What is Gaunilo’s main objection to Anselm’s argument?

Gaunilo's main objection to Anselm's argument is centered on the idea that imagined perfection does not guarantee existence. He offers this critique through a thought experiment famously known as the "Lost Island" argument. Gaunilo suggests that just because one can conceive of the most perfect island — that is, an island than which no greater can be conceived — it does not follow that this island exists in reality. This is a crucial point because it challenges the leap from conceptual understanding to real-world existence. Gaunilo's argument emphasizes that existence is not a predicate or a property that can be appended to the concept of a perfect being or thing. Thus, he effectively undermines the claim that Anselm's definition of God as the greatest conceivable being necessarily entails actual existence. This objection illustrates that mere conceptualization cannot be equated with actual existence, highlighting a significant flaw in Anselm's reasoning.