Ontological Argument

An ontological argument is any one of a category of arguments for the existence of God appearing mainly in Christian theology. The exact criteria for the classification of ontological arguments are not widely agreed, but the arguments typically start with the definition of God and conclude with his necessary existence, using mostly or only a priori reasoning and little reference to empirical observation.

It is widely accepted that the first ontological argument was proposed by Anselm of Canterbury in 1078 in his Proslogion. Anselm defined God as "that than which nothing greater can be conceived", and then argued that this being could exist in the mind. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it only exists in the mind, a greater being is possible—one which exists in the mind and in reality. Seventeenth century French philosopher René Descartes deployed a similar argument. Descartes published several variations of his argument, each of which centered on the idea that God's existence is immediately inferable from a "clear and distinct" idea of a supremely perfect being. In the early eighteenth century, Gottfried Leibniz augmented Descartes' ideas in an attempt to prove that a "supremely perfect" being is a coherent concept. A more recent ontological argument came from Kurt Gödel, who proposed a formal argument for God's existence. Norman Malcolm revived the ontological argument in 1960 when he located a second, stronger ontological argument in Anselm's work; Alvin Plantinga challenged this argument and proposed an alternative, based on modal logic. Attempts have also been made to validate Anselm's proof using an automated theorem prover. Other arguments have been categorised as ontological, including those made by Islamic philosopher Mulla Sadra.

The first critic of the ontological argument was Anselm's contemporary, Gaunilo of Marmoutiers. He used the analogy of a perfect island, suggesting that the ontological could be used to prove the existence of anything. This was the first of many parodies, all of which attempted to show that it has absurd consequences. Thomas Aquinas later rejected the argument on the basis that humans cannot know God's nature. David Hume offered an empirical objection, criticising its lack of evidential reasoning and rejecting the idea that anything can exist necessarily. Immanuel Kant's critique was based on what he saw as the false premise that existence is a predicate. He argued that "existing" adds nothing (including perfection) to the essence of a being, and thus a "supremely perfect" being can be conceived not to exist. Finally, philosophers including C. D. Broad dismissed the coherence of a maximally great being, proposing that some attributes of greatness are incompatible with others, rendering "maximally great being" incoherent.