Explain and critically evaluate the Ontological argument for the existence of God.

The Ontological argument (in its most famous form) was originally proposed by Saint Anselm of Canterbury. He argued that if we define God as "a being than which no greater can be conceived", God must necessarily exist. His essential argument was that existence is obviously preferable to non existence, and so existence is a quality that the greatest possible being must posses. God must exist else we would be able to imagine something greater than him, which is impossible on the current definition of God. One of the initial objections to this argument was Gaunilo's parody case, "the greatest island". He argued that simply because the greatest possible island can be conceived of, doesn't grant it actuality. This attempted reductio ad absurdum was however easily rejected by Anselm, who simply pointed out the enormous semantic gulf between the statements, "An island greater than any other" and "That than which no greater can be conceived". It also must be pointed out that the definition "greatest possible island" could vary wildly from person to person, whereas Anselm's definition of God is far narrower in scope. In theory, the greatest possible entity or being is something that can be identified empirically. Immanuel Kant was also a fierce critic of the Ontological argument. He believed that it missed the essential point that "existence is not a predicate". In other words, existence is not a quality that something can posses, and so the ontological arguments talk of such is incoherent. This is a much more successful objection than Gaunilo's, yet via an appeal to modality and "necessary existence", the Ontological argument is still defensible. Necessary existence (existence in all possible worlds) is a quality that something can or cannot posses, and so a modal version of the Ontological argument seems plausible. Perhaps the best argument left to the non theist is an attack on Anselm's implicit model of God as a perfect being, something many argue is an impossibility.