Introduction

Anselm tells how he searched long and hard for a single argument that by itself would suffice to prove that God exists, and how one day at matins it suddenly came to him. This is the famous 'ontological argument': God is 'that than which nothing greater can be conceived'; if he existed only in the mind, one could conceive of something greater, existing in reality as well, which is a contradiction; therefore God truly exists. This short treatise, written as a prayer, has been disputed ever since it appeared: the monk Gaunilo at once wrote a reply, Aquinas rejected it, Descartes and Leibniz took it up again, and Kant made a celebrated critique of it. Philosophers are arguing about it still.