Arguments against abortion can take a number of forms, but there seem to be two which ground their resistance in the alleged right to life of the embryo, what I will call the equivalence argument and the potentiality argument. The equivalence argument claims that, at least in respect to killing, that at some point in gestation, the embryo acquires the same moral properties and so the same rights as a typical adult human being, and so it is wrong to kill it in exactly the same way as it is wrong to kill a typical adult human being. The point in gestation varies across versions of the argument: fertilisation, acquisition of sufficient neural infrastructure to feel pain, and viability outside the womb are some of the more common. The potentiality argument claims, rather than the embryo acquiring, in this respect, the same moral properties as a typical adult human being and so having a right against being killed, that the embryo gains a right to life not because of properties it has now, but properties it will have in the future. Again, this right can be gained at a variety of points in the 'lifecycle' of the embryo. The point though, is that the arguments are different, at least in their form, because one invokes properties which exist in the present, and one invokes properties which may exist at some point in the future.
This might seem to be to the advantage of the equivalence argument: properties which may exist at some point in the future are fairly slippery things, since not only do they not exist now, but they may never exist. Michael Tooley has offered a rather devastating critique of the potentiality argument, which, ignoring the quite obvious problem of regress in the invoking of potential, asks us rather pointedly whether we should care about potential at all. Imagine that there are kittens which, with a single injection, could be made to develop in such a way to come to possess whatever properties ground the basic moral status of adult human beings. So far as Tooley is concerned, if we have a duty of some sort to bring the potentialities of a human embryo into being, then we have a duty of an identical sort to bring the potentialities of these kittens into being on the simple ground of consistency, since in terms of the potentialites of becoming an agent relevantly like a typical human adult, the two are identical. Most of us, I think, feel that we don't have a duty to inject the kittens and turn them into cats with the intellectual faculties of the typical adult human being, so, presumably, we don't have a duty to do the equivalent with human embryos, which must mean that invoking their potentialities as a ground for a duty not to destroy them rests on a mistake.
The equivalence argument, which relies on the claim that, at least at some point during pregnancy, the embryo acquires the properties which ground moral concern about taking the lives of typical human adults, however, also has a number of faults. The most obvious of these is that human embryos are not typical human adults, and have few of the properties which distinguish typical human adults from other animals, making it rather difficult to claim that they share whatever property or set of properties it is that grounds moral concern about taking the lives of typical human adults. It seems unlikely that the simple capacity to feel pain grounds the moral concern we have about taking the lives of typical human adults, since it appears that most other animals feel pain, and most of us tend to think that killing other animals is probably morally acceptable, as long as it is done humanely. Likewise, viability outside of the womb wouldn't seem to ground moral concern about taking life, since we would neither regard the taking of a human life which relied on the direct interference of others as necessarily any differently from the taking of any other human life, nor do we take whether they can survive on their own or not to be an important consideration in the slaughter of livestock. Finally, fertilisation - at which point the embryo is a single celled organism - would by the logic of equivalence imply serious moral concern for other single celled organisms, like amoeba.
Thus, it seems that there is nothing to be said, from the perspective of the rights of the embryo, against abortion. This would not of course necessarily exhaust reasons against abortion. It might be possible to make arguments on the grounds of responsibility and maybe also from other, non-rights-based, grounds, grounds of the desirability of the attiitudes towards the embryo and sexual behaviour in general which abortion might embody. This, of course is not to endorse any of these arguments, merely to mention that, however good or bad they might be, they perhaps could be made. Still, even if such arguments could be made, I think it remains rather shocking to think that an embryo never has a right to life, if for no other reason than it implies that infants do not either. Michael Tooley, in the article where he makes the kittens-embryos analogy, in fact does argue, and on much the same grounds that I have presented here, that infanticide and abortion are morally equivalent. After all, infants lack many of the capacities typical of adult humans, just as embryos do, most obviously those associated with rational agency, which at least since Kant has often been invoked as the grounds of moral concern for typical adult human beings.
Infanticide, at least in Western societies, is regarded as utterly taboo, yet according to Tooley, that taboo is baseless. Cheeringly, I disagree. I think Tooley's dismissal of the potentiality argument misses something important about the right to life of typical adult human beings, and that because of what it misses, the dismissal may not work. Not only that, it may indicate something about the limits of moral reasoning. What Tooley misses about the right to life of typical adult human beings is that right to life seems to rest on wholly future-related considerations. The badness of death, as separate from the badness of the pain associated with it, seems to rest, at least for the person who dies, in the plans they don't carry through, and can't make, in the way that it can act as an artificial break on their authoring of their life. I don't think we'd care so much, if much at all, about death, if people were very quickly reincarnated as themselves, able to continue living their lives as a meaningful and roughly coherent whole. Those future-related considerations, though, are all about potentialities: the other things that could have happened in that life, the other shapes it could have been given granted more time.
If that's true though, the potentialities argument for a right to life of embryos and infants could surely also be made to work: just as we are concerned about the things that could have happened in the life of an adult human being when it is ended, we can likewise be concerned about the things that could have happened in the life of an embryo or infant in the event that it is ended. This brings us to the other problem with the potentialities argument though, that if we are concerned with potentialities, there is something of a regress. Surely no-one is obliged to procreate as much as possible in order to realise the potential of all the children that could be born, yet, if abstract potential matters as it would seem the argument suggests it does, then it appears to imply it. It is not abstract potential that matters though: it is the potential of a human life, a concrete thing with particular limits. Think of it the other way: there's something unsettling, not quite right, about the idea of immortality, as though the lack of at least a rough time-frame in which to construct a life subtly undermines the notion of constructing a life, leaves it nothing against which to struggle, to define itself. We do not have a duty to maximise potential in the abstract, but not to constrain potential in the concrete, in what exists anyway, which I think is enough to suggest a cut-off point for the right to life embryos derive from their potentialities, that of viability outside of the womb, of an existence independent from their mother. Before that point, they are not really a separate existence, since their lives are totally tied up with that of the mother, but afterwards, they are a separate existence, a concrete thing in the world, a thing of its own.
Tooley of course denies the moral relevance of viability, but I think that's part of a more general mistake that he makes of failing to situate his examples in the linguistic practices of moral assessment, of not thinking about eminently normative categories like 'a human life', of rather concentrating on examples filled with categories and concepts taken from their normal contexts, stripped of the connections and interlinkings with provide them with meaning. I'm not entirely sure what we should do with kittens we could make develop into cats with all the moral features of typical adult humans, but the idea that they would be equivalent, in all the relevant ways, to infants or embryos fails to grasp that infants and embryos, once they have passed the relevant developmental point, are a human life, whereas what we are doing with the kittens is choosing to make them into something similar in one very important respect to a human life, and so they cannot already be a human life. Moral reasoning comes up against limits like these, and for all their alterations across time and space, they must be at least confronted and explored before being dismissed.