Using Aristotle’s Physics II.8 to Elucidate the Fifth Way

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.