If the burden of proof rests on those making positive moral claims, moral skeptics are not obligated to provide positive arguments for their skepticism.

Moral skeptics, in response, sometimes try to shift the burden of proof to their opponents. Anyone who makes the positive moral claim that sodomy is morally wrong seems to need some reason for that claim, just as someone who claims that there is life on Mars seems to need evidence for that claim. If the presumption is always against those who make positive moral claims, then it is opponents of moral skepticism who must carry the burden of proof. Or, at least, moral skeptics can deny that the bur