 Ross rejects all naturalistic definitions of moral terms, including ‘right’ and (intrinsic) ‘good’. In RG, he argues (following Moore 1912 and Sidgwick 1907) that the moral terms ‘right’ and ‘ought’ are incapable of definition: ‘right’ is an ‘irreducible notion’ (RG 12). In FE, he suggests again ‘right’ is indefinable (FE 42), though he is sympathetic to the idea that ‘right’ is definable in terms of ‘suitable’ (FE 52–55). On this view, ‘this act is right’ means ‘this act has “the greatest amount of suitability possible in the circumstances”’ (FE 53; also 55). This is not a naturalist definition, since ‘suitability’ is itself a ‘unique and indefinable’ ethical notion (FE 146; also 159).