Inspired by a recent difficulty articulating an answer to this question
In the conversation in question, I was asked about my field of study. When I answered "moral theory", I was met by surprise that such a field existed and, given its existence, could be of serious interest.
One component of my interlocutor's surprise was a familiar claim: that the truth of relativism rendered pointless the study of moral theory, that no right answer was to be had. "Ultimately, morality is a point of view, an opinion."
The second component of his surprise was different. It was the claim--phrased as a commonsensical, scientific view--that morality was determined by our biological history. We have the morality we have because our ancestors had the morality they had. Moral claims and moral norms are a matter of convention and of biology. There's no right answer to moral questions; it's just a brute fact that we think of as right what we think of as right and as wrong what we think of as wrong. Moral experience, if it's studied at all, should just be a topic of study for anthropologists, discovering and explaining what environmental and social conditions gave rise to what moral beliefs in people adapting to those conditions.
What I'd like to be able to do, in future, faced with these or similar charges, is to make vivid what's lacking in those two claims, what they fail to capture about moral experience, and why moral philosophy offers a fruitful, valuable way of thinking about moral experience.
The relativist charge
I certainly should have done better responding to this charge. I think the first standard answer is to say that relativism is self-defeating. If relativism is true--if my belief that x is true says nothing about whether you ought to believe that x is true--then it seems that there's no reason to believe the relativist thesis. The relativist thesis is itself subject to its claims, so there's no reason to accept its truth.
But I don't at the moment see how this strategy answers a strictly moral relativist. You could be a relativist only about moral claims. You could think that my claim that x is (morally) right gives you no reason to think that x is (morally) right, but not think that the truth of some x, or a justified x, gives you no reason to think that x. The moral relativist thesis is a claim about moral claims, and not itself a moral claim. If it's justified, then it's not vulnerable to the self-defeating objection.
A second strategy, then, is a denial that the relativist thesis is true or justified. This has the virtue of making the relativist think about what sort of relativist he is. For people clearly converge in their moral beliefs (and in their wider moral experience). Nobody thinks that wanton murder is right (and everyone experiences disgust or horror at it). How can that be explained, if relativism is true? The relativist can here (i) bite the bullet and say that someone who thinks wanton murder is fine can't be criticized, (ii) retreat from individual-level relativism to some kind of group or cultural relativism, (iii) give an explanation of the necessity of convergence in evolutionary terms, or (iv) be a nihilist, not a relativist, and claim that nothing really has moral force, not even subjectively.
I'll deal with (iii) below, since it retreats into the second component of the surprise. I think I'm comfortable responding to (ii), along the usual lines that we seem seem to be able, independently of our own group or culture, to criticize its moral beliefs. (Perhaps also with some puzzles about the difficulty of reconciling intolerant moral systems with relativism's message of tolerance, c.f. de Marco 80, though I'm not sure how convincing I find those as arguments against cultural relativism.) So cultural relativism is not a convincing explanation for moral convergence.
As for (iv), I like de Marco's (74) suggestion that it refuses to take seriously moral experience. What evidence, we can ask the nihilist, has he that the fact of moral experience is best explained as an illusion? It's OK to say that it has another explanation--an evolutionary one, etc.--but to say it has no need of explanation is too expensive a conclusion; it should be a last resort, and we're not there yet.
That leaves (i). How to respond to the idea that different moral experiences converge on the same belief by chance or by caprice--maybe by laziness--and that while belief in the wrongness of wanton murder may be true for one person or severally for many, nothing follows about the rightness or wrongness of wanton murder, objectively speaking, or for everyone?
I think (i) picks out one strand of an important bifurcation among people suspicious of morality. The intuition is that you can't just pluck morality out of thin air--that would be antiquated, possibly religious, mumbo-jumbo. Moral beliefs have to come from somewhere. And that leads to two paths. One path is to say that they come from nowhere (which is (i), the caprice argument). The second is to say they come from an evolutionary or a cultural source. Now, the caprice argument is hard to distinguish from (iv). So the defender of (i) has to be careful not to become a nihilist. He started off wanting to explain moral experience convincingly, but if he retreats to nihilism, he dismisses it. Both we and he should find that unsatisfactory. But suppose he holds to (i) and avoids shading into nihilism: he thinks moral experience has force, but only subjectively, so that we can't criticize the wanton murderer, even though we think we ought not wantonly murder. What can we say to him?
De Marco's (73) suggestion is that this kind of individualistic relativism, or extreme subjectivism, forgets the "social function" of morality. That may be right, but I think that would be unconvincing to the tough-minded subjectivist. "So what?", he might say. "It would be nice if morality were the sort of thing that could serve a social function, and that may have been true in the past, but we have an obligation to be honest in our inquiry. Saying that my position won't work because it doesn't allow morality to do something we want it to do gets things backwards. We should look at bare moral experience and deduce what it can do from the bare experience, not assume that its past functions, social or otherwise, are an essential part of it."
I think here a better strategy may be to cast the situation in scientific-explanatory terms. We agree that moral experience exists and stands in need of explanation. One candidate explanation is that moral beliefs get their force by whim or caprice, subjectively. An alternative explanation is, for example, moral realism: that convergence is explained by moral beliefs often responding to moral facts. Now we can ask: "Which explanation is more fruitful?" and perhaps make progress that way.
A third strategy, it occurs to me, or a way of bringing out the force of the above strategy, is the denial that moral experience just consists of moral beliefs or moral feelings. Most people feel the force of moral reasoning. If the existence of moral reasoning is conceded, then there are good grounds to think that morality is not just an arbitrary edifice of pronouncements and exhortations, learned at mothers' knees. Reasoning obliges us to believe some y if we believe x, and so moral beliefs are not arbitrary, subjective, or relativistic.
One reply to this strategy might be that moral reasoning, granted it exists, can't lead all the way to moral theory, that reasoning under-determines what's right and wrong. I might legitimately reason my way to x, you might legitimately reason your way to not-x, and neither of us can be criticized. Again, relativism or subjectivism seems best to describe the situation.
This is a nice objection from my point of view, in that it leads right into the practice of moral philosophy. Because that's exactly what we often do! We try to see to what someone who affirms some moral principle is committed, we expose inconsistencies or contradictions, we offer counterexamples to principles, and so forth. That is, we try to show that moral reasoning does offer definite conclusions--that it leads to moral theory.
I'm going to split this post in two and deal with the second, evolutionary charge in a second part.