An idea · Medieval · first attested 1078
The Ontological Argument
To prove God's existence from the concept of God alone.
Anselm's argument from the second chapter of the Proslogion is the most audacious in the history of thought, because it leans on no fact about the world. God is "that than which nothing greater can be conceived." Even the fool who denies God understands the phrase, so such a being exists at least in his thought. But existing in reality is greater than existing in thought alone; so if God were only a thought, something greater could be conceived, and that is a contradiction. Therefore God must exist. Gaunilo objected with the image of a most perfect island; Aquinas rejected the argument; Descartes revived it in the Meditations; Kant landed the famous blow that existence is not a property; Gödel, in the twentieth century, restated it in modal logic. Nine centuries of replies to three pages.