, Michael Tooley, and Mary Anne Warren, among others, and if successful, abortion is morally unproblematic. Second, others argue (most famously Judith Jarvis Thomson) that even if all human beings are persons, sometimes (or even all the time) the rights of the mother outweigh the rights of the unborn person, and therefore the choice to abort is morally acceptable.

Professor Christopher Kaczor’s The Ethics of Abortion: Women’s Rights, Human Life, and the Question of Justice is now the most current and comprehensive philosophical defense available of the claim that both these strategies fail, and that therefore abortion is morally unacceptable. Happily, Kaczor’s clear and even-handed writing makes it an excellent resource for anyone who wishes to think carefully about the significant arguments on either side. The bulk of The Ethics of Abortion takes on these two types of argument and their many permutations, and Kaczor then concludes with a chapter on difficult cases and a chapter on the possibility that artificial wombs might end the abortion debate.

For those who claim that some human beings are not persons and therefore have no significant moral status, the central philosophical challenge is to identify non-arbitrary criteria for personhood that manage to include all those human beings that are undoubtedly persons and exclude only those that might plausibly fail to be persons. Many potential criteria, as Kaczor carefully shows, fail because they are arbitrary: birth (there is surely no good reason to think a fetus is a non-person one minute before birth but a person one minute after birth, or, for that matter, that merely the location of a being can determine whether or not it is a person), viability (besides being relative to the state of technology, why should dependence on another human being make one a non-person; after all, we do not think this is true of severely conjoined twins), human appearance (looks can be deceiving; a burn victim charred beyond recognition is nevertheless a person), implantation (were we to develop artificial wombs, the absence of implantation would certainly not result in the absence of personhood—why should mere attachment to someone make such a difference?).

The danger of arbitrariness forces a move toward criteria chosen because of their obvious connection to the powers and abilities of mature human persons. But the threat here is that we will include too many things as persons, or exclude beings that are obviously persons. We therefore cannot look, for example, to sentience (the ability to feel pain and pleasure), because this would include leeches and wasps, but exclude rational but unfeeling aliens and angels, as well as human beings suffering from a rare but very real condition that makes one insensitive to pain. The most promising and powerful strategy here is to focus on those characteristics that are obviously connected to personhood, and the most plausible of these is “consciousness.” Consciousness lies at the foundation of a Lockean conception of personhood, for example, and it would be difficult to imagine any creature (animal, alien, or angel) that had no connection to consciousness and was nevertheless a person.

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But now we face a different set of difficulties. If we take the criterion as it stands, that one must be conscious to qualify as a person, all sleeping and sedated human beings will fail to be persons, a reductio ad absurdum. And if we instead claim that you are a person only if you have an immediately exercisable capacity for self-awareness, we will include sleeping adults, but exclude those in temporary comas (who might take years to recover, and who at any rate almost never have an “immediately exercisable capacity” for consciousness). Though philosophers like Peter Singer and Michael Tooley do not consider this a reason to reject this strategy, whatever interpretation we give the criterion will justify not just abortion at any point during pregnancy but also infanticide (at least for days or weeks after birth, and for some philosophers, even months).

Instead, Kaczor defends an endowment account of human personhood over against the performance accounts defended by Singer, Tooley, and others. A performance account of human personhood “holds that a being is to be accorded respect if and only if the being functions in a given way,” whereas an endowment account “holds that each human being has inherent moral worth simply by virtue of the kind of being it is.” And by “endowment” Kaczor means “an intrinsic, dynamic orientation towards self-expressive activity [like] . . . rationality, autonomy, and respect.” Are you a person because you are something that actually engages in rational and free conscious activity, or are you a person because you are the kind of thing that engages in rational and free conscious activity?

It is only, argues Kaczor, if we look to the kind of thing that you are rather than your actual activity that we will be able to escape many serious moral difficulties. For example, any plausible performance criterion (rationality, self-awareness, sentience, and so on) implies qualitative measurements by which all of us can and do differ dramatically; if the criteria for personhood come in degrees, it looks like personhood will have to come in degrees as well, and this immediately destroys any foundation for equal human dignity and rights (those of us who happen to be smarter are therefore persons to a greater degree and our rights should therefore weigh more in the balance against those who are less “personal”—a sadly recurring theme in human history, and surely one that should make defenders of abortion nervous). Again, performance accounts of human personhood are plagued by the “episodic problem.” Performance account criteria may, by definition, be absent while biological life is present, but if the organism then regains its previous powers, the same human being has gone in and out of “personhood,” and could do so many times over the course of one lifespan. But surely personhood cannot be so trivial that it can come and go episodically in this way. (Imagine, for example, that our technology improves such that we can heal those in a persistent vegetative state—we will be forced to decide whether or not to make this organism a person again.)

If, on the other hand, we accept that all human beings are persons, there are still those who argue that abortion is nevertheless acceptable because the rights of the mother outweigh the rights of the unborn person. Here Kaczor focuses almost exclusively on Judith Jarvis Thomson’s 1971 article “A Defense of Abortion” (an extraordinarily influential article—an influence unmatched, I might add, by the power of its arguments). It was Thomson’s central claim that women who do not wish to be pregnant have no duty to maintain the life of the unborn person within them. This is because we do not have special responsibilities towards particular individuals unless we voluntarily assume them, and therefore a woman who does not implicitly or explicitly assume responsibility for her child does no wrong in separating herself from it through abortion.

For example (and the example is Thomson’s), if you are abducted against your will and hooked up to a sick but famous violinist who needs to share your kidneys for nine months, you do no wrong in detaching yourself, even if it means his death. As critics of Thomson have often pointed out (pro-choice critics as well as pro-life critics), it looks like this will justify abortion only in cases of rape (only about 1% of abortions in the United States).  If one willingly engages in activity whose natural outcome is a new human person, the analogy no longer applies.  And even in cases of rape, it follows that one may only “remove” the unborn person, but not directly dismember it. (Suppose you are justified in detaching yourself from the violinist—this does not mean you are justified in shooting him dead.)

Even if Thomson and others can somehow defend the usefulness of the analogy, it is nevertheless quite unreasonable to think that special responsibilities arise only if we voluntarily assume them. After all, that way of thinking would allow any reluctant father to deny child support, for he never agreed to assume the special responsibilities attached to fathering his child. More generally, family relationships are precisely the sort of relationships that do ground special responsibilities we bear towards others, even if we never agree to them. This is why I am at fault, and obviously so, if I take no interest in the support of my elderly parents and justify my actions by saying that I never voluntarily assumed responsibility for them.

Contrary to Thomson, if unborn human beings are persons, and if we must weigh the right to life of an innocent person against various other rights of the mother, Kaczor argues that the right to life wins out. This is both because mothers bear special responsibilities towards their children, and because various lesser rights do not outweigh the right to life. (It is grossly implausible, for example, that one’s right to privacy entails that one may also kill innocent human persons.) The one potential exception (a very rare one, given modern technology) is when the life of the mother is at stake, and Kaczor agrees that sometimes it will be acceptable in such circumstances to bring about the death of the fetus in order to save the mother, though even here one must do so in a manner that does not include directly intending to kill the unborn person. (If a pregnant woman discovers she has a cancerous uterus, it is permissible to surgically remove the uterus, even though this will result in the death of her child; neither she nor her doctors ever directly aim at the death of her child—it is instead an unintended side effect.)

Kaczor concludes, therefore, that both overall strategies fail, and so the inescapable conclusion is that abortion is morally unacceptable. And even if one is not necessarily convinced by Kaczor’s positive arguments in favor of the moral status of the unborn, his devastating critiques of the philosophical defenses of abortion show that the burden of proof lies squarely with them.