Good is the fulfillment of final causes. Badness is the lack of fulfillment of final causes.
Would you have an explanation of why final causes in nature so often fail? In other words, would you have an explanation for why natural evil exists?
If it is necessary to go further afield, to revelation (e.g., the fall) by all means, feel free to do so.
The example I’m thinking of sex being ordered towards procreation. It fails in this mightily, much of the time. The typical sexual encounter – without birth control, and where both partners have healthy reproductive systems – does
not end in procreation. A naturalist would observe, in fact, that nature is “hedging her bets” in the ironic sense of Christ’s parable of the sower (some seed fell on rocky soil; some seed fell by the wayside; but some seed…). Having millions (billions?) of sperm would be another means of “hedging one’s bets” in terms of increasing one’s chances, in what is a very imperfect process (thankfully, humans are willing to try it more than once, if they don’t at first succeed

).
What I think is logically incontrovertible is that there is no
imperative that the creation of a perfect Creator
not be as perfect as itself (indeed, Heaven would refute anyone who would claim the contrary).
So what is Aquinas’ explanation for the reason why a perfect Creator made an imperfect Creation? With humans, we are talking about free will. But I am talking about nature here.
Again, if revelation is necessary, by all means invoke it. But if Aquinas is saying that nature – which is not God –
must contain imperfection and evil (privation), then he is making an assumption that does not logically follow. I see no logical reason why nature would not be a perfect
chip off the old block of a perfect Creator, especially an omnipotent one. There is no logical reason
why the created realm of an absolutely perfect creator would need to be a faded facsimile of the perfection of the creator.
I suppose the Aquinas answer is in the distinction between potential and actuality – potential is, by definition, imperfect. But this is more a description of the way things
are, not a description of
why they are, as they are. On an
a priori basis, the real paradox is how the creation of an utterly perfect Creator should not be as
perfect as itself. In fact, once you acknowledge imperfection and privation, you’re actually giving carte blanche to creation’s being almost infinitely
more imperfect than it is. Creation could be much more imperfect than it is – say, with the human race dying out because it can no longer reproduce; and, indeed, many species
did fail so utterly to achieve their final causes, that they became extinct – and yet it would not impeach this idea that the Creator is perfect.
I don’t know, it seems like a pretty amusingly rigged standard of measurement

But perhaps our human expressions, “the apple doesn’t fall far from the tree” and “a chip off the old block” are not valid; nor, the expression, “by a tree’s fruits, you will know the tree… No good tree can produce bad fruits.”