I can gain no clear sense from your letter of whether you really think infanticide for normal healthy babies is in principle morally wrong. You seem to suggest it is not intrinsically wrong, since the baby "cannot be said to have an interest in its continued existence. It acquires such an interest only retrospectively, if it survives. But the adult who develops from the neonate cannot remember being the neonate, and has no mental links with it--nor with the fetus." I am not sure I follow your reasoning here, but you are forgetting one highly significant fact: You were once a neonate. You now have a strong interest in the survival of your earlier infant self. And it has an interest in surviving because it will become the individual you are now (this is not to say that it is aware of that interest). As we philosophers like to say, you and the neonate are numerically identical, however much you may have forgotten that early stage of your life. You do have mental links with the infant you once were: There is a continuous causal mental stream between you now and it then. The fundamental point here is that it is good for the infant that it not be killed, since it will go on to lead a worthwhile human life--even judged in the most simple-minded utilitarian terms. Killing it is robbing it of that future life.
I assume you are not being serious when you say that for me "something is prima facie wrong if it offends a strongly held moral sentiment." Of course, the fact that a moral view is strongly held is no ground for accepting it. The reason I hold the view strongly is that it is based on a sound moral judgement: that it is prima facie wrong to take the lives of persons and potential persons. I could as well "argue" the same way against you: it is just a strongly held moral sentiment with you that pain and the frustrating of desires matter morally. All moral systems must rest on basic moral judgements, including the (undisputed) moral system that regards killing rational self-conscious beings as prima facie morally wrong. In moral theory what you want is a set of principles that conform as well as possible with your considered moral judgements; what you don't want is a set of theoretical dogmas that declare permissible what you know very well is wrong.
Do I, as you suggest, "deduce this reason from a moral absolute-the sanctity of the life of a person"? I don't know what you mean by an absolute, but in one clear sense I do not believe in such absolutes. The sanctity of life can be overridden, for example, in cases of mortal self-defence. It can also be overridden if the life in question is so miserable as to be unbearable to the person whose life it is--with that person's express consent. What I do think is that the idea of the sanctity of life--the idea that this is a basic moral value--is a sound idea in moral theory; just as I think prohibitions against stealing, rape, bribery, etc., are sound moral ideas. I would add that I think it is a complete mistake--a characteristically philosophical mistake--to insist that all our sound moral judgements should be codifiable under some one moral precept (as utilitarianism attempts to do). Our moral judgements concern a variety of values, linked in various ways.
You say my argument against Singer is also based on "an ad hoc metaphysical theory of 'degrees of potentiality.'" No, it is based on some clear distinctions in the notion of potentiality, of the kind I illustrated by reference to the tree and Mozart. In fact, the idea of different strengths of modality is a commonplace in analytical philosophy. If you want to refute me you need to explain why the distinctions I made are unreal. But maybe you are just playing "devil's advocate"; in which case I say, the devil doesn't need any more advocates.
You ask what I think of Singer's Great Ape Project. At last I can strike a positive note (do you think I enjoy raking you and Singer over the coals?): It is a wonderful idea. In fact, I was more than happy to contribute an essay to the volume Singer edited on the topic, entitled Apes, Humans, Aliens, Vampires and Robots . I fully endorse all such efforts to curtail the abuse of higher animals, and I particularly abhor the treatment of apes and chimpanzees that we humans have routinely meted out. I am, as I hope I have made clear, against killing sensitive sentient conative beings in general.