Anselm’s ontological argument described in part (a), was refuted in his own lifetime, by Gaunilo, who demonstrated in a reduction ad absurdum of his own, that if the logic of the argument were applied to things other than God, it led to invalid conclusions. Gaunilo didn’t identify any specific fault with the argument, but argued that something must be wrong with it, because if there wasn’t anything wrong, then we can use its logic to prove anything, which we may have no reason to believe to be true.
For instance, Gaunilo argued that it’s possible to construct an argument in the exact same form as the ontological argument, that claims to prove the existence of the perfect island: this island must exist for if it did not then it would be possible to conceive of an island greater than that island than which no greater can be conceived, which is absurd.
Therefore, if the ontological argument works, then the argument for the existence of the perfect island must work too. They are both logically similar, so they stand, or fall together. However, the argument for the island is clearly spurious, as we have no reason for this ‘perfect island’ to exist. Unless a theist can point to some relevant difference between his argument for the existence of God, and Gaunilo’s argument for the existence of the perfect island, the theist will have to abandon the ontological argument for the existence of God, as well as Gaunilo’s, because if one fails, then they both do, as they are so similar. This weakness of the ontological argument adds fuel to the fire for atheists through Gaunilo putting forward his argument for the existence of the perfect island as an objection to the ontological argument. He knew that they would both fall, which shows that neither argument works which further supports atheists. Another weakness of the ontological argument which gives support to atheists is