One of the most familiar arguments for expressivist metaethics is the claim that the rival theory, moral realism, cannot provide a satisfying explanation of why moral properties supervene on natural properties. Non-cognitivism, however, has its own problems explaining supervenience. Expressivists try to establish supervenience either by second-order disapproval of type-inconsistent moral evaluations or by pragmatic considerations. But disapproval of inconsistency is merely a contingent attitude that people happen to have; and pragmatic justification does not allow for appraisers to take their (...) own moral attitudes seriously enough. What has been overlooked is a third alternative. The metaethical theory that can best account for supervenience is neither realist nor non-cognitivist but an objectivist version of constructivism. On the constructivist theory, right and wrong are determined by the principles that people would (hypothetically) consent to under ideal conditions. Type-consistency is a required feature of any principles regulating our conduct, if they are to be freely agreed to by ideally rational people. (shrink)