I should say that I’m not at all confident that I correctly interpreted Anselm’s argument in the previous post. But at least one major interpreter–namely, Charles Hartshorne–agrees that chapter III of the Proslogion is where the action really starts; he refers to the (more famous?) iteration of the argument in chapter II as “but a preliminary try, and an unsuccessful one–elliptical and misleading at best–to state the essential point, which is first explicitly formulated in Proslogium III, and reiterated many times in the Apologetic I, V, and IX” (Hartshorne, “Introduction to the Second Edition,” Basic Writings of St. Anselm, Open Court edition, 1962, p. 6). I’m not well read enough on more recent Anselm scholarship to say if this represents the consensus view or not, but it’s nice to get a little validation.
For Hartshorne, the key insight underlying Anselm’s argument is that divinity must be thought of as existing necessarily:
Anselm’s proposal can therefore be put thus: the contrasts, creature-creator and contingently-necessarily existent, should be seen as one and the same contrast, somewhat differently expressed. All things, except God are contingent–of course, says the theist, since they exist only thanks to the fact that it pleased God (and it might not have) to make their existence possible! But surely God does not exist because it pleased Him (and might not have) to make His own existence possible! In this and many other ways it may be shown that God cannot be contingent in a sense parallel to the contingency of ordinary existents.
To talk about “perfect islands” as analogous to deity is mere trifling, unless insular “perfection” (which, nota bene, is not Anselm’s word) is taken to include the status, “creator of all things else.” But then “insular” loses its meaning! When one reads some cheap and easy refutations of the Proof one gets the impression of the following strange course of thought: if God can exist necessarily, surely many other things can too; or, what God can do, others can do also; or, surely His unique excellence cannot go so far that His very mode of existing, His very relation to the category of existence, is unique also. Ah, but can it not? This may turn out to be the same as the question whether or not theism (and not simply an argument for theism) is logically possible. And if the logical possibility of theism is what the critic impugns, by all means let him say so! (pp. 6-7)
What Hartshorne is saying, I think, is that, for Anselm, if it is possible that God exists, then it is necessary that God exists. That is, necessary-existence is an essential part of the concept “God,” and so if that concept is internally consistent or coherent, then it must be instantiated in reality. The “logic” of the God-concept, so to speak, follows different rules than any other.