 Moreover, serious questions regarding a priori moral knowledge and justification arise only for non-error-theoretic versions of cognitivism. Error theories, such as J.L. Mackie’s, hold that moral statements express propositions, and hence are true or false, but that these propositions are all false. Moral propositions are systematically false because they presuppose something radically mistaken. Mackie claims they presuppose the existence of objectively prescriptive moral facts, when there are no such facts (1977: Ch. 1; but see Berker 2019, which contests the standard interpretation of Mackie presented here). A person could be justified in believing a false proposition, but one could not know it. And it is difficult to see how one could even be justified a priori in believing a proposition of a type that is systematically false because it makes a radically mistaken presupposition. Thus, this entry focuses on non-error-theoretic versions of cognitivism.