According to Falcon, Aristotle proposes that we have knowledge of a thing only when we understand its causes, and he proposes four possible causes: material, formal, efficient, and final.
The material cause: “that out of which”, e.g., the bronze of a statue.
The formal cause: “the form”, “the account of what-it-is-to-be”, e.g., the shape of a statue.
The efficient cause: “the primary source of the change or rest”, e.g., the artisan, the art of bronze-casting the statue, the man who gives advice, the father of the child.
The final cause: “the end, that for the sake of which a thing is done”, e.g., health is the end of walking, losing weight, purging, drugs, and surgical tools.
The final cause for a statue is the statue itself.
With the final cause, Aristotle's theory is teleological; the purpose of a thing is a kind of cause of it. Because of this, he has been accused of anthropomorphizing nature, attributing to it psychological reasons for the way things are. However, while the theory can take desire, intention, etc., into account, the final causes of natural things don't require psychological causes. For example, Aristotle explains that the 'final cause' for why frontal teeth are sharp and back teeth are flat is because that is the best arrangement for the survival of the animal.
A couple of other key points are that, in studying nature, we should look for generalities. We aren't concerned about exceptions; we are trying to discover the rules. He also doesn't require that we use all four causes. In some cases, like the bronze statue, the formal and final causes are the same. In some cases the efficient cause is enough. For example, Aristotle explains a lunar eclipse with an efficient cause: the earth comes between the moon and sun.
The idea of a final cause was controversial in Aristotle's day. (I think it became controversial again in the 18th or 19th century.) Many philosophers* proposed that material and efficient causes were good enough. Aristotle claimed that material and efficient causes alone failed to account for regularity. If we ask, why are front teeth sharp and back teeth flat, material and efficient causes alone leave us with coincidence; animals produce offspring like themselves, and that's it. There is no reference to this arrangement making survival easier. Final causes, on the other hand, allow us to say that teeth are arranged that way to make survival easier; they explain the regularity in ways that material and efficient causes do not.
Leaving Falcon behind for a moment, why did final causes become controversial in the 18th or 19th century? Because teleological explanations seem to imply Nature has a personality. Before this time, Christian philosophers who adopted Aristotle would often point to God to provide final causes: Why were front teeth sharp and back teeth flat? God designed the animal that way to improve its chances of surviving. But God had to be killed in the Enlightenment (Hegel proclaimed it long before Nietzsche) and all that sort of thing removed. Biology today still uses teleological terms, but they intend them in reverse: the animal survives better because the teeth are arranged that way, and survival means a better chance of reproducing, which produces offspring with teeth arranged that way.
Next up, a dive into the Physics.
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* Science as the organized / methodical study of nature was a branch of philosophy up through the Scientific Revolution, and some branches of science (such as physics) were still called 'natural philosophy' up into the 19th century.
