Anselm used the Ontological Argument to prove that God’s existence is necessary. The origins of this argument are found in Anselm’s writings, he began with a quotation from a Psalm “The fool says in his heart, ‘there is no God’…” and then reflected on the truthfulness of this. Anselm defined God as ‘that than which nothing greater can be conceived’ assuming you accept this a priori definition Anselm went on to state that anyone who denies God’s existence is a fool. He observed that it is greater to exist in reality than in the mind alone (you would prefer to have a real £20 note than an imaginary one). Someone who doubts God’s existence must have a concept of God in order for them to be able to reject it therefore if the concept of God is the greatest conceivable being then he must exist. He argued that existence was part of perfection, so if God is the greatest being (with all perfections) then it is necessary that he exists otherwise he cannot be the greatest conceivable being and that is not – to Anselm – the definition of God, the very fact that we can conceive of him as the greatest being proves, for Anselm, that he exists. For Anselm someone who denies God’s existence is essentially saying “God, who exists, does not exist”.
Gaunilo responded to Anselm’s argument with ‘On behalf of the fool’. He insisted that if Anselm’s argument is correct it should prove the existence of all other perfect things, he gave the example of a perfect island observing that if someone tried to prove to him the existence of such an island using the same reasoning that it would not mean that the island does in fact exist. Anselm then in response went on to develop his argument further he stuck with his definition of God as the greatest conceivable being and still argued that it was greater to exist in reality than in the mind alone but he went on to ask if it is greater for a being to have