What conclusions can we draw concerning the moral status of abortion?  I have focused mainly on the central type of argument for an extreme anti-abortion position, an argument which involves the claim that abortion is always in itself at least prima facie very seriously wrong on the grounds that all human beings, including zygotes, embryos, and fetuses, have a right to life that is just as weighty as that possessed by normal, innocent, adult human beings.  The crucial question that arises for this type of argument is this: What property is possessed by human zygotes, embryos, and fetuses, and which is such that any entity that possesses that property has a right to life?   As regards responses to that question, we have canvassed the following five possibilities:

(1)  The possession of an immaterial, immortal soul gives an entity a right to life;

(2)  The possession of the capacity for relevant higher mental functioning - such as the capacity for thought - gives an entity a right to life;

(3)  Membership in the biologically defined species, Homo sapiens, gives an entity a right to life;

(4)  The possession of the active potentiality for acquiring the capacity for relevant higher mental functioning - such as the capacities for thought and self-consciousness - gives an entity a right to life;

(5)  Membership in a species whose normal adult members possess the capacity for relevant higher mental functioning - such as the capacities for thought and self-consciousness - gives an entity a right to life.

Let us now briefly consider how those five suggestions fare.

(1)  The Immaterial, Immortal Soul Criterion

        If human beings possessed immaterial souls that (1) were present from conception, and (2) involved the capacity for relevant higher mental functioning - such as the capacity for thought - then it might well be argued that human beings have a right to life from conception onward.  But the problem with this criterion is, first, that there is no plausible argument for the presence of an immaterial soul even in the case of adult human beings; and secondly, that what we know about neurophysiology, both in the case of humans and other animals, provides positive evidence against the hypothesis that mental functioning is based upon capacities that reside in an immaterial substance.

(2)  The Capacities Criterion

        The objection to using this criterion to support the claim that human zygotes, embryos, and fetuses have a right to life is, first, that there is no behavioral evidence for the claim that humans at such stages in their development have, for example, the capacity for thought, and, secondly, that what we know about the dependence of the capacity for thought not only upon the existence of a brain, but also upon the presence in the brain of certain very complex neuronal circuitry provides very strong evidence that human beings in the early stages of their development do not possess any capacity for thought.

(3)  The Membership in Homo Sapiens Criterion

        Appeal to this criterion is, as we have seen, exposed to two main objections.  First, there are excellent reasons for holding that the principle that all innocent members of the biologically defined species Homo sapiens have a right to life cannot be a basic moral principle: there must, for example, be some principle that does not refer to particular species that explains why members of other species - consider the imaginary case of ET - also have a right to life.  But if the principle in question cannot be basic, then one needs to go back to the underlying, basic principle, and show that that principle supports the claim that humans have a right to life from conception onward.  But this, of course, takes one back to the other criteria that we have been considering, and if none of those criteria is sound, then neither is the derived criterion that appeals to membership in the biologically defined species, Homo sapiens.

        The second objection is that the claim that all innocent members of the biologically defined species, Homo sapiens, have a right to life is exposed to counterexamples - such as the cases of anencephalic infants, and adults who have suffered whole brain death or upper brain death.

(4)  The Potentiality Criterion

        The objection to the claim that having certain potentialities - such as the potentiality for acquiring the capacity for thought - is that, on the one hand, none of the arguments that have been offered in support of this criterion is sound, and, on the other hand, that there are very plausible arguments in support of the claim that this criterion is unsound.

(5)  The 'Normal Members of a Species' Criterion

        This final proposal is very closely related to the third criterion above, and is exposed to the same types of objection.  Thus, in the first place, just as there are strong reasons for holding that a basic moral principle will not contain any reference to a particular species, so there are strong reasons for holding that it should not involve the general concept of a species.  In the second place, the present criterion entails that all members of the biologically defined species, Homo sapiens, have a serious right to life, and so it is exposed to precisely the same counterexamples as the third criterion - namely, the cases of anencephalic infants, and adults who have suffered whole brain death or upper brain death.

        The upshot is, first, that none of the arguments that advocates of an extreme anti-abortion position offer in support of the claim that all humans from conception onward have a right to life stands up to critical scrutiny, and, secondly, that that claim has itself been shown to be false in view of the counterexamples provided by the cases of humans that either lack a brain, or that suffer from brain defects or brain damage so sever that no mental life at all is present.

        The extreme anti-abortion position is, therefore, refuted.  What, then, is the correct position on abortion?  Answering that question would require much more discussion.  In particular, one would need to find satisfactory answers to the following two questions:

(1)  What property, or properties, are the basis of an entity's having a right to life?

(2)  At what stage in the development of a human being does it acquire the property, or properties, in question?

        The first of these questions is a question in ethical theory.  We have seen that some answers - e.g., answers to appeal to species membership, or to an immaterial soul, or to potentialities - are unsound.  But what is the correct answer?

        Unfortunately, this is one of the most difficult issues in moral theory, and so we cannot even begin to address it here.  But in thinking about this issue, it is important to have in mind the general sort of view that many liberals on abortion would hold is correct:

First, a developing entity does not have a right to life until it acquires relevant psychological capacities - such as the capacity for thought and/or self-consciousness.  Secondly, once an entity acquires the right to life, it does not lose that right if it temporarily loses the relevant capacities:  if it could in principle regain those capacities, then it continues to possess a right to life.  Thirdly, an entity does lose the right to life if it can never again have the relevant capacities.

        Though I myself favor a slightly different criterion, I think that the personhood criterion has much to recommend it.  First, it explains the intuitions that most people have both about the cases of anencephalic human infants, and about adult human beings who have suffered whole brain death or upper brain death.

        Secondly, it enables one to offer a non-arbitrary, species-free account of the differences in moral status of normal adult members of different species, both real and imaginary.  Mice do not have a right to life, not because they are not members of our species, but because they do not have the capacity for thought and/or self-consciousness.  Chimpanzees, on the other hand, may have a right to life, even though they are not members of our species, since they may have the relevant psychological capacities, and the relevant sort of mental life.  And beings such as the imaginary ET would certainly have a right o life completely on a par with that of normal adult members of our species, since such beings clearly would possess the capacity for thought and self-consciousness.

        Thirdly, a plausible view of rights is that they function to protect interests, and if this is correct, then one can offer an argument for the personhood criterion, since one can argue that individuals who possess the capacity for thought and self-consciousness will be able to envisage their own continued existence, and will be able to have desires about that, and those desires in turn will make it the case that their continued existence is, in general, in their own interest.  Attribution of a right to life to entities that satisfy the personhood criterion is therefore justified, because that attribution will contribute to the protection of an interest that such entities have in their own continued existence.

        If something like the personhood criterion is correct, which position on the moral status of abortion is correct: a moderate view, or a liberal one?  This will depend on the correct answer to the second question mentioned above - that is, the question:

(2)  At what stage in the development of a human being does it acquire the property, or properties, in question?

        Unlike the first question, this is not a philosophical question as such, but a scientific question, and only careful scientific investigation can settle at what point, for example, a developing human being first acquires the relevant capacities - such as the capacity for thought and the capacity for self-consciousness.  If those capacities are acquired, say, at some point during fetal development, then fetuses will have a right to life from that point onward, and so abortion would in itself be prima facie very seriously wrong from that point onward.   Presumably, then, a moderate view on abortion would be correct.  If, on the other hand, the relevant capacities are not acquired prior to birth, then a liberal view on the moral status of abortion will be correct.

        In short, while there strong reasons for holding that the extreme anti-abortion view is untenable, the choice between  moderate views and an extreme liberal view is, at present, a matter for further investigation, since that choice depends both upon the answer to some difficult philosophical questions concerning the precise properties that are the basis of the right to life, and also upon a scientific determination of the point in the development of human beings when the relevant properties are first acquired.  But answers to both of these questions are well within reach.  Accordingly, it should not be long before the difficult question of the moral status of abortion is settled, once and for all.