In a debate with Patricia Churchland, Peter Singer, Sam Harris, and Lawrence Krauss, Simon Blackburn explains why Harris simply has it wrong on whether science can provide substantive guidance on morality:
There is no doubt, he notes, that "science can inform our values" (and I would add that this goes trivially for many other types of knowledge). But "as to whether you need nothing but science", "I don't agree with Sam about that and neither do the other three speakers we've heard so far."
Blackburn grounds this in the traditional fact/value distinction but focuses on the functions of mental states rather than on ontology: where beliefs represent the world as it is, desires and concerns represent it as one would like it to be. Values represent a specific kind of desire that you are prepared to make public and insist upon from others.
Blackburn goes on to say that the idea that ethics is about promoting welfare and avoiding suffering is a commonplace of every moral philosopher, not a discovery of science (as he claims Sam Harris has it): "Where the moral philosophers find the going difficult is in having an adequate conception of human flourishing."
One might think the good life is merely about dreamlike states of contentedness -- something that could be achieved by lies and drugs, the subject of Huxley's objections in Brave New World. This leads Blackburn to a "fools paradise" objection (one might also call this the Truman Show objection): one might be happy merely out of ignorance and delusion, including deception by others on a grand scale. According to Aristotle such a person is not flourishing -- although this is something no neurological measure could tell us. If wisdom and knowing the truth are measures of flourishing, then mere knowledge about mental states (and accompanying brain states) is not enough. The same brain state accompanies a true belief as a false one, when they have the same content (and all other things being equal).
According to another conception, flourishing is a matter of self-mastery, of suppressing rather than satisfying desire, as in Buddhism. The litmus test for the debate might be whether a scientist could ever tell us whether was Buddha was right. According to Blackburn, that simply will never be the case.
Finally, he notes that "philosophy problematizes question of well-being," and that saying everyone should be happy provides absolutely no guide to how to live one's life. He further seems to suggest that what constitutes flourishing might vary radically from one individual to another.
(I'm not endorsing all of these arguments -- I still have to think them over; although I've described my objections to Harris in previous posts (and hopefully will be able to do so in more detail in the near future)).