The Fifth Way and John of Damascus

The text of the Fifth Way in the Summa Theologiae (I Q2 a3) is quite elliptical. It's not easy to extract an argument from it. The first part of the Fifth Way seems to give us the sub-conclusion that there is, for each unintelligent natural body, a directing intelligence. The argument then concludes with the statement: "Therefore, some intelligent being (intelligens) exists by whom all natural things are directed to their end." How does Thomas bridge the gap from an intelligent being for each natural body to the conclusion that there is some intelligence that directs all natural things to an end? 

I find it helpful to look at parallel passages in other Thomistic texts. Here, for example, is a passage from the Summa Contra Gentiles, Book 1, chapter 13:

[35] Damascene proposes another argument for the same conclusion taken from the government of the world (rerum) [ De fide orthodoxa I, 3]. Averroes likewise hints at it [In II Physicorum ]. The argument runs thus. Contrary and discordant things cannot, always or for the most part (pluries), be parts of (concordare) one order except under someone’s government, which enables all and each to tend to a definite end. But in the world (mundo) we find that things of diverse natures come together (concordare) under one order, and this not rarely or by chance, but always or for the most part (maiori parte). There must therefore be some being by whose providence the world (mundus) is governed. This we call God.

This argument explicitly appeals to the coherent order of the universe as a whole. Thomas explicitly cites an earlier argument in John of Damascus's work, On the Orthodox Faith. Here is my translation of the crucial paragraphs there (Chapter 3):

"But the very permanence and conservation and governance of creation teaches us that it is God who makes that universe exist continually and be conserved, and who always provides for it. For how do the contraries of nature (I mean fire, water, air, and earth) come together in the consummation of the world and remain indestructible, unless some omnipotent power brings them together and conserves them in their indestructibility?"

​"And what is it that establishes the heavenly and terrestrial things, and whatever is throughout (per/because of?) the air, and whatever attends (secundam/results from) the water, and, even more so, the things of water that are before those—the heaven and the earth and the air, the nature of fire and water? What is it that drives them with a restless and uninterrupted motion?"

"Isn’t it a craftsman of these things, who imposes a rational order on all of them, by whose design this cosmic device or machine is produced and conducted? Isn’t this same craftsman the one who makes these things and brings them into existence? Therefore, we will not attribute such power to chance. Suppose, however, that the elements were generated by chance: who so orders them that they go on existing? And even supposing that it did seem that the elements arose chance, we would have to ask: whose calculations are responsible for these things' being preserved and cared for, that is, the things that came to being in place of the original elements? An intelligent being, and not chance. Who could this be if not God?"

John's argument seems to be an early version of the fine-tuning argument for design. He is suggesting that rational design is needed to explain why the various elements of the world can coexist peacefully, resulting in a relatively stable and long-lasting cosmos. Recent work in cosmology has vindicated John's intuitions. Unless the universe's initial conditions and the fundamental constants governing the various forces were precisely adjusted to one another, one force or another would come to dominate, resulting in a world consisting entirely of black holes or entirely of a uniform gas cloud.

That Thomas accepts and uses this sort of argument is also confirmed by the following passage from De Veritate:

De Veritate Q5 a2 (Is the world governed by providence?):
"Material and efficient causes, as such, cause only the existence of their effects. They are not sufficient to produce goodness in them so that they be aptly disposed in themselves, so that they could continue to exist, and toward others so that they could help them. Heat, for example, of its very nature and of itself can break down other things, but this breaking down is good and helpful only if it happens up to a certain point and in a certain way. Consequently, if we do not admit that there exist in nature causes other than heat and similar agents, we cannot give any reason why things happen in a good and orderly way."