ProslogionAnselm of Canterbury
About Proslogion
Anselm composed the around 1077–1078 as a prayerful meditation seeking a single argument for God's existence and attributes. After a chapter of spiritual preparation, chapters 2 and 3 present what later writers call the ontological argument. God is that than which nothing greater can be thought. Such a being exists in the understanding even of the fool who denies God. If it existed only in the understanding, a greater could be thought, namely one existing in reality. Therefore it exists in reality. Chapter 3 adds that this being cannot even be thought not to exist. The remaining chapters derive divine attributes from the same formula and close in praise.
Gaunilo of Marmoutier replied on behalf of the fool; Anselm answered. Aquinas rejected the argument as moving illicitly from concept to existence. Descartes and Leibniz revived forms of it. Kant named and criticized the ontological proof. The remains the short text that forces every later theist and atheist to say whether existence belongs to the idea of a greatest being.