Anselm received an immediate reply from a monk named Gaunilo: you could prove anything perfect must exist by this argument! I can conceive of the perfect island, greater than which cannot be conceived. And so such an island must exist, because it would be less great if it didn’t. But this is ridiculous, so the ontological argument must be flawed. You can’t infer the existence of something, Gaunilo argues, from the idea of its being perfect.
Anselm replied that the ontological argument works only for God, because the relation between God and greatness or perfection is unique. An island wouldn’t cease to be what it is – an island – if it wasn’t perfect; of course, it wouldn’t then be a perfect island. But islands aren’t perfect by definition; perfection is something an island can have or not have. It is an ‘accidental’ not an ‘essential’ property of islands. It’s perfectly coherent to think of an island that isn’t perfect.
(An essential property is one that something must have to be the thing that it is. Islands must be areas of land surrounded by water. Can we say that ‘perfect islands’ are islands that are essentially perfect? Not convincingly, because perfect islands aren’t a different kind of thing from islands, but a type of island. So they are still only essentially islands, and accidentally perfect.)
By contrast, God, argues Anselm, must be the greatest conceivable being – God wouldn’t be God if there was some being even greater than God – it’s incoherent to think of God as imperfect. Being the greatest conceivable being is an essential property of God. But then because it is better to exist than not, existence is an essential property of God. So to be the greatest conceivable being, God must exist.