For my MCCC Philosophy of Religion Students. Here is what I want you to be able to say for the oral exams.
Anselm's contemporary Gaunilo thought Anselm was a fool for believing that you could just think of something in your mind, and it would then actually exist in reality.
Gaunilo said, if that's true, then I can think of a great island, and because it is greater to exist in reality than just in the mind, my "greatest island" must exist.
Our objections to Gaunilo are these:
1) Gaunilo misunderstands and misquotes Anselm. Gaunilo writes: "How is the fact that this greater being has been proved to be greater than everything else supposed to show me that it exists in actual fact."
But Anselm is not talking about "a greater being," or a "being greater than everything else," but rather a "greatest possible being".
2) Even if Gaunilo had correctly understood Anselm's "greatest possible being," there would still be a problem, which is: Contingent things like islands have no intrinsic maximums. For example, you could not think of a "greatest possible number," or a "greatest possible pizza." So, Gaunilo cannot think of of a "greatest possible island."
3) Even if "greatest possible island" was conceivable (which it is not), it would be a subjective thing. For example, my "greatest possible island" would include sushi at every meal with the music of David Hasselhoff playing 24/7. Presumably your greatest possible island would not.