Sam Harris has responded to Russell Blackford's review--or, rather, to Jerry Coyne's summation of Russell's review--at Why Evolution Is True. I've joined the discussion with a number of elaborate comments. I'm reposting my most recent comment below (with slight modification), since I think it is of the most general interest, and because it develops some of my own ideas in interesting ways:
About defining “morality,” I think there are two assumptions in play here:
1) Evolution has favored ways of thinking which promote general well-being.
2) What people call “morality,” though different in many details, is just those ways of thinking which promote general well-being.
If we accepted these two premises, we could argue that what is moral is just what promotes general well-being. There is no sense in saying something could be moral without promoting general well-being, because the definition of “morality” doesn’t allow it.
I think (1) is possibly true, but it hasn't been established beyond a reasonable doubt. In contrast, (2) does not seem true at all. Russell’s criticism of Harris is worth repeating here (actually, I’ll rephrase it slightly, but the substance is the same): If “x ought to y” just means “If F(x,y), then general well-being will be promoted,” then the statement “we ought to promote general well-being” would be a tautology. It would just mean “by promoting general well-being, we promote general well-being.” This is not something we could logically refute. Yet, we can say “you ought to promote general well-being” without saying anything apparently tautological. We can, in fact, rationally wonder whether or not promoting general well-being is something we ought to do. This makes the proposed definition of “morality” very unattractive.
There are other reasons to suppose that our moral thinking is not just about promoting well-being. I tend to think of moral thinking as more about fostering a sense of one’s own dignity. Our moral impulse is our impulse to be dignified rational agents. So, “x ought to y” really means “If F(x,y), then x will foster x’s dignity.” The statement “x should foster x’s dignity” would be tautological, and this isn’t so hard to accept. For it would seem illogical to say to somebody that they should not foster their own dignity. So I would rather define “morality” as “the process of fostering dignity,” and not “ways of thinking which promote general well-being.”
Follow-up: A Science of Dignity