Aquinas’s fifth way is short, sweet, and misunderstood. I’ve been thinking about it as Dr. Ed Feser advises—not in terms of complexity à la Paley and the ID movement, but in terms of order (see here).
Here is what Aquinas actually says (emphasis mine):
The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God (ST I.2.3).
So it is clear that Aquinas, like Aristotle, does not equate all forms of final causality with intelligent agency. Natural things are directed by what they are towards specific ends. What’s more, Aquinas emphasizes that the evidence that “we see”, i.e. the empirical data for the proof, is regularity. He isn’t making a watch-maker argument that the design is complex.
Here are some further thoughts:
We find in Metaphysics 983a30-984a1 that final cause is the opposite of efficient cause. Indeed, on the broadest level, final cause can be defined in relation to efficient cause (combined with formal cause) such that if x, by its nature, is the efficient cause of something y, then the final cause of y is that aspect of the essential nature of x by which x is directed toward the generation of y.
The carpenter has a desire to sit, and it is that aspect of the carpenter that is the final cause of the chair. The heart requires fresh oxygenated blood to beat, so the final cause of blood being circulated from the lungs to the heart is the nature of the heart itself, which uses oxygen to help it pump blood through the circulatory system to sustain its nature of rhythmic beating. Of course other things can exploit final causality. The carpenter exploits the cellular structure of woody material, which directs it towards solidity and also makes it workable. That woody nature is directed towards the high growth of trees, which in turn allows trees to gather sunlight above other plants and nourish the woody cells. So the woody cells are the efficient cause of growth, and the final cause of growth is to sustain the woody cells. With respect to the carpenter, it could be said that he functions in a wider economy to provide chairs for other people who want to sit. So the carpenter responds to their desires to sit as a cause to make more chairs (in exchange for money or goods which the carpenter needs to live). Likewise, other organs, besides the heart, need oxygen to pump. Those organs, in exchange, help to cause the heart to pump by providing neural input, or the proper hormone levels, etc. So there are systems upon systems of interlocking law-like causes. Those causes depend upon a constancy in the nature of things and the way they behave (just as economy depends upon regularity in human laws—hence the carpenter won’t act for the ends of other humans if they are inconstant and cheat him). Nothing in physical nature seems to determine this regularity in themselves. Thus, with the elimination of formal and final causality, Hume found the problem of induction. But with a supreme mind that sustains and orders the nature of all things, the problem of induction disappears and the regularity and intelligibility of the cosmos can be accounted for. Human economy can fail because our intelligence doesn’t guarantee law-like behavior. But physical laws won’t fail because the intelligence which orders and moves all things is immutable, though particular instances of natural kinds can be corrupted such that the function eventually ceases. Thus chairs break, and hearts cease to pump. The law-like behavior of essential natures are regulated, but evil can deform individual things and break down particular systems.
In Christian theology, God is the efficient cause of all things, and it is God’s Goodness that is the essential aspect of his nature which liberally and graciously generates all things. So God is both the first efficient cause and the final cause of all things. This is because God is Being itself, and so all beings are brought into existence through his essential nature, and they are directed towards Being insofar as Being is that which perfects their nature.
Perhaps, then, the fall can be understood as a metaphysical breakdown between individuals and their essences. That breakdown can allow the accumulation of accidental changes to disrupt systems within a natural individual to the point where it ceases to be what it is and a substantial change occurs—cataclysm and death. But God still sustains essences, and the promise of Christ is that there will be a new creation. Perhaps this means that individual things will be perfected, and function according to their natures perfectly.