  The proper way to state Anselm’s argument is a matter of dispute, and any detailed statement of the argument will beg interpretative questions.  But on a fairly neutral or consensus reading of the argument (which I shall go on to reject), Anselm’s argument goes like this. God is “that than which a greater cannot be thought”; in other words, he is a being so great, so full of metaphysical oomph, that one cannot so much as conceive of a being who would be greater than God. The Psalmist, however, tells us that “The fool has said in his heart, ‘There is no God’ ” (Psalm 14:1; 53:1). Is it possible to convince the fool that he is wrong? It is. All we need is the characterization of God as “that than which a greater cannot be thought.” The fool does at least understand that definition. But whatever is understood exists in the understanding, just as the plan of a painting he has yet to execute already exists in the understanding of the painter. So that than which a greater cannot be thought exists in the understanding. But if it exists in the understanding, it must also exist in reality. For it is greater to exist in reality than to exist merely in the understanding. Therefore, if that than which a greater can be thought existed only in the understanding, it would be possible to think of something greater than it (namely, that same being existing in reality as well). It follows, then, that if that than which a greater cannot be thought existed only in the understanding, it would not be that than which a greater cannot be thought; and that, obviously, is a contradiction. So that than which a greater cannot be thought must exist in reality, not merely in the understanding.