Proslogion

The Proslogion is a short treatise written by Anselm of Canterbury at the abbey of Bec around 1077 and 1078. It is structured as an address to God, and the famous argument for God’s existence appears in its second and third chapters. The book is short enough that it can be read in a single sitting and is written in a register closer to prayer than to systematic philosophy.

The argument it contains, later called the Ontological Argument, proceeds from the definition of God as “that than which nothing greater can be conceived.” Anselm argues that anyone who understands this definition must concede that the thing so defined exists in the understanding. He then argues that if it existed only in the understanding and not also in reality, it would not be that than which nothing greater can be conceived, since something existing both in the understanding and in reality would be greater. Therefore that than which nothing greater can be conceived must exist in reality.

The structure is so simple that it can fit on a page, and that simplicity has been part of the trouble. Critics from Gaunilo onward have suspected that something must be wrong with it, and the history of the argument is the history of attempts to specify what.