Gaunilo (flourished 11th century) was a Benedictine monk of the Marmoutier Abbey near Tours, France, who opposed St. Anselm of Canterbury’s ontological argument for God’s existence.
Gaunilo’s Liber pro insipiente (“In Defense of the Fool”) was a critique of the rationality of Anselm’s assertion that the concept of “that than which nothing greater can be thought” (i.e., God) implies God’s existence. Gaunilo argued by analogy, pointing out that one’s concept of a “perfect island” does not imply that such a place exists. The title of his book was taken from Anselm’s reference to the atheistic “fool” of the 14th Psalm.