The arguments of the Proslogion
Looking back on the sixty-five chapters of complicated argument in the Monologion, Anselm found himself wishing for a simpler way to establish all the conclusions he wanted to prove. As he tells us in the preface to the Proslogion, he wanted to find
a single argument that needed nothing but itself alone for proof, that would by itself be enough to show that God really exists; that he is the supreme good, who depends on nothing else, but on whom all things depend for their being and for their well-being; and whatever we believe about the divine nature. (P, preface)
That "single argument" is the one that appears in chapter 2 of the Proslogion. (We owe the curiously unhelpful name "ontological argument" to Kant. The medievals simply called it "that argument of Anselm's" [argumentum Anselmi].)
The argument goes like this. God is "that than which nothing greater can 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 definition of God as "that than which nothing greater can 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 nothing greater can 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 t ...