Recent proponents of the ontological argument have leamed an important lesson from Leibniz: the argument requires the assumption, or premise, that God is possible. In one form or another this premise appears in the versions of the argument endorsed by Hartshome, Malcolm, and Plantinga.1 But Leibniz's lesson has not been taken fully to heart by his modem followers. He thinks that to bring the argument to a triumphant conclusion one needs to prove that God is possible. For without this proof, we have no assurance that the idea of God is noncontradictory.2 So Leibniz struggled vainly I think to produce two proofs of the possibility of God.3 His modem followers, however, have on the whole simply assumed the truth of the critical premise.4 The fact that there has been relatively little effort to show that the concept of God is not coherent probably contributes to whatever plausibility this procedure has.5 But, plausible or not, the procedure is mistaken. The concept of God is contradictory as I shall argue shortly. Establishing this would have consequences that go beyond the ontological argument: the entire edifice of orthodox natural theology would fall at a stroke.6 In arguing that the idea of God is contradictory it is important to be clear about what idea of God I have in mind. My target is the standard Judeo-Christian theological conception of divinity, a being who is by definition absolutely perfect. 'Absolute perfection' is to be taken in a sense strong enough to involve the properties of omniscience, omnipotence, and complete moral goodness. The idea of an absolutely perfect being is that of one who knows all things, has unrestricted power, possesses the maximum amount of virtue, and is free of any sort of defect or limitation. There have been other accounts of deity, and the argument I am going to propose does not apply to all of them. One cannot refute, with a single argument, the existence of such diverse Gods as have been conceived. God has, for example, sometimes been said to be limited in his power, or finite in some other way.