[M]oral nonnaturalism faces the challenge of explaining the normativity of morality just as much as does moral naturalism. If normativity needs to be explained, it is not explained by giving up on naturalistic ways of explaining it. Antireductionist forms of nonnaturalism that view moral properties as sui generis face an especially difficult problem, for they appear simply to postulate normativity. It is unclear how they could explain it.
David Copp, Morality in a Natural World: Selected Essays in Metaethics, Cambridge, 2007, p. 282