Ontological argument — An a priori argument for the existence of God that starts only from the concept of God, without appeal to experience. The classic formulation is by Anselm of Canterbury, in the Proslogion (c. 1077–1078, chs. 2–3): God is “that than which nothing greater can be thought” (aliquid quo nihil maius cogitari possit); if this being existed only in the understanding, another greater one could be thought, existing in reality as well; therefore it exists necessarily in reality. The monk Gaunilo, in On Behalf of the Fool, objected that the same reasoning would prove the existence of a perfect island; Anselm replied that the argument holds only for the being than which nothing greater is conceivable. Thomas Aquinas rejected the argument (Summa Theologiae I, q. 2, a. 1), since it assumes that we know the divine essence. Descartes, in the Fifth Meditation (1641), argues that existence belongs to the essence of a supremely perfect being, as the sum of its angles belongs to a triangle. Leibniz judged the argument incomplete until it is shown that such a being is possible. In the Critique of Pure Reason (1781), Kant criticized it, claiming that existence is not a real predicate adding something to the concept of a thing; it was he who gave the argument the name “ontological.” In the twentieth century, modal logic renewed the topic: Alvin Plantinga (The Nature of Necessity, 1974) proposed a modal version, and Kurt Gödel left a formal proof, drafted around 1941 and published only posthumously (1987; Collected Works, vol. III, 1995). The debate involves the relation between essence and existence and ontology.
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