  Finally, more sophisticated forms of reductionist naturalism hold that while moral predicates do refer to natural properties, they do not so refer in virtue of any sort of analytic equivalence between moral predicates and non-moral predicates. The analogy here is with certain theories of reference borrowed from the discussion of proper names and natural kinds (see, e.g., Boyd 1988). Some philosophers have held that identifications of natural kinds like ‘water=H2O’ are necessarily true but not analytic. Rather, they are a posteriori necessary truths that hold in virtue of ‘water’ bearing the right causal relation (it has not been easy to say what causal relations are of the right kind) to H2O. Water and H2O may well be the same property but a competent user of ‘water’ need not know this. Indeed, before the discovery of modern chemistry most users of ‘water’ did not know this and it is not obvious that the meaning of the word changed in light of this discovery. If this is a plausible semantic theory and if a similar theory is true of moral predicates (a thesis that requires further argument) then the Open Question Argument is in trouble. For given this semantic theory, just as a competent user of ‘water’ and ‘H2O’ could without confusion say, “I know it is H2O but is it water?” a competent user of ‘good’ and ‘pleasant’ could say, “I know it is pleasant but is it good?” without confusion. Whether this sort of causal theory of reference is plausible in the case of moral vocabulary is more controversial, but the defender of the Open Question Argument needs some reason to reject the analogy.