This paper concerns the relation between two metaethical theses: moral naturalism and moral skepticism. It is important that we distinguish both from a couple of methodological principles with which they might be confused. Let us give the label “Cartesian skepticism” to the method of subjecting to doubt everything for which it is possible to do so—usually by introducing alternative hypotheses that are consistent with all available evidence (e.g., brains in vats). Let us give the label “global naturalism” to the principle that requires of any item which we admit into our ontology that it “fits” (in some manner or cluster of manners to be specified) with our naturalistic scientific worldview. One might be both a Cartesian skeptic and a global naturalist, if the latter principle is something that has survived the former test procedure. Alternatively, one might have adopted global naturalism for some other reason, while having little patience for the Cartesian method of doubt. Moral naturalism is the metaethical view that moral entities (e.g., properties like goodness and evil) fit within the scientific image of the world. The moral naturalist will probably be a global naturalist, but need not be: It is consistent with allowing non-natural entities into one’s ontology that one happens to think that moral properties are of the natural variety. Moral skepticism denies that moral entities fit within our scientific worldview. One way of denying moral naturalism is to be a moral error theorist: to hold that our moral discourse attempts to make reference to moral properties, but these properties do not exist.1 Another way of denying moral naturalism is to be a noncognitivist: to hold that our moral discourse was never really in the business of referring to moral facts or properties in the first place, and ipso facto such facts or properties are not naturalistic. In this paper, the label “moral skepticism” denotes the disjunction of these two theses. Neither the error theorist nor the noncognitivist must be committed to global naturalism, but usually will be; indeed, this commitment will often be a motivating factor of their metaethical views...