condition of an organism’s having a right to life. But this won’t do, because one certainly wants to ascribe a right to life to adult humans who are completely paralyzed. Maybe the suggestion is rather that motility is a sufficient condition of something’s having a right to life. However, it is clear that motility alone is not sufficient, since this would imply that all animals, and also certain machines, have a right to life. Perhaps, then, the most reasonable interpretation of the claim is that motility together with some other property is a sufficient condition of something’s having a right to life, where the other property will have to be a property possessed by unborn members of the species Homo sapiens but not by unborn members of other familiar species.
The central question, then, is what this other property is. Until one is told, it is very difficult to evaluate either the moral claim that motility together with that property is a sufficient basis for ascribing to an organism a right to life or the factual claim that a motile human fetus possesses that property while a motile fetus belonging to some other species does not. A conservative would presumably reject motility as a cutoff point by arguing that whether an organism has a right to life depends only upon its potentialities, which are of course not changed by its becoming motile. If, on the other hand, one favors a liberal view of abortion, I think that one can attack this third suggested cutoff point, in its unspecified form, only by determining what properties are necessary, or what properties sufficient, for an individual to have a right to life. Thus I would base my rejection of motility as a cutoff point on my claim, defended above, that a necessary condition of an organism’s possessing a right to life is that it conceive of itself as a continuing subject of experiences and other mental states.
The second suggested cutoff point – the development of a recognizably human form – can be dismissed fairly quickly. I have already remarked that membership in a particular species is not itself a morally relevant property. For it is obvious that if we encountered other “rational animals,” such as Martians, the fact that their physiological makeup was very different from our own would not be grounds for denying them a right to life.24 Similarly, it is clear that the development of human form is not in itself a morally relevant event. Nor do there seem to be any grounds for holding that there is some other change, associated with this event, that is morally relevant. The appeal of this second cutoff point is, I think, purely emotional.
The overall conclusion seems to be that it is very difficult to defend the cutoff points traditionally advanced by those who advocate either a moderate or a liberal position on abortion. The reason is that there do not seem to be any basic moral principles one can appeal to in support of the cutoff points in question. We must now consider whether the conservative is any better off.
V Refutation of the Conservative Position
Many have felt that the conservative’s position is more defensible than the liberal’s because the conservative can point to the gradual and continuous development of an organism as it changes from a zygote to an adult human being. He is then in a position to argue that it is morally arbitrary for the liberal to draw a line at some point in this continuous process and to say that abortion is permissible before, but not after, that particular point. The liberal’s reply would presumably be that the emphasis upon the continuity of the process is misleading. What the conservative is really doing is simply challenging the liberal to specify the properties a thing must have in order to be a person, and to show that the developing organism does acquire the properties at the point selected by the liberal. The liberal may then reply that the difficulty he has meeting this challenge should not be taken as grounds for rejecting his position. For the conservative cannot meet this challenge either; the conservative is equally unable to say what properties something must have if it is to have a right to life.
Although this rejoinder does not dispose of the conservative’s argument, it is not without bite. For defenders of the view that abortion is always wrong have failed to face up to the question of the basic moral principles on which their position rests. They have been content to assert the wrongness of killing any organism, from a zygote on, if that organism is a member of the species Homo sapiens. But they have overlooked the point that this cannot be an acceptable basic moral principle, since difference in species is not in itself a morally relevant difference. The conservative can reply, however, that it is possible to defend his position – but not the liberal’s – without getting clear about the properties a thing must possess if it is to have a right to life. The conservative’s defense will rest upon the following two claims: first, that there is a property, even if one is unable to specify what it is, that (i) is possessed by adult humans, and (ii) endows any organism possessing it with a serious right to life. Second, that if there are properties which satisfy (i) and (ii) above, at least one of those properties will be such that any organism potentially possessing that property has a serious right to life even now, simply by virtue of that potentiality, where an organism possesses a property potentially if it will come to have that property in the normal course of its development. The second claim – which I shall refer to as the potentiality principle – is critical to the conservative’s defense. Because of it he is able to defend his position without deciding what properties a thing must possess in order to have a right to life. It is enough to know that adult members of Homo sapiens do have such a right. For then one can conclude that any organism which belongs to the species Homo sapiens, from a zygote on, must also have a right to life by virtue of the potentiality principle.
The liberal, by contrast, cannot mount a comparable argument. He cannot defend his position without offering at least a partial answer to the question of what properties a thing must possess in order to have a right to life.
The importance of the potentiality principle, however, goes beyond the fact that it provides support for the conservative’s position. If the principle is unacceptable, then so is his position. For if the conservative cannot defend the view that an organism’s having certain potentialities is sufficient grounds for ascribing to it a right to life, his claim that a fetus which is a member of Homo sapiens has a right to life can be attacked as follows. The reason an adult member of Homo sapiens has a right to life, but an infant ape does not, is that there are certain psychological properties which the former possesses and the latter lacks. Now, even if one is unsure exactly what these psychological properties are, it is clear that an organism in the early stages of development from a zygote into an adult member of Homo sapiens does not possess these properties. One need merely compare a human fetus with an ape fetus. What mental states does the former enjoy that the latter does not? Surely it is reasonable to hold that there are no significant differences in their respective mental lives – assuming that one wishes to ascribe any mental states at all to such organisms. (Does a zygote have a mental life? Does it have experiences? Or beliefs? Or desires?) There are, of course, physiological differences, but these are not in themselves morally significant. If one held that potentialities were relevant to the ascription of a right to life, one could argue that the physiological differences, though not morally significant in themselves, are morally significant by virtue of their causal consequences: they will lead to later psychological differences that are morally relevant, and for this reason the physiological differences are themselves morally significant. But if the potentiality principle is not available, this line of argument cannot be used, and there will then be no differences between a human fetus and an ape fetus that the conservative can use as grounds for ascribing a serious right to life to the former but not to the latter.
It is therefore tempting to conclude that the conservative view of abortion is acceptable if and only if the potentiality principle is acceptable. But to say that the conservative position can be defended if the potentiality principle is acceptable is to assume that the argument is over once it is granted that the fetus has a right to life, and, as was noted above, Thomson has shown that there are serious grounds for questioning this assumption. In any case, the important point here is that the conservative position on abortion is acceptable only if the potentiality principle is sound.
One way to attack the potentiality principle is simply to argue in support of the self‐consciousness requirement – the claim that only an organism that conceives