  Very roughly, non-naturalism in meta-ethics is the idea that moral philosophy is fundamentally autonomous from the natural sciences. More accurately, a family of related but distinct doctrines has gone under the heading ‘non-naturalism’. In some contexts, ‘non-naturalism’ denotes the semantic thesis that moral predicates cannot be analyzed in non-normative terms (see Shaver 2000 and Gibbard 2002: 153). In other contexts, ‘non-naturalism’ denotes the epistemological thesis that knowledge of basic moral principles and value judgements are in some sense self-evident (see Frankena 1963: 85–86). However, this view (which some self-styled naturalists would actually accept) is more often and more usefully referred to as ‘intuitionism’ and I shall henceforth also refer to it as such. Most often, ‘non-naturalism’ denotes the metaphysical thesis that moral properties exist and are not identical with or reducible to any natural property or properties in some interesting sense of ‘natural’. However, just which sense of ‘natural’ is most apt in this context is highly controversial and I shall return to this point shortly. Understood in this way, non-naturalism is a form of moral realism and is opposed to non-cognitivist positions according to which moral utterances serve to express non-cognitive attitudes rather than beliefs that provide their truth conditions and is also opposed to error-theoretical positions according to which there are no moral facts. Moreover, each of these different conceptions of non-naturalism bears interesting relations of support to the others. For example, a prima facie plausible explanation of the alleged immunity of moral predicates from analysis in non-normative terms (non-naturalism in the first sense) would be that moral predicates denote non-natural properties (which entails non-naturalism in the third sense). Perhaps unsurprisingly, Moore accepted non-naturalism in all three of these senses. Because non-naturalism is far more often understood in the third of these three ways I shall henceforth use ‘non-naturalism’ with the third of these three meanings unless otherwise noted.