Like many of the proponents of "evolutionary psychology," Thayer assumes that biological science cannot explain moral experience because science is concerned with factual claims rather than value judgments, and he attributes this fact/value distinction to David Hume. But Thayer misses Hume's point. Hume distinguishes is and ought in order to show that moral assessments are derived not from pure reason alone but from moral emotions. Yet far from denying that moral judgments are judgments of fact, Hume claims that moral judgments are accurate when they correctly report what our moral judgments would be in a given set of circumstances. Correct moral judgments are factual statements about the species-typical pattern of moral sentiments in specified circumstances.
There are, of course, a number of complications; but as a three-sentence summary of Hume's rather complex view of moral judgments, the end of this paragraph is quite good. As I noted in a previous post, Hume is quite clear that the distinction is to show that "moral assessments are derived not from pure reason alone" but from moral sentiments; although, due to complexities in Hume's theory of taste, things are not as straightforward as the second sentence makes it sound, it is still pretty much right, and Hume does treat moral judgments as a kind of factual statement about moral taste; and Hume is quite clear about the "species-typical" part. And Arnhart seems to be right, too, when he says that Darwin recognized that "the ethical naturalism of Smith and Hume allowed morality to become an object of scientific study, because scientists could study the natural roots of moral judgment in the evolved moral emotions of the human animal."
I hadn't originally intended to blog more on this subject, but I'm thinking at present of writing a post on my own view of the ought/is question -- the things I think Hume gets quite right, the points at which my view diverges from his, etc. If I do, in fact, write it, it will probably be up at some point in the next week.
