The famous argument of St. Anselm (1033-1109) of Canterbury (1033-1109) for the existence of God. It is an argument a priori, drawn from the idea of God rather than a posteriori, from the works of God. Anselm used it to refute the fool who says there is no God (Psalm 13). He argues as follows: We call God a being so great that nothing greater can be conceived. This definition is accepted even by the atheist, who admits that God exists at least in the mind of the believer. But that which is so great that one cannot imagine anything greater, cannot exist only in the mind. why not? Because on this supposition one could think of something greater, namely, the same being existing outside the mind, i.e., in reality. Therefore, God exists both in mind and reality. Anselm's argument depends on the realistic metaphysics of Plato and has been the subject of learned discussion over the centuries.