Gaunilo was a contemporary of St Anselm, and, though a monk, is best remembered for his criticism of Anselm’s ontological argument for God’s existence. Gaunilo argues that if the ontological argument for the existence of God is sound, then its logic can be used to prove the existence of any perfect thing, e.g. the perfect island. It is absurd to think, however, that every perfect thing exists; the ontological argument must therefore be unsound. Gaunilo’s objection thus attempts to show that there is something wrong with the ontological argument, without specifying precisely what it is.

Gaunilo’s rebuttal of Anselm’s argument On Behalf of the Fool, gets its name from the fools of Psalm 14:1 and Psalm 53:1 who say in their hearts that there is no God, to whom Anselm refers as he develops the argument in his Proslogium. The dialogue between Anselm and Gaunilo can be followed into Anselm’s Reply to Gaunilo.