“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. (…) 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.” (wiki)
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