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Who was Gaunilo of Marmoutier?

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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 of Marmoutier was an 11th-century Benedictine monk based at the Marmoutier Abbey near Tours, France. He is best known for his critique of the ontological argument for God's existence proposed by St. Anselm of Canterbury.12

In his work titled Liber pro insipiente ("In Defense of the Fool"), Gaunilo challenged Anselm’s assertion that the concept of "that than which nothing greater can be thought" necessitates God's existence. He famously argued by analogy, suggesting that simply having a concept of a "perfect island" does not prove that such an island actually exists in reality. Gaunilo denied that an idea of a being inherently includes its existence in the objective order. His critique prompted Anselm to write a rebuttal, the Liber apologeticus contra Gaunilonem ("Book of Defense Against Gaunilo"), in which the archbishop reaffirmed his original argument.12