St. Thomas’s Fifth Way remains something of a mystery to me. Clearly it begins with the fact that unintelligent natural bodies act for determinate ends, and it reaches the conclusion that the first cause of these bodies is intelligent. But what is the basis for introducing the concept of intelligence? What for Thomas is the essence of being intelligent?
 
We may get some help on this question by looking at Thomas’s commentary on Aristotle’s Physics, in particular, Physics II.8, where Aristotle defends the application of teleology to the world of nature. It is clear, I think, that both Aristotle and Thomas have a somewhat deflationary understanding of ‘acting for a purpose’ (telos or finis). Any entity that, by virtue of its nature, has a set of causal powers and potentialities can be said to be acting ‘for the sake of’ manifesting those powers in the appropriate circumstances. See, for example, Summa Theologiae I.II Q1, a2:
 
“But an agent does not move except from intention of an end; for if an agent were not determined to some effect it would not do this rather than that. Therefore, to produce a determinate effect it must be determined to something certain which has the nature of an end.”
 
So, why think that when some natural thing is ordered to an end that intelligence is somehow involved? In Physics II.8, Aristotle notes that natural things pursue their ends “without deliberation.” This is how Aquinas analyzes the point in Lecture 13, paragraph 259: “But because they [natural bodies] always act in the same way it is clear that they do not act by intellect but by nature [non ex intellectu sed per naturam]. The artisan [in contrast] judges the form of the thing built and can vary it.” So, the key idea is the variability of the products of an intelligent agent. An unintelligent agent always pursues its end in the same way, regardless of circumstances. An intelligent agent, in contrast, can pursue the same end in many different ways, selecting a satisfactory way in light of varying circumstances.
 
So, in order to infer the existence of an intelligent cause behind the phenomena of nature, we need two things: (i) some evidence of constant ends or goals across some range of natural species, and (ii) extreme variability in the way that these differing species attain these common goals.
 
As Thomas points out in the Fifth Way, natural bodies do not just seek their proper end—they achieve that end very frequently (frequentius). Achieving this end requires the cooperation of the body’s environment. For a natural substance to exercise its natural powers, there must be mutual manifestation partners (as C. B. Martin labeled them). For each of a substance’s natural active powers, there must be substances with complementary passive potentialities. To exercise a power to heat, there must be something with the potential of being heated. And, conversely, for each natural passive power, there must be substances in the environment with the corresponding active power. Each domain of mutual manifestation partners exhibits a single, common end: the end of mutual cooperation in the realization of the constituent partners’ natural ends. This common end is achieved in a variety of ways, one way for each species of substance in the domain.
 
Consequently, the fine-tuning of each domain requires an intelligent cause. Since the observable universe constitutes a system of harmonious, mutually tolerant domains, we can reasonably infer that there is a single intelligence at the root of the universe.

I assume that, upon reaching the Fifth Way, we are supposed to be able to presuppose the truth of the conclusion of the earlier Ways, especially the Second Way. So, I take it for granted that we already know that all natural things in creation are caused to be exist (Second Way) by some eternal (First Way) and necessary-per-se (Third Way) being or beings. What the Fifth Way is supposed to tell us is that this necessary, eternal first cause is intelligent—in fact, that it has knowledge that encompasses all the facts about the operations of nature.
 
This immediately distinguishes the Fifth Way from many cases of the so-called design argument, as considered by Hume in his Dialogues and as defended by William Paley and many contemporary apologists. These non-Thomistic arguments are vulnerable to Richard Dawkins’s (and Hume’s) Who designed the designer? objection. This objection won’t work against the Fifth Way, since we’ve already established the existence of an uncaused cause of finite things. The only question for us is: is this uncaused cause something that could reasonably be considered as having intelligence and foresight?
 
In my view, the Fifth Way relies on two separate facts about natural ends: (i) the fact that natural substances have ends at all, and (ii) the fact that the world’s substances collaborate in such a way that most of them are generally able to achieve their ends most of the time. Some recent commentators (including Ed Feser and Gavin Kerr) have completely ignored the second fact, which is clearly indicated by the text (“not fortuitously, but designedly, do they achieve their end,” “ex intentione perveniunt ad finem”). In addition, this second argument is explicit in the version of the Fifth Way in the Summa Contra Gentiles (I.13, par. 35):
 
“Contrary and discordant things cannot, always or for the most part, be parts of one order except under someone’s government, which enables all and each to tend to a definite end. But in the world we find that things of diverse natures come together under one order, and this not rarely or by chance, but always or for the most part.”
 
It is also strongly suggested by a parallel passage in 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."
 
Powers can be thwarted or blocked. Many powers require appropriate conditions for their manifestation. E.g., active powers require their corresponding passive power. Imagine a charged particle in a world without other charged particles, or a massive particle in a world of massless particles. This is, I think, the point of Thomas’s example from De Veritate. Fire has the power to destroy things. This power must be limited in order to allow other bodies to persist and to exercise their powers and dispositions.
 
Before presenting the arguments, we need to consider what it means for something to act “providentially”, i.e., intelligently and with foresight. I propose the following definition:
 

• A thing x is intelligent iff (i) x knows a number of essences (or accidental quasi-essences)
  (ii) x is capable of per se causing things to exist with these essences,
  (iii) each of these essences is per se ordered to some particular end, and
  (iv) x is disposed to cause things of these kinds only if their per se ends are (a) individually good and (b) generally mutually consistent.

 
In the case of human beings, a human being is intelligent by virtue of knowing the essences of certain human actions (which includes knowing the per se effects of these actions), and by virtue of being able to cause the existence of such actions (i.e., by choosing to do them). In addition, human beings are generally disposed to act in ways that are individually good (in some respect) and mutually consistent in their per se effects.
 
I will also assume that if x is an immaterial thing and the per se cause of the existence of some y, then x can be said to “know” the essence of y (including y’s per se effects). There is for Aquinas no further condition required for knowledge. Any immaterial thing automatically knows the natures of all the things that it can cause per se. This could be taken as a philosophical definition of knowledge. The greater the number and variety of things that some immaterial being can cause, the greater its knowledge. (See, for background, De Veritate Q2 A4; Summa Contra Gentiles I.44; and Summa Theologiae I Q14, a1.)
 
Here's my reconstruction of the argument of the Fifth Way:
 
1. There are one or more uncaused things that are immaterial (because immutable), necessarily existing, and necessarily the cause of the existence of all other things (Ways One through Three).
 
2. There are material things (contingent and mutable) that are naturally ordered to determinate ends.
a.This is true because we see these things behaving in consistent and regular ways. (See Summa Theologiae I-II, Q1, a2: “An agent does not move except out of an intention for an end. For if the agent were not determinate to some particular effect, it would not do one thing rather than another: consequently, in order that it produce a determinate effect, it must, of necessity, be determined to some certain one, which has the nature of an end.”)
 
3.The natural end of anything is (by definition) good.
 
4. The first cause (or causes, if there are more than one) knows (individually or collectively) the essences of all contingent things, and is capable of causing per se the existence of things with those essences.
a. Because the first causes are immaterial causes of those things.
b. And because the first causes did in fact cause per se the existence of those things.
c. By the definition of knowledge, this suffices for the first causes to know these essences.
 
5. Since every natural end is good (by 3), the first causes are disposed to cause per se the existence only of things with good ends.
 
6. Many things in nature are such that their ends are mutually consistent.
a.We can infer this from the fact that most things generally achieve their natural ends.
 
7. If the first causes were not disposed to cause the existence of things with mutually consistent ends, there would not exist so many things in nature with ends that are mutually consistent.
 
8. Therefore, the first causes are disposed to cause the existence of things only when their ends are mutually consistent. (From 6, 7)
 
9. Therefore, the first causes are (individually or collectively) intelligent. In fact, very intelligent, given the large number of possible essences that are capable of causing to be exemplified. (From 4, 5, 8 and the definition of intelligent)