On Christian Doctrine continues, as St. Augustine divvies up purely human institutions and learning into what he thinks useful for Christians, or for Bible study, and what is not. (Kinda Plato’s Republic-y, when you think about it.)
Seeing as all that Dan Brown stuff is sure that Christianity hates science, it’s rather amusing to point out that St. Augustine, like most of the early Fathers, takes it for granted that the study of the natural sciences is the easiest part of worldly knowledge to justify to Christians — much easier than philosophy or rhetoric. Even though he can’t find strict utility for most of astronomy and even though it has so many pagan and divinatory associations, it’s not long before he’s using astronomy as an example again. The tie between close study of Creation and appreciative love of the Creator is obvious to him, and he expects it to be obvious to anyone who puts any effort at all into thinking.