This chapter deals with the relation between two metaethical theses: moral naturalism and moral skepticism. Moral naturalism and moral skepticism are certainly not contradictories; they do not exhaust the space of metaethical possibilities. It is usual to think of them as contraries, for surely to embrace one position is to reject the other. The chapter explores the possibility of an irresolvable indeterminacy between moral naturalism and moral skepticism. David Lewis located one potential node of indeterminacy: between moral naturalism and error theoretic moral skepticism. The chapter investigates the potential undecidability of the debate between cognitivism and non-cognitivism. Rudolf Carnap invites open competition with his cognitivist rival. Having discussed Lewis and Carnap, it seems apt to end with the philosopher who best connects them: W. V. Quine. Faced with the nodes of indeterminacy, the author counsels neither sectarianism nor ecumenicalism in particular, but rather what might be called "metaethical ambivalence".

This chapter explains moral naturalism from Richard Joyce's contention that any philosopher with broadly naturalist inclinations should reject moral naturalism in favor of the error theory, according to which there are no moral facts at all. Moral naturalism of the sort comprises two claims. The first is that there are moral facts. The second claim is that the Humean theory of reasons is true. According to the Humean theory, nothing is a reason for an agent unless and because she has desires of the appropriate types. Moral naturalists might claim that thinking of some moral reasons as being categorical is not only something one find very natural, but also the best way to acquire moral reasons. A single line of argument would be sufficient to dispose of moral naturalism, expressivism, moral fictionalism, moral subjectivism, and other views, since they all fail to describe a system of moral concepts.