  Like the non-cognitivist, the error theorist denies that there any moral properties, but unlike the non-cognitivist maintains that the legitimacy of moral discourse presupposes that there are such properties. This trivially entails that whenever there is a moral difference there must be some non-moral difference for the simple reason that there never can be any moral differences between cases given the error theory. Typically, the error theorist agrees with the non-naturalist (implicitly, anyway) about the semantics of moral predicates but holds that moral predicates do not refer precisely because moral properties to which they purport to refer would have to be so queer (see Mackie 1977). One aspect of the putative queerness of moral properties is the idea that they necessarily supervene on the natural (and non-moral) properties without being reducible to those properties.