How does Plantinga respond to Gaunilo's example of the perfect island?

Alvin Plantinga responds to Gaunilo's famous example of the perfect island primarily by arguing that islands do not possess the same kind of intrinsic maximums as God does. In Gaunilo's argument, he claims that one could conceive of a perfect island in the same way that Anselm conceived of a perfect God, thus suggesting that the reasoning used to demonstrate God's existence could also be used to demonstrate the existence of such an island.

Plantinga counters this by explaining that the nature of God is fundamentally different from that of any created entity, including islands. God, as defined in classical theism, is a necessary being, meaning that His existence is not contingent on anything else. He embodies maximal greatness and perfection in a unique manner that does not apply to islands or other contingent beings. While one can imagine an island, it lacks the necessary qualities that are intrinsic to the concept of God, such as omnipotence, omniscience, and moral perfection.

Therefore, the distinction between the nature of divine perfection and the qualities of temporal, contingent entities like islands is central to Plantinga's response, validating the idea that one cannot simply substitute islands for God in the ontological argument.