Subjective views of morality and the burden of proof

Most atheists recognize that the existence of objective moral facts tells against atheism, and so they reject objective moral facts. On this view there is no objective moral fact about the woman who killed and ate part of her three week old baby. Thus the only moral facts are those relative to the judgments of individuals. Most of us disapprove of this action and thus call it "evil" but others of us are free to dissent and there is no objective fact either way to settle the issue. In other words, everybody is right.

Next, many atheists who take this moral subjectivist position do a most curious thing: they attempt to turn the argument around on the moral objectivist (theist, atheist or otherwise) by asking a question of moral epistemology: "well how do you know that x is always right while y is always wrong?"

The problem with this tactic is that the burden of proof lies not with the ethical objectivist but rather with the ethical subjectivist and here's why.

Consider the analogy with the idealist. This is the person who rejects the view that there is a three dimensional world of substance. Instead, according to the idealist, all that exists are minds and their sensations, sort of like the "matrix". The idealist then attempts to challenge the realist who believes that there is a world of extended substance by asking us how we know that such a world exists since all we ever get is more sensations.

In one sense we might concede that we have no smoking gun to convince the idealist that there is in fact a world of extended substance. Nonetheless, we always begin with our common sense beliefs and treat them as innocent until proven guilty. (For instance, I cannot convince the solipsists that I exist. But too bad for them; they don't know what they're missing.)

So we have basic intuitive beliefs that we exist, that others exist, and that an external world exists which we grasp through our sensations. So here as elsewhere the burden of proof lies not with the advocate of common sense but rather with the dissenter: why doubt our intuitions?

The exact same argument applies to the moral question. Why accept the moral subjectivist's denial of objective moral facts? After all, we have overwhelmingly strong intuitions to believe that a mother killing and eating her infant constitutes not simply a different taste in dinner, but a gross moral evil. If the moral subjectivist would have us reject this fundamental conviction, she surely owes us an argument. And not just any argument but one that is more persuasive than the intuitions she would have us reject.