Gaunilo of Marmoutiers

Gaunilo or Gaunillon (fl. 11th century) was a Benedictine monk of Marmoutier Abbey in Tours, France. 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, 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. An empiricist, Gaunilo thought that the human intellect is only able to comprehend information provided by the senses.

Little beyond this essay is known of Gaunilo; no other extant writings bear his name. Anselm wrote a reply to it, essentially arguing that Gaunilo had missed his point.

Anselm claimed his ontological argument as proof of the existence of God, whom he described as that being for which no greater can be conceived. A god that does not exist cannot be that than which no greater can be conceived, as existence would make it greater. Thus, according to St. Anselm, the concept of God necessarily entails His existence. He denies Gaunilo a Godless epistemology.

Gaunilo criticised Anselm's argument by employing the same reasoning, via reductio ad absurdum, to "prove" the existence of the mythical "Lost Island", the greatest or most perfect island conceivable: if the island of which we are thinking does not exist, it cannot be the greatest conceivable island, for, to be the greatest conceivable island, it would have to exist, as any existent island would be greater than an imaginary one. This, of course, is merely a direct application of Anselm's own premise that existence is a perfection. Since we can conceive of this greatest or most perfect conceivable island, it must, by Anselm's way of thinking, exist. While this argument is absurd, Gaunilo claims that it is no more so than Anselm's.

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