  Versions of this argument have been defended and criticized by a succession of philosophers from Anselm’s time through the present day (see  ontological arguments).  Our concern here is with Anselm’s own version, the criticism he encountered, and his response to that criticism. A monk named Gaunilo wrote a “Reply on Behalf of the Fool,” contending that Anselm’s argument gave the Psalmist’s fool no good reason at all to believe that that than which a greater cannot be thought exists in reality. Gaunilo’s most famous objection is an argument intended to be exactly parallel to Anselm’s that generates an obviously absurd conclusion. Gaunilo proposes that instead of “that than which a greater cannot be thought” we consider “that island than which a greater cannot be thought.” We understand what that expression means, so (following Anselm’s reasoning) the greatest conceivable island exists in our understanding. But (again following Anselm’s reasoning) that island must exist in reality as well; for if it did not, we could imagine a greater island – namely, one that existed in reality – and the greatest conceivable island would not be the greatest conceivable island after all. Surely, though, it is absurd to suppose that the greatest conceivable island actually exists in reality. Gaunilo concludes that Anselm’s reasoning is fallacious.