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Gaunilo (or Gaunilon) of Marmoutiers was an 11th-century Benedictine monk, best known for his criticism of St Anselm's ontological argument for the existence of God. His thesis On Behalf of the Fool takes its name from the fools mentioned in Psalms 14:1 and Psalms 53:1, who say in their hearts that there is no God. Anselm referred to them in developing his ontological argument in the Proslogion. Little beyond this essay is known of Gaunilo; no other extant writings bear his name.


The "Lost Island" refutation

Anselm claimed his ontological argument as proof of the existence of God, whom he described as that being than 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. 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.

Philosophers often attempt to prove the ontological argument wrong by comparing Anselm's with Gaunilo's. The former runs thus:

  1. God is that being than which no greater can be conceived.
  2. It is greater to exist in reality than merely as an idea.
  3. If God does not exist, we can conceive of an even greater being, id est one that does exist.
  4. Therefore, God must indeed exist in reality.
  5. Therefore, He exists.

Gaunilo's argument runs along the same lines:

  1. The Lost Island is that than which no greater can be conceived.
  2. It is greater to exist in reality than merely as an idea.
  3. If the Lost Island does not exist, one can conceive of an even greater island, id est one that does exist.
  4. Therefore, the Lost Island exists in reality.

If one of these arguments is sound, it has been asserted, they must both be sound. By Gaunilo's reckoning, however, one (and, therefore, the other, too) is unsound. The Lost Island does not exist, so there is something wrong with the logic that proves that it does. Because the argument proves true in one case that which is patently false (the Lost Island), it is fair to ask whether it may fairly be regarded as proving true the other case. The fact that there is no perfect island is put forth by Gaunilo as showing that Anselm's argument for God's existence is flawed.

Such objections are called overload objections: they do not claim to show where or how the argument goes wrong; they merely argue that, if it is unsound in one application, it is unsound in all others. Simply put, they are arguments that would overload the world with an indefinitely large number of things, like perfect islands.


Gaunilo's objection to the ontological argument has been criticised on several grounds. One concerns the very idea of a perfect island, which, presumably, has an abundance of lush trees and pristine beaches. The more of these that an island has, the criticism continues, the better the island is.

But there is no intrinsically maximum number of trees or beaches that an island could possibly have: for any one conceivable island, there is another, even greater, with one more palm tree and one more beach. Ergo, there is no island than which no greater can be conceived, because, the critique insists, more trees and more beaches are better, and the island thereby is argued to move without end toward infinity. Therefore concept of the perfect island is incoherent, and therefore there is and can be no such thing. Of course, Gaunilo was not so foolish as to claim that a higher tree density or total would be "greater", just that its existence would be greater. However, the criticism attempts to burden the island with boundless trees and beaches nonetheless.

Alvin Plantinga tendered a reply to Gaunilo's remonstrance by arguing that the concept of "that than which nothing greater can be conceived" is not applicable to an island, or any other object, in the special way that it is applicable to God. Plantinga defends Anselm's proof by averring that it applies exclusively to Him, a viewpoint that Anselm himself had stated but failed to elaborate. A necessary being is both existent and the greatest conceivable and greatest possible being. Only God, as Anselm defines him, meets all of those criteria and can, therefore, be dubbed a necessary being.

Another criticism of Gaunilo's argument points out that, whereas God is that thing than which no greater can be conceived, Gaunilo's is that island than which no greater can be conceived. Thus, while no island may exceed it in greatness, it is perfectly reasonable to suppose that some non-island could. "Consequently," wrote William L. Rowe in his summary of the polemic, "if we follow Anselm's reasoning exactly, it does not appear that we can derive an absurdity from the supposition that the island than which none greater is possible does not exist."[1]

Gaunilo's criticism of St Anselm's argument may be seen as either making it absurd or not, but it does succeed in raising doubt about the logical structure of Anselm's proof.


David and Marjorie Haight took a very similar tack with Anselm's proof attempt as did Gaunilo. However, whereas Gaunilo changed the target noun of Anselm's proof, "God", to an alternate noun that he felt was more obviously absurd, a "Lost Island", the Haights inverted the adjective in Anselm's reasoning. Where Anselm used the word "greater" to define god into existence, the Haights point out that the logic can be inverted by replacing "greater" with "worse". The statement then follows to a conclusion that the very most bad thing has to be an existent bad thing, because it would be worse for this bad thing to exist than to not exist, therefore it must exist in its absolute badness. Therefore, the Devil must also exist, so long as Anselm's proof is held as consequential.

Both Gaunilo and the Haights arguments point out that there may be other nouns, and other bivalent adjectives that when conceived as an Anselm proof (in an extreme that demands existence) could also be argued to necessitate their existence as well. For example, with cold or heat: Surely an absolutely cold being that exists in reality is more absolutely cold (or hot) than one that only exists in imagination. Therefore it must indeed exist in reality. And so on. The Haights show that the word "great" may not be the only adjective that pushes for existence when conceived in the extreme, just as the phrase "that God thing" may not be the only noun interacting with "great" in this way, as Gaunilo observed.


  • Haight, Frederick David; Haight, Marjorie A.: The scandal of reason: or shadow of God. (University Press of America, March 15, 2004).


  1. ^ Rowe: "The Ontological Argument" in Feinberg; Shafer-Landau: Reason & Responsibility, p. 15.


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