He is best known for his contemporary criticism of the ontological argument for the existence of God which appeared in St. Anselm's Proslogion.
In his work In Behalf of the Fool,[a] Gaunilo contends that St. Anselm's ontological argument fails because logic of the same kind would force one to conclude many things exist which certainly do not.
[2] An empiricist, Gaunilo thought that the human intellect is only able to comprehend information provided by the senses.
[citation needed] Philosophers often attempt to prove the ontological argument wrong by comparing Anselm's with Gaunilo's.
[citation needed] Plantinga defends Anselm's proof by averring that it applies exclusively to Him.
"Consequently", wrote William L. Rowe in his summary of the polemic, "if we follow Anselm's reasoning exactly, it does not appear that we can derive an absurdity from the supposition that the island than which none greater is possible does not exist.
"[5] Gaunilo's refutation is also criticized on the grounds that it misinterprets the argument set forth by Anselm.
Thus, altering Anselm's formula but adopting his premises, entails that the Lost Island both can and cannot be thought not to exist.