Here's a capsule history of the transition from medieval Scholasticism to the Enlightenment. Following Aristotle, Scholastics like Thomas Aquinas held that a complete understanding of any natural phenomenon requires attention to each of its four causes. Consider a cat. There is its material cause, which is what it's made out of—flesh, as opposed to the inorganic matter of which stones and dirt are composed. There is its formal cause, which is its nature or essence—felinity, with all the catlike qualities and behaviors that that entails. There is its efficient cause, which is what brought it into existence—its parents. And there is its final cause, the battery of ends or goals toward which it tends—chasing mice, making other cats, and so on.
Of course, there is more to the story. What exactly distinguishes flesh from other kinds of matter? What exactly is the mechanism by which cats make new cats? These questions too require answers, but finding those answers will entail identifying further causes of each of the four kinds referred to. How do we determine these causes? Through observation, naturally. It is through our experience of cats that we know that they come from other cats, that they tend to seek out mice, and so forth.
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