“That there are certain truths about God that totally surpass man’s ability appears with the greatest evidence. Since, indeed, the principle of all knowledge that the reason perceives about some thing is the understanding of the very substance of that being (for according to Aristotle “what a thing is” is the principle demonstration), it is necessary that the way in which we understand the substance of a thing determines the way in which we know what belongs to it. Hence, if the human intellect comprehends the substance of some thing, for example, that of a stone or of a triange, no intelligible characteristic belonging to that thing surpasses the grasp of human reason. But this does not happen to us in the case of God. For the human intellect is not able to reach a comprehension of the divine substance through its natural power.” (Summa Contra Gentiles, I.3)
We don’t come to know God like we come to know other things, because the substance of a thing determines the way in which we can know it. If God’s substance is unlike created things, which the intellect only knows in part, how much less can we comprehend God’s substance? The only being who can comprehend God’s substance is God.
(Incidentally, ontotheological skepticism toward Thomas is completely unwarranted here, as I’m inclined to think it always is with him. God “has” a substance, but his substance is no different than his own self. For Thomas, it seems to me, denying that God has a substance—a reality, or a ‘that-which-God-is’—would be tantamount to denying that God exists. And denying that God exists while remaining a Christian in the name of piety is silly.)
Having spent the last year reading a lot of Barth and talking to a lot of Barthians who are skeptical of negative theology (saying things like “of course we know God’s essence, it is found in Jesus Christ,” etc. etc.), I’m interested to see the way in which negative knowledge of God actually functions in the rest of SCG. I suspect it is designed precisely to ensure the Christian’s assurance of her relationship with the living Jesus Christ. Whether it “works” is another question, of course, but I am optimistic, as the first three chapters of SCG have been great.