  In the first three chapters Moore sets out his criticisms of ‘ethical naturalism’. At the core of these criticisms is the thesis that the position involves a fallacy, the ‘naturalistic fallacy’, of supposing that goodness, which Moore takes to be the fundamental ethical value, can be defined in naturalistic terms, in terms, say, of pleasure or desire or the course of evolution. As against all such claims Moore insists that goodness is indefinable, or unanalysable, and thus that ethics is an autonomous science, irreducible to natural science or, indeed, to metaphysics. Moore's main argument against the possibility of any such definition of goodness is that when we confront a putative definition, such as that to be good is to be something which we desire to desire, we can tell that this is not a claim that is true by definition because its truth remains for us an ‘open question’ in the sense that it remains sensible to doubt it in a way which would not be possible if it were just a definition which makes explicit our understanding of the words. The merits of this argument are questionable; in many cases we can sensibly doubt the truth of a definition, especially where the definition makes use of discoveries that have not been part of our ordinary understanding, as is normally the case with definitions in the natural sciences. But there is, I think, a way of modifying Moore's argument which takes it around this objection, namely by taking it to rest on the epistemological thesis that ethical questions cannot be answered without the explicit involvement of ethical beliefs. The reason that this thesis is inimical to naturalistic definitions of ethical values is that an important role of definitions in the natural sciences and elsewhere is that they enable one to answer questions in new ways that would not otherwise be possible: it is thanks to the definition of water as H2O, and not in terms of its familiar phenotype, that we can be confident that some comets are mainly composed of water. But there is no way in which we would accept that the answer to a novel ethical question, for example concerning the acceptability of the use of human embryos for stem cell research, is to be found by finding a definition of ethical values which enables us to answer this question without drawing on beliefs that are expressed with the familiar ethical ‘phenotype’, i.e., explicitly ethical concepts.