 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 unanalyzable in non-moral terms. This meant the property is “non-natural,” or distinct from any of the natural properties studied by science, though he also thought it distinct from metaphysical properties. Views that denied this thesis committed what he called “the naturalistic fallacy” (1903: 10/1993: 62), which he found in hedonists such as Jeremy Bentham, evolutionary ethicists such as Herbert Spencer, and metaphysical ethicists such as T.H. Green. His main argument against their view was what has come to be known as the “open-question argument,” though he actually stated in two slightly different ways. Consider a particular naturalist claim, such as that “x is good” is equivalent to “x is pleasant” or “x is pleasure.”If this claim were true, he 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. Alternatively, if this naturalist claim were true, “x is pleasant but x is not good” would be self-contradictory. Once it was established that x is pleasant, the question whether it is good would then be closed, or not worth considering, whereas, he argued, it remains open. 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 of not being 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 (1903: 10–17/1993: 62–69). Sidgwick had used one form of this argument against Bentham and Spencer, but only in passing (1907: 26n, 109); Moore developed it a greater length and made it central to his metaethics. And though he applied it specifically to “good,” it can equally well be used about other moral or normative concepts such as “ought” and “duty.”