Sean Carroll in Cosmic Variance:
See especially the third one there, where I try to be relatively careful about what I am saying. (Wouldn’t impress a philosopher by a long shot, but by scientist/blogger standards I was careful.) Upshot: concepts relevant to morality aren’t empirical ones, and can’t be tested by doing experiments. Morality depends on science (you can make moral mistakes if you don’t understand the real world), but it isn’t a subset of it. Science describes what happens, while morality passes judgments on what should and should not happen, which is simply different.
By now Harris’s book The Moral Landscape has appeared, so you can read for yourself his explanations in full. In a different world — one where I had access to a dozen or so clones of myself with fully updated mental states, willing to tackle all the projects my birth-body didn’t have time to fit in — I would read the book carefully and report back. This is not that world.
Happily, Russell Blackford has written a longish and very good review, in the Journal of Evolution and Technology. He also blogged about it, and Jerry Coyne blogged about Russell’s review. As far as I can tell, Russell and I basically agree on all the substantive points, and he’s more trained in philosophy than I am, so you’re actually doing a lot better than something one of my clones would have been able to provide. It’s an extremely generous review, always saying “I liked the book but…” where I would have said “Despite the flaws, there are some good aspects…” So you’ll find in the review plenty of lines like “Unfortunately, Harris sees it as necessary to defend a naïve metaethical position…”