An ontological argument for the existence of Go attempts the method of a priori proof, which uses intuition and reason alone. In the context of the Abrahamic religions, ontological arguments were first proposed by the medieval philosophers Avicenna (in The Book of Healing) and Anselm of Canterbury (in his Proslogion). Important variations were developed by later philosophers like René Descartes, Gottfried Leibniz, Norman Malcolm, Charles Hartshorne, and Alvin Plantinga. A modal logic version of the argument was devised by the mathematician Kurt Gödel.