Any kind of moral naturalist realist needs to reject Moore’s open question argument. There are two alternatives here. One is to insist that Moore’s posited openness is relatively superficial, and that there is no principled barrier to inferring moral facts a priori from the non-moral natural facts, even if such inferences will sometimes require a significant amount of information and reflection. The other is to argue that the constitution of moral facts by non-moral natural facts is an a posteriori matter, akin to the relation between water and H2O, and that therefore Moore’s openness only points to a conceptual gap, not a metaphysical one (Ridge 2014: Section 2).