The Ontological Argument
The ontological argument attempts to prove God’s existence through abstract reasoning alone. The argument is entirely a priori, i.e. it involves no empirical evidence at all. Rather, the argument begins with an explication of the concept of God, and seeks to demonstrate that God exists on the basis of that concept alone. Whether such a proof is possible even in principle is questioned by Hume.
The argument is ingenious. It has the appearance of a linguistic trick, but it is a difficult task to say precisely what, if anything, is wrong with it. All forms of the argument make some association between three concepts: the concepts of God, of perfection, and of existence. Very roughly, they state that perfection is a part of the concept of God, and that perfection entails existence, and so that the concept of God entails God’s existence.
The ontological argument was first formulated in the eleventh century by St Anselm in his Proslogium, Chapter 2. Anselm was a Benedictine monk, Archbishop of Canterbury, and one of the great medieval philosopher-theologians. Anselm’s ontological argument rests on the identification of God as “that than which no greater can be conceived”. Once it is understood that God is that than which no greater can be conceived, Anselm suggests, it becomes evident that God must exist.
A form of the ontological argument also constitutes the crux of Rene Descartes‘ Meditations. Having presented the argument from dreamingâ€”the sceptical argument that we are not justified in believing that there exists an external world on the basis of sense-perception because one might have the same sense-perceptions in a dreamâ€”Descartes rescues himself from scepticism on the basis of his belief in God. God is no deceiver, Descartes argues, and so our clear and distinct perceptions of the external world can be trusted. Descartes arrives at the belief that there exists a trustworthy God via a form of ontological argument.
The most prominent modern advocate of the ontological argument is Alvin Plantinga. Plantinga is best-known for his defence of the view that religious belief is foundational, i.e. that religious belief does not stand in need of external justification, but is also known for his work on modal logic, i.e. on the logic of possibility and necessity. Plantinga applies his approach to modal logic to the ontological argument, presenting it in a revised form.
The critics of the ontological argument are no less distinguished than are its advocates. Among them is St Thomas Aquinas, the thirteenth century Dominican and the arguably greatest philosopher of religion of all. Aquinas was canonised in the fourteenth century, when he was said by the Pope to have met the criterion for canonisation of having performed miracles in virtue of the answers that he had given to perplexing philosophical questions about God. Aquinas rejected the ontological argument in his Summa Theologica, First Part, Question Two.
The earliest critic of the ontological argument, though, was a contemporary of Anselm, the monk Gaunilo of Marmoutiers. Gaunilo objected to the ontological argument on the ground that it seemed possible to use its logic to prove the existence of any perfect thing at all. Gaunilo sought to demonstrate this by constructing an ontological argument for the existence of the perfect island. This argument, he suggested, is clearly fallacious, and so the ontological argument for the existence of God, which relies on precisely the same logic, must be fallacious too.
The most vaunted criticisms of the ontological argument, however, are those of Immanuel Kant. Kant argued against the ontological argument on the grounds that existence is not a property of objects but concepts, and that whatever ideas may participate in a given concept it is a further question whether that concept is instantiated. Whether his criticisms are sufficient to undermine all forms of the ontological argument remains a matter of much dispute.