Probably the most significant was that I found myself asking the question, "What if the problem isn't with me? What if this is incomprehensible...because it is incomprehensible?" It turns out in retrospect that my objection was really about 700 years old, because St. Thomas had already been there: having an idea of a thing in my mind doesn't imply anything about whether that thing actually exists outside of my mind. Consequently there doesn't seem to be any useful apologetic force to the argument.
(As an aside, I'm still pretty shocked that Sproul seriously thought it could be useful in proving God's existence to the unbeliever. I recall that he denied it was legerdemain or smoke and mirrors, but I couldn't see any way to clear the air and get out of the funhouse.)
I'm not a philosopher by training, but it seems to this armchair analyst (perhaps after having read Knowles) that St. Anselm's argument hinges upon a view of creation with Platonist hues. The whole thing might make more sense if we were to concede the notion of innate ideas: if the ideas in our brains had some original, external existence apart from us, then it might make more sense to talk about a necessary Being who must exist if I can conceive of Him.
Knowles has some useful things to say about the argument in pp. 102-106 of The Evolution of Medieval Thought (heh. It's amusing that he too can't avoid the rhetoric of magic when discussing it: "[M]any will have an uneasy feeling that a logical sleight of hand has been brought off at their expense." p. 103). One particularly interesting thing is that
Anselm himself specifically, and in the same context [of defending the argument], recognized the difference between a mental and a real existence.Aha! Well, that might explain the whole thing a bit better, even if I don't agree with the saint about Platonic realism. I'd much rather that the nature of the disagreement be one of such substance as this than be forced to wonder whether Anselm was a poor logician.
Realization of this has led some to maintain that the argument is not a merely logical one - the analysis of a concept - but an epistemological one. It is said that Anselm held so firmly the Platonic realism, according to which the idea is the only reality directly cognisable by the mind, that for him the presence of the idea of God in the mind implied the existence of the subsistent idea of God - that is, of God Himself, since there is no distinction of any kind in God. [p. 104; emphasis added]
Knowles proceeds to discuss other possible resolutions to the question, including Gilson's.
Anselm, he points out quite rightly, held that the mind could arrive at truth, which was the adequacy of the mental concept to express the being external to the mind. In other words, the external being is the cause of the concept and its truth, not vice versa. If therefore a being is not only existent, but necessarily so (as is God by definition), and is in fact the only being necessarily existent ('than which no greater can be conceived') then the only true concept of this Being will be one that agrees with its definition, and the mind will recognize the adequacy of the concept and the necessity of the existence of its object as soon as it as been presented to the mind. [p. 105; no reference to where in Gilson's work this hypothesis is presented]This seems to maybe put a Thomistic-ish spin on things, and I'm not really sure that it is as satisfactory as the prior one I discussed. Certainly I don't think that on Gilson's reckoning Anselm's argument has any useful force with non-Christians. Knowles goes on (ibid) to say that it was not really intended for that purpose, though.
Interestingly, he also says that "Bonaventure and the Augustinians in general accepted it, as strengthened by the doctrine of the divine illumination of the intellect, while Aquinas and the Aristotelians rejected it...Ever since, indeed, it has been a touchstone of Realism, and as such accepted in one form or another by Descartes, Leibniz and Hegel, and rejected by Locke, Kant, and Thomists of every kind" (ibid).
This suggests to me that locating the difficulties about the argument for me in the fact that I'm much more inclined towards Thomism than towards Platonic realism is the Right Thing to Do. It's unlikely that I'll be the one to find out by researching the question, but it would be interesting to know if Sproul is more Platonist than Aristotelian in his epistemology. Given his enthusiasm for Anselm's argument, I suspect that he is.