Old dialogue from Slate between Peter Singer and Richard Posner. My favorite passage from Posner:

Now you may reply that these are just facts about human nature; that they have no normative significance. But they do. Suppose a dog menaced a human infant and the only way to prevent the dog from biting the infant was to inflict severe pain on the dog—more pain, in fact, than the bite would inflict on the infant. You would have to say, let the dog bite (for “if an animal feels pain, the pain matters as much as it does when a human feels pain,” provided the pain is as great). But any normal person (and not merely the infant’s parents!), including a philosopher when he is not self-consciously engaged in philosophizing, would say that it would be monstrous to spare the dog, even though to do so would minimize the sum of pain in the world.

I do not feel obliged to defend this reaction; it is a moral intuition deeper than any reason that could be given for it and impervious to any reason that you or anyone could give against it. Membership in the human species is not a “morally irrelevant fact,” as the race and sex of human beings has come to seem. If the moral irrelevance of humanity is what philosophy teaches, and so we have to choose between philosophy and the intuition that says that membership in the human species is morally relevant, then it is philosophy that will have to go.