The existence of god has always been a matter of great debate in philosophy of religion. Arguments have been put forward both for and against god’s existence. One of such arguments is the Ontological Argument. The term ‘ontological’ comes from the Greek ontos, meaning ‘essence’, ‘existence’, ‘being’. The Ontological Argument claims that: a. The proposition ‘God exists’ is a priori/deductive – it can be known to be true without reference to sense experience, just by thinking about God’s nature. b. In the proposition ‘God exists’, the subject ‘God’ contains the predicate ‘exists’, so God must exist. c. God’s existence is a necessary truth, not a contingent one. 

Anselm’s eleventh-century ontological argument was the first of its kind and continues to resurface in different forms. Anselm was a Benedictine monk, Archbishop of Canterbury and a saint of the Church. His Ontological Argument appears in Proslogium. Here is a summary of his argument-

P1 God is a being than which nothing greater can be conceived.

P2 This is a definition which even a fool understands in his mind, even though he does not understand it to exist in reality.

P3 There is a difference between having an idea in the mind and knowing that this idea exists in reality.

P4 For example, a painter has an idea in his mind of what he wants to paint; but when he has painted it, that idea now exists both in his mind and in reality.

P5 It is greater to exist both in the mind and in reality than to exist only in the mind.

P6 If God existed only in the mind, I could think of something greater, namely a God who existed in reality also.

C Therefore in order to be the greatest conceivable being (P1), God must exist both in the mind and in reality.

We can therefore reduce Anselm’s arguments to two essential premises and a conclusion.

P1 God is the greatest conceivable being.

P2 It is greater to exist in reality than to exist only in the mind. 

C Therefore, as the greatest conceivable being, God must exist in reality.

Anselm’s argument was criticised by a fellow monk, Gaunilo. Gaunilo was a contemporary of Anselm. He wrote On Behalf of the Fool, which essentially rejected Anselm’s attempt to give an a priori proof of the existence of God.

The following puts Gaunilo’s argument in parallel with that of Anselm.

P1 It is possible to conceive of the most perfect and real lost island.

P2 It is greater to exist in reality than to exist only in the mind.

C Therefore the most perfect and real lost island must exist in reality.

Gaunilo clearly believes that the concept of ‘the most perfect and real lost island’ makes no sense, since we know that such an island cannot exist. Gaunilo is using a method of argument called ‘reductio ad absurdum’, which is Latin for ‘argument to absurdity’. He is suggesting that Anselm’s argument can be used to prove the existence of an endless number of perfect objects and so the real fool would be anybody who argued in this way.