 Mackie’s discussion provides a first illustration of how accounts of moral motivation have been deployed to defend or rebut broader positions in metaethics. According to Mackie, the motivating power of objective values, if there were such values, would have to be just as Plato depicted it. “Plato’s Forms give a dramatic picture of what objective values would have to be. The Form of the Good is such that knowledge of it provides the knower with both a direction and an overriding motive; something’s being good both tells the person who knows this to pursue it and makes him pursue it. An objective good would be sought by anyone who was acquainted with it, not because of any contingent fact that this person, or every person, is so constituted that he desires this end, but just because the end has to-be-pursuedness somehow built into it” (Mackie 1977, 40). Mackie contends that the moral sentences we utter when we make moral judgments in fact express propositions about just such “objectively prescriptive” properties; as a result, our moral judgments can be true or false. So moral cognitivism—the view that moral judgments and beliefs, and the sentences that express them, can be true or false—provides the correct account of moral semantics, of what our moral judgments mean. Given that our moral discourse is cognitivist, it would seem to presume the correctness of moral realism, the view, roughly, that moral judgments and beliefs are truth evaluable, and some of them are literally  true.[2]  But moral discourse suffers from what is called “presupposition failure,” according to Mackie: moral discourse presupposes objectively prescriptive properties, but there aren’t any; such properties would have to be “queer entities” unlike anything else in the world. Talk about morality is, Mackie evidently thinks, rather like talk about unicorns. Our “unicorn talk” expresses propositions (at least assuming it follows medieval legend) about horse-like creatures, tamable only by virgins, whose spiral horns possess magical powers. But there are no such creatures, and so our unicorn talk is systematically in error, though few of us any longer succumb to the error. In denying the existence of moral properties, Mackie rejects moral realism, combining a cognitivist moral semantics with an error theory. According to the error theory, “although most people in making moral judgments implicitly claim…to be pointing to something objectively prescriptive, these claims are all false” (Mackie 1977,  35).[3]