Proslogion

A short meditation in twenty-six chapters, written around 1077–1078 when Anselm was prior of the abbey of Bec in Normandy. In the preface he says he looked for a single argument that would prove, by itself, that God exists and has the attributes Christians believe. Chapter 1 is a prayer that ends with the motto of faith seeking understanding: he does not seek to understand in order to believe, but believes in order to understand. Chapter 2 gives the famous argument. God is that than which nothing greater can be thought. Even the fool of the Psalms, who says there is no God, understands this phrase, so the thing exists in his understanding; but if it existed only there, something greater could be thought, namely the same thing existing in reality. Chapter 3 adds that God cannot even be thought not to exist. The remaining chapters derive God's attributes and end in prayer. Most editions include the monk Gaunilo's reply, with its lost island, and Anselm's answer.

It contains the ontological argument, which philosophers from Aquinas to Descartes, Kant and Plantinga have attacked or defended, and it is short enough to read in an evening.