ABSTRACT
The Ontological Argument for the existence of God is an a priori argument. The Ontological Argument claims the concept of God is different: from the definition of the concept of God, the existence of God is supposed to follow. Gaunilo concluded that Anselm's argument must have a mistake in it as well. Gaunilo was suggesting that since his Island Argument is invalid, so too is Anselm's argument about God, since they have the same logical form. When Anselm dismisses Gaunilo's Island Argument, he seems to be suggesting that the Ontological Argument is deductively valid, while the Island Argument is not. However, maybe Anselm is claiming that the premises of the Island Argument are implausible, though the premises of the Ontological Argument are quite plausible. Anselm’s argument makes use of the idea that God is perfect. Parallel remarks apply to defining unicorns or golden mountains.
