Gaunilo's Cogito Argument

Gaunilo presents Anselm with a dilemma in section 7 of his Responsio: I know most certainly that I exist. But If I cannot think my non-existence at the same time, then Anselm's claim in Proslogion 3 (that my inability to think God's non-existence, while knowing most certainly that He exists, is a unique property of God) would be false. If I can do so, however, then I should also be able to know most certainly that God exists and, at the same time, think his non-existence. I will show that Anselm's response to Gaunilo's attack is not adequate because it does not address the issue of certainty, which is at the heart of Gaunilo's objection.