But of course just stopping there wouldn't be any fun so Aquinas allows that we can deduce some things, based upon the fact that whatever mystery created the universe created it in a certain way.
The mystery of existence may be beyond knowledge and proof, but we can still recognize it and we can talk about it. How? The way we talk about other patterns, visually, by saying what it is like. And we can do this in a couple of ways. We can talk about God figuratively, by saying things like, "A mighty fortress is our God" or "God is the light of the world." But Aquinas wants to be much more rigorous than that. Moving beyond the poetic, he wants to deduce things that are true about God in the same way that we deduce other complex things that are true in life, using pattern recognition.
pattern recognition and deduction
Aquinas spends a lot of time recognizing patterns in the universe and then trying to deduce information about the mysterious source of these patterns. But he does warn us that this method in the end will tell us more about the universe than about its mysterious source.
And as is befitting a fellow who launches his theology from mystery and from the theological tradition of an unknowable God, Aquinas looks at patterns and begins his deductions not with what God is like, but with what God is not like. He looks at life as we know it, and says that God is not anything like that.