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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 of itself as a continuing subject of experiences has a right to life. For this requirement, when taken together with the claim that there is at least one property, possessed by adult humans, such that any organism possessing it has a serious right to life, entails the denial of the potentiality principle. Or at least this is so if we add the uncontroversial empirical claim that an organism that will in the normal course of events develop into an adult human does not from the very beginning of its existence possess a concept of a continuing subject of experiences together with a belief that it is itself such an entity.

I think it best, however, to scrutinize the potentiality principle itself, and not to base one’s case against it simply on the self‐consciousness requirement. Perhaps the first point to note is that the potentiality principle should not be confused with principles such as the following: the value of an object is related to the value of the things into which it can develop. This “valuation principle” is rather vague. There are ways of making it more precise, but we need not consider these here. Suppose now that one were to speak not of a right to life, but of the value of life. It would then be easy to make the mistake of thinking that the valuation principle was relevant to the potentiality principle – indeed, that it entailed it. But an individual’s right to life is not based on the value of his life. To say that the world would be better off if it contained fewer people is not to say that it would be right to achieve such a better world by killing some of the present inhabitants. If having a right to life were a matter of a thing’s value, then a thing’s potentialities, being connected with its expected value, would clearly be relevant to the question of what rights it had. Conversely, once one realizes that a thing’s rights are not a matter of its value, I think it becomes clear that an organism’s potentialities are irrelevant to the question of whether it has a right to life.

But let us now turn to the task of finding a direct refutation of the potentiality principle. The basic issue is this. Is there any property J which satisfies the following conditions: (1) There is a property K such that any individual possessing property K has a right to life, and there is a scientific law L to the effect that any organism possessing property J will in the normal course of events come to possess property K at some later time. (2) Given the relationship between property J and property K just described, anything possessing property J has a right to life. (3) If property J were not related to property K in the way indicated, it would not be the case that anything possessing property J thereby had a right to life. In short, the question is whether there is a property J that bestows a right to life on an organism only because J stands in a certain causal relationship to a second property K, which is such that anything possessing that property ipso facto has a right to life.

My argument turns upon the following critical principle: Let C be a causal process that normally leads to outcome E. Let A be an action that initiates process C, and B be an action involving a minimal expenditure of energy that stops process C before outcome E occurs. Assume further that actions A and B do not have any other consequences, and that E is the only morally significant outcome of process C. Then there is no moral difference between intentionally performing action B and intentionally refraining from performing action A, assuming identical motivation in both cases. This principle, which I shall refer to as the moral symmetry principle with respect to action and inaction, would be rejected by some philosophers. They would argue that there is an important distinction to be drawn between “what we owe people in the form of aid and what we owe them in the way of non‐interference,”²⁵ and that the latter, “negative duties,” are duties that it is more serious to neglect than the former, “positive” ones. This view arises from an intuitive response to examples such as the following. Even if it is wrong not to send food to starving people in other parts of the world, it is more wrong still to kill someone. And isn’t the conclusion, then, that one’s obligation to refrain from killing someone is a more serious obligation than one’s obligation to save lives?

I want to argue that this is not the correct conclusion. I think it is tempting to draw this conclusion if one fails to consider the motivation that is likely to be associated with the respective actions. If someone performs an action he knows will kill someone else, this will usually be grounds for concluding that he wanted to kill the person in question. In contrast, failing to help someone may indicate only apathy, laziness, selfishness, or an amoral outlook: the fact that a person knowingly allows another to die will not normally be grounds for concluding that he desired that person’s death. Someone who knowingly kills another is more likely to be seriously defective from a moral point of view than someone who fails to save another’s life.

If we are not to be led to false conclusions by our intuitions about certain cases, we must explicitly assume identical motivations in the two situations. Compare, for example, the following: (1) Jones sees that Smith will be killed by a bomb unless he warns him. Jones’s reaction is: “How lucky, it will save me the trouble of killing Smith myself.” So Jones allows Smith to be killed by the bomb, even though he could easily have warned him. (2) Jones wants Smith dead, and therefore shoots him. Is one to say there is a significant difference between the wrongness of Jones’s behavior in these two cases? Surely not. This shows the mistake of drawing a distinction between positive duties and negative duties and holding that the latter impose stricter obligations than the former. The difference in our intuitions about situations that involve giving aid to others and corresponding situations that involve not interfering with others is to be explained by reference to probable differences in the motivations operating in the two situations, and not by reference to a distinction between positive and negative duties. For once it is specified that the motivation is the same in the two situations, we realize that inaction is as wrong in the one case as action is in the other.

There is another point that may be relevant. Action involves effort, while inaction usually does not. It usually does not require any effort on my part to refrain from killing someone, but saving someone’s life will require an expenditure of energy. One must then ask how large a sacrifice a person is morally required to make to save the life of another. If the sacrifice of time and energy is quite large it may be that one is not morally obliged to save the life of another in that situation. Superficial reflection upon such cases might easily lead us to introduce the distinction between positive and negative duties, but again it is clear that this would be a mistake. The point is not that one has a greater duty to refrain from killing others than to perform positive actions that will save them. It is rather that positive actions require effort, and this means that in deciding what to do a person has to take into account his own right to do what he wants with his life, and not only the other person’s right to life. To avoid this confusion, we should confine ourselves to comparisons between situations in which the positive action involves minimal effort.

The moral symmetry principle, as formulated above, explicitly takes these two factors into account. It applies only to pairs of situations in which the motivations are identical and the positive action involves minimal effort. Without these restrictions, the principle would be open to serious objection; with them, it seems perfectly acceptable. For the central objection to it rests on the claim that we must distinguish positive from negative duties and recognize that negative duties impose stronger obligations than positive ones. I have tried to show how this claim derives from an unsound account of our moral intuitions about certain situations.

My argument against the potentiality principle can now be stated. Suppose at some future time a chemical were to be discovered which when injected into the brain of a kitten would cause the kitten to develop into a cat possessing a brain of the sort possessed by humans, and consequently into a cat having all the psychological capabilities characteristic of adult humans. Such cats would be able to think, to use language, and so on. Now it would surely be morally indefensible in such a situation to ascribe a serious right to life to members of the species Homo sapiens without also ascribing it to cats that have undergone such a process of development: there would be no morally significant differences.

Secondly, it would not be seriously wrong to refrain from injecting a newborn kitten with the special chemical, and to kill it instead. The fact that one could initiate a causal process that would transform a kitten into an entity that would eventually possess properties such that anything possessing them ipso facto has a serious right to life does not mean that the kitten has a serious right to life even before it has been subjected to the process of injection and transformation. The possibility of transforming kittens into persons will not make it any more wrong to kill newborn kittens than it is now.

Thirdly, in view of the symmetry principle, if it is not seriously wrong to refrain from initiating such a causal process, neither is it seriously wrong to interfere with such a process. Suppose a kitten is accidentally injected with the chemical. As long as it has not yet developed those properties that in themselves endow something with a right to life, there cannot be anything wrong with interfering with the causal process and preventing the development of the properties in question. Such interference might be accomplished either by injecting the kitten with some “neutralizing” chemical or simply by killing it.

But if it is not seriously wrong to destroy an injected kitten which will naturally develop the properties that bestow a right to life, neither can it be seriously wrong to destroy a member of Homo sapiens which lacks such properties, but will naturally come to have them. The potentialities are the same in both cases. The only difference is that in the case of a human fetus the potentialities have been present from the beginning of the organism’s development, while in the case of the kitten they have been present only from the time it was injected with the special chemical. This difference in the time at which the potentialities were acquired is a morally irrelevant difference.

It should be emphasized that I am not here assuming that a human fetus does not possess properties which in themselves, and irrespective of their causal relationships to other properties, provide grounds for ascribing a right to life to whatever possesses them. The point is merely that if it is seriously wrong to kill something, the reason cannot be that the thing will later acquire properties that in themselves provide something with a right to life.

Finally, it is reasonable to believe that there are properties possessed by adult members of Homo sapiens which establish their right to life, and also that any normal human fetus will come to possess those properties shared by adult humans. But it has just been shown that if it is wrong to kill a human fetus, it cannot be because of its potentialities. One is therefore forced to conclude that the conservative’s potentiality principle is false.

In short, anyone who wants to defend the potentiality principle must either argue against the moral symmetry principle or hold that in a world in which kittens could be transformed into “rational animals” it would be seriously wrong to kill newborn kittens. It is hard to believe there is much to be said for the latter moral claim. Consequently one expects the conservative’s rejoinder to be directed against the symmetry principle. While I have not attempted to provide a thorough defense of that principle, I have tried to show that what seems to be the most important objection to it – the one that appeals to a distinction between positive and negative duties – is based on a superficial analysis of our moral intuitions. I believe that a more thorough examination of the symmetry principle would show it to be sound. If so, we should reject the potentiality principle, and the conservative position on abortion as well.