In the Proslogion, St. Anselm presents a philosophical argument for the existence of God. Anselm's proof, known since the time of Kant as the ontological argument for the existence of God, has played an important role in the history of philosophy and has been incorporated in various forms into the systems of Descartes, Leibniz, Hegel, and others. Included in this edition of the Proslogion are Gaunilo's "A Reply on Behalf of the Fool" and St. Anselm's "The Author's Reply to Gaunilo." All three works are in the original Latin with English translation on facing pages. Professor Charlesworth's introduction provides a helpful discussion of the context of the Proslogion in the theological tradition and in Anselm's own thought and writing.