Anselm's Ontological Argument and Gaunilo's Response

In this argument, Anselm starts by defining God as that than which nothing greater can be conceived (TTWNGCBC). He then goes on to say that everyone has an idea of God as TTWNGCBC as by using the word ‘God’ it demonstrates an understanding and recognition of the principle of God. This means that even the fool that "says in his heart, ‘there is no God'" (psalms 14:1 and 53:1) is contradicting himself as he is showing that he holds a concept of what God is. Anselm uses the example of the painter to further explain this point as the painter has an idea of a painting, which after he paints it exists not only in his mind but also reality. Anselm states that it is greater to exist in reality than in concept. In order for God to fulfil his definition as TTWNGCBC, he must exist in reality rather than just in concept. Therefore, God exists. 

However, Gaunilo responded to Anselm in his argument ‘on behalf of the fool’ which is a reduction ad abusrdem argument. Gaunilo used the concept of a perfect island ‘that than which nothing greater can be conceived’ to show how a word alone cannot explain its existence if we have not experienced it. He argued that just because you can conceive of something, does not mean it exists in reality.

Anselm developed his argument in order to overcome Gaunilo’s criticisms. He argues that comparing God to an island is an unfair comparison as an island is contingent (can be thought of to not exist) whereas God’s existence is necessary (cannot be thought of as not existing). Nothing on this Earth can be compared to God due to the contingent nature of it. A God who cannot be thought of as not existing is greater than a God who can be thought of as not existing; therefore, Anselm proves that God exists necessarily.