I stumbled upon the following argument a few moments ago, against the possibility of being justified in believing in moral facts.
(1) We’re justified in believing in some fact only if it plays a role in the explanation of our observations and other non-moral facts.
(2) Moral facts don’t play this role.
(3) We are not justified in believing moral facts.
This was reproduced by Matt DeStefano on the Secular Outpost, though the argument comes from Gilbert Harman. By way of brief response, against (1) we should note that some of our observations include moral observations. What I mean by a moral observation is analogous to what I would mean by a physical observation – that I have an experience of the world which I have to categorize as physical/moral (respectively). I have no more reason, it seems to me, to put my moral experience into question then I have to put my physical experience into question. Thus, when I know by moral experience that there are moral facts (about objective duties, obligations, rights and values), in just the same way as I know by physical experience that there are physical facts (about physical objects and relations between them), I find myself without any reason to call one into question which couldn’t easily inspire a parody to call the other into question.
Moreover, (1) seems to be too restrictive, as it may imply that we are not justified in having any metaphysical beliefs (such as a belief in the reality of the past). There is a much deeper problem, however; namely, that this argument cannot be rationally affirmed. This can be demonstrated thus: suppose I believe that the argument is sound – does my belief that the argument is sound play any role in the explanation of our observations and other non-moral facts? It seems to me that it cannot. Therefore, if the argument is sound one cannot accept it. If the argument is sound, then I ought not to believe that the argument is sound. Or, more weakly, if the argument is sound then I can never be justified in believing that the argument is sound. In other words, if one is not justified in believing in moral facts for this reason (Premise 1), then for this same reason (Premise 1) one cannot be justified in believing this argument sound.
Another problem may be that, while we might be tempted to slip and say, as I did above, that “If the argument is sound, then I ought not to believe that the argument is sound,” we would, if the argument is correct, be wrong to say this. If there are no moral facts, then epistemology has no moral implications. In other words, it may make it trivially true that belief in moral facts is unjustified, since it is not the case that we ‘ought‘ not to believe that for which we have not epistemic justification. The whole ‘weight’ of epistemic justification is measured by its moral implications.
[Edit: this post has been significantly revised, and hopefully improved].