Moore’s non-naturalism comprised two main theses. One was the realist thesis that moral and more generally normative judgements – like many of his contemporaries, Moore did not distinguish the two — are objectively true or false. The other was the autonomy-of-ethics thesis that moral judgements are sui generis, neither reducible to nor derivable from non-moral, that is, scientific or metaphysical judgements. Closely connected to his non-naturalism was the epistemological view that our knowledge of moral truths is intuitive, in the sense that it is not arrived at by inference from non-moral truths but rests on our recognizing certain moral propositions as self-evident.
Moore expressed the realist side of his non-naturalism by saying that fundamental moral judgements ascribe the property of goodness to states of affairs, though especially in Principia Ethica he tended not to distinguish moral concepts and moral properties. Like others of his time, he seems to have taken the realist view that moral judgements are objectively true for granted; he certainly did not defend it extensively against anti-realist alternatives. In this he was doubtless influenced by the grammar of moral judgements, which have a standard subject-predicate form. But it may also be relevant that, at least early on, the only subjectivist view he seems to have been aware of was the naturalist one according to which to say “x is good” is to report some psychological fact such as that you approve of x or that most people in your society do. In his 1912 book Ethics he showed that this view does not allow for moral disagreements, since, for example, my report that I approve of x and your report that you disapprove of it can both be true. Late in life he encountered the non-cognitivist emotivism of C.L. Stevenson, which says that moral judgements express rather than report feelings and therefore can conflict. He initially conceded that this anti-realist view had as good a claim as his own to be true,but shortly after reverted to his earlier non-naturalism, saying he could not imagine what had induced him to consider abandoning it.
Especially in Principia Ethica, Moore spent much more time defending his other non-naturalist thesis, about the autonomy of ethics, which he expressed by saying the property of goodness is simple and unanalyzable, and in particular is unanalyzable in non-moral terms. This meant the property is “non-natural,” which means that it is distinct from any of the natural properties studied by science. Views that denied this committed what he dubbed “the naturalistic fallacy,” which he found in hedonists such as Jeremy Bentham, evolutionary ethicists such as Herbert Spencer, and metaphysical ethicists such as T.H. Green. Moore’s main argument against their view was what has come to be known as the “open-question argument,” though he actually stated in a couple of slightly different ways. Consider a particular naturalist claim, such as that “x is good” is equivalent to “x is pleasure.” If this claim were true, Moore argued, the judgement “Pleasure is good” would be equivalent to “Pleasure is pleasure,” yet surely someone who asserts the former means to express more than that uninformative tautology. The same argument can be mounted against any other naturalist proposal: even if we have determined that something is what we desire to desire or is more evolved, the question whether it is good remains “open,” in the sense that it is not settled by the meaning of the word “good.” We can ask whether what we desire to desire is good, and likewise for what is more evolved, more unified, or whatever Sidgwick had used one form of this argument against Bentham and Spencer, but only in passing; Moore spent much more time on it and made it central to his metaethics.
The open-question argument was extensively discussed in the 20th century and met with several objections. One said the argument’s persuasiveness depends on the “paradox of analysis”: that any definition of a concept will, if successful, appear uninformative. If an analysis does capture all its target concept’s content, the sentence linking the two will be a tautology; but this is hardly a reason to reject all analyses. Moore could respond that in other cases accepting a definition leads us to see that the sentence affirming it, while seeming informative, in fact is not. This does not happen, however, in the case of “good.” Even if we agree that only pleasure is good, no amount of reflection will make us think “Pleasure is good” equivalent to “Pleasure is pleasure”; Ross took this line.
Another objection, made later in the century, said that while the argument may show that the concept “good” is distinct from any non-moral concept, it cannot support a similar conclusion about the property of goodness. Science, the objection runs, uncovers many non-analytic property-identities; for example, water is identical to H2O even though the concepts “water” and “H2O” are distinct. By analogy, the property of goodness could be identical to that of pleasure even if “good” and “pleasure” have different meanings. Again, however, Moore could respond to this objection. The property of being water is that of having the underlying structure, whatever that is, of the stuff found in lakes, rivers, and so on; when this structure turns out to be H2O, the latter property “fills a gap” in the former and makes the two identical. But this explanation does not extend to the case of goodness, which is not a higher-level property with any gap needing filling: to be good is not to have whatever other property plays some functional role. If goodness is analytically distinct from all natural properties, therefore it is metaphysically distinct as well. It is worth noting, however, that Moore did not explain the open-question argument in the way many later non-cognitivists would. Following Hume, they said that moral judgements are intrinsically motivating, so sincerely accepting “x is good” requires a commitment or at least some motivation to pursue x if that is possible. But then no definition of “good” in purely natural terms can succeed, since it cannot capture the term’s action-guiding force; nor can an evaluative conclusion be validly inferred from premises none of which have such force. Whatever the merits of this Humean explanation, Moore did not give it. On the contrary, the question whether moral judgements are intrinsically motivating is not one on which he expressed clear views or apparently thought important. In Principia Ethicahe remarked casually that we “hardly ever” think something good without having some attitude of will towards it, but he denied that this is true universally. Whether it is true universally isn’t something he thought worth considering further.