  The first argument against normative non-naturalism concerns normative supervenience. The normative supervenes on the natural; in all metaphysically possible worlds in which the natural facts are the same as they are in the actual world, the moral facts are the same as well. This claim has been called the “least controversial thesis in metaethics” (Rosen forthcoming); it is very widely accepted. But it is also a striking fact that stands in need of some explanation. For naturalists, such an explanation is easy to provide: the moral facts just are natural facts, so when we consider worlds that are naturally the same as the actual world, we will ipso facto be considering worlds that are morally the same as the actual world. But for the non-naturalist, no such explanation seems available. In fact, it seems to be in principle impossible for a non-naturalist to explain how the moral supervenes on the natural. And if the non-naturalist can offer no explanation of this phenomenon that demands explanation, this is a heavy mark against non-naturalism (McPherson 2012).