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How did Gaunilo challenge St. Anselm of Canterbury’s ontological argument?

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Luca della Robbia: St. Anselm of Canterbury — St. Anselm of Canterbury St. Anselm of Canterbury (center), terra-cotta altarpiece by Luca della Robbia; in the Museo Diocesano, Empoli, Italy.

Gaunilo, an 11th-century Benedictine monk, challenged St. Anselm of Canterbury’s ontological argument in his work Liber pro insipiente ("In Defense of the Fool").

Anselm had argued that because God is defined as "that than which nothing greater can be conceived," God must exist in reality; otherwise, a being that exists in both thought and reality would be greater than one existing only in thought.12

Gaunilo critiqued the rationality of this assertion by using an analogy: he argued that having a concept of a "perfect island" does not imply that such an island must exist in reality. He denied that the mere idea of a being necessarily includes its existence in the objective order, nor that a direct intuition of such a concept guarantees the being's actual existence. By invoking the "fool" mentioned in the 14th Psalm, Gaunilo sought to demonstrate that Anselm’s logic could be used to "prove" the existence of any perfect thing, which he considered an invalid leap from thought to reality.32