Thinking about a necessary being › Gaunilo's lost island

Gaunilo's response, usually titled On Behalf of the Fool, challenges the reasoning through a lost island. He describes an island surpassing other lands in riches and delights, then asks whether its being understood obliges us to accept its real existence. The intended pressure comes from trying to reproduce the disputed move in a less plausible case (section 6).

The example is a parody of an inference, rather than geographical evidence against an island. Its force does not depend on a survey of every ocean. Gaunilo asks whether someone would have supplied a proof by reasoning from the island's described superiority to its existence. If that procedure appears inadequate, the advocate of Anselm's proof must explain the relevant difference.

Calling the example a refutation without qualification would move too quickly. A successful parody must preserve the feature on which the original inference depends. If it changes that feature, the defender can reject the analogy without accepting the island. The disagreement therefore concerns the scope of the reasoning as well as the plausibility of its conclusion.

Gaunilo also questions whether understanding words about an unsurpassable being gives the required understanding of their subject. That challenge reaches an earlier stage of the argument than the existence comparison. It is useful to separate the challenges, since defending the intelligibility of the description would not automatically validate the inference from it.