This chapter introduces classical non-naturalism and then outline the chief doctrines that render the G. E. Moore of Principia a paradigm member of that school:namely, his minimalism and cognitivism in moral semantics, non-naturalistic realism in moral metaphysics, and intuitionism in moral epistemology. Critics were also skeptical about the intuitionist moral epistemology of non-naturalism, which some took to presuppose the existence of aquasi-perceptual faculty of moral knowledge for which there is no evidence. The decline of interest in non-naturalism went hand in hand with the rise of ethical doctrines that appear more congenial to philosophical naturalism. Philip Stratton-Lake and Robert Shaver are among those who disagree with the common charge that Moore’s Open Question Argument (OQA) founders on his conflation of concepts and properties. Moore, like other classical non-naturalists, did not draw any sharp lines of the sorts that are now familiar in moral semantics between ethical and moral concepts, the Ought of morality, rationality, or prudence, and thick and thin normative concepts.