Gaunilo of Marmoutiers
Notes
Gaunilo (eleventh century), Benedictine monk of Marmoutiers Abbey, remembered for one short text: On Behalf of the Fool, a reply to Anselm of Canterbury's Ontological Argument. He pointed out that by the same reasoning one could prove the existence of a perfect island, since an island that existed would be greater than one that did not. The objection has never been satisfactorily answered.
Wikipedia: https://en.wikipedia.org/wiki/Gaunilo_of_Marmoutiers