  Alternatively, perhaps the best explanation of the apparent “openness” of the relevant questions to competent users is that the predicates do not refer to properties at all. For it might be held that moral predicates do purport to refer to non-natural properties but fail to refer precisely because those properties would have to be so queer. This would be to defend a kind of error theory, as defended by John Mackie (see Mackie 1977 and Joyce 2001). In effect, this would be to grant that the non-naturalist is right about the semantics (moral predicates do purport to refer to non-natural properties) but wrong about the metaphysics (the non-naturalist is wrong to suppose moral predicates refer to anything).