In the light of the recent discussion of these issues sparked by Sam Harris - and reminded by the fact that I am giving a paper at this year's AAP Conference - I dug out my paper from last year's conference. This extract seems salient:
What I want to say has two components. First, it is not true that the actual religions have any special authority in the realm of ethics — though they might have if their supernatural claims had actually been true. Second, it is not true that science has no authority in this realm.
The situation is this: science cannot tell us the ultimate point of morality, but neither can the actual religions. Let me explain what I mean.
The ultimate point of morality cannot be obedience to the will of a god or a group of gods. This would raise the notorious Euthyphro problem: does conduct become morally correct because it is in accordance with a god's commands, or should we obey the god's commands because they track the independent requirements of morality? If the former, we seem to be stuck with the idea that murder and rape are wrong only because of the arbitrary commands of a powerful being (this being could have made murder morally right simply by commanding it). If the latter, then why not find out what the independent requirements of morality actually are, i.e. the requirements that are independent of the god's will?
So what is the ultimate point of morality? I don't see how either religion or science can tell us! Some candidates include: Individual flourishing (for which we'd need a definition that is not already moralised); social survival or something like escape from a Hobbesian state of nature; the reduction of suffering; or the maximisation of overall happiness. There may be other candidates, or perhaps we could adopt some combination. But we can reach a conclusion on this kind of question only by rational reflection on our values, the realm of secular ethical philosophy. When we so do, we can never step entirely out of all our values at once, so there always remains an irreducible element of what we really do most deeply desire the world to be like. As Hume argued, "oughts" can not ultimately be derived from reason alone without that element of desire — but nor can they be derived by solely from revelation.
However, once we know what morality is aimed at, a god or angel or wise ancestor could be a reliable moral advisor. I.e., such a cognitively superior being might well be able to tell us what kind of moral code (social or personal) best conduces to, say, human functioning (again, in some sense that is not already moralised).
If prophets were genuinely receiving information from a god or other supernaturally knowledgeable being, this might well have included such reliable information on how we should act to obtain our ultimate moral goal or goals.
The difficulty is this: in the real world the holy books seem no more reliable about ethical matters than they are about empirical matters such as the age of the Earth. Sophisticated religious adherents tend to interpret the holy books more in keeping with what they know from elsewhere about what conduces to flourishing (or social survival, reduction of suffering, and other such goals). Far from being authoritative, the holy books end up needing to be interpreted in the light of secular wisdom about what actually conduces to such goals as flourishing or happiness.
I conclude that one of the actual religions could have a degree of moral authority if it were actually tapping into the knowledge of a cognitively superior being such as a god. In principle, however, science can have exactly the same kind of moral authority.
Once again, science cannot settle ultimate "ought" questions, such as what should morality be aiming at. But if we can answer that question, perhaps through some process of rational reflection on our values, science can, in principle, give us information about how to achieve our goal. Say that we are trying to come up with a set of virtues to teach our children and aspire to ourselves, with the aim of advancing human flourishing (again, in some sense that is not already moralised). In principle, what we now need is information about the world, including information about human nature.
As to the latter, various fields of science (and not just the controversial field of evolutionary psychology) can now study human nature, and the outcomes may have implications for how we should try to constrain our own conduct. As we learn more, this can feed back into our moral understanding. Hume believed in moral progress, as understanding increased and civilization developed; and, in principle, I see no reason to think that he was wrong about this.
However, science, like the religions, is limited in the ethical realm. Given the current state of sciences such as psychology, the quality of any advice coming from science leaves much to be desired. We do not have an exact science of what best contributes to, say, individual and collective human flourishing.
For the moment, at least, we must rely to a large degree on such things as historical experience, folk understandings of what makes people happy, our own experience as individuals, and so on. Moral philosophers need to reflect on all of these things. They can also reflect on religious texts from various traditions, of course, since these may contain some wisdom, albeit seemingly not supernatural wisdom.
And a bit later:
Once we have an answer to the ultimate questions in the ethical domain, science can investigate such questions as how human flourishing or the minimisation of suffering (for example) is best achieved. However, we also need to rely on personal and historical experience, etc., since the most relevant sciences (such as psychology) are relatively imprecise and at an early stage of development. Religion has failed to give us reliable information about these questions, though it could have done so in principle if it had obtained the information from a cognitively superior being such as an angel or a god.
Thus, science and religion are both are unable to answer the ultimate questions in the ethical domain. However, in principle, both can answer less-than ultimate ethical questions, as well as empirical questions. The poor record of religion in answering empirical questions, and the poor record in improving on other sources (such as literature and human experience) in the ethical domain suggests that the actual religions are not in receipt of information from cognitively superior beings.