An ontological argument is a philosophical argument for the existence of God that uses ontology. Many arguments fall under the category of the ontological, and they tend to involve arguments about the state of being or existing. More specifically, ontological arguments tend to start with an a priori theory about the organization of the universe. If that organizational structure is true, the argument will provide reasons why God must exist.
The first ontological argument in the Western Christian tradition was proposed by Anselm of Canterbury in his 1078 work Proslogion. Anselm defined God as “a being than which no greater can be conceived”, and argued that this being must exist in the mind, even in the mind of the person who denies the existence of God. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it exists only in the mind, then an even greater being must be possible—one which exists both in the mind and in reality. Therefore, this greatest possible being must exist in reality. Seventeenth-century French philosopher René Descartes employed a similar argument. Descartes published several variations of his argument, each of which centred 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 philosophers Mulla Sadra and Allama Tabatabai.
Since its proposal, few philosophical ideas have generated as much interest and discussion as the ontological argument. Nearly all of the great minds of Western philosophy have found the argument worthy of their attention, and a number of criticisms and objections have been mounted. 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 argument could be used to prove the existence of anything. This was the first of many parodies, all of which attempted to show that the argument has absurd consequences. Later, Thomas Aquinas rejected the argument on the basis that humans cannot know God’s nature. Also, 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.