An Advantage of Aquinas' Fifth Way over the Design Argument

David Hume’s critique of the design argument in his Dialogues Concerning Natural Religion is so powerful that it is considered by many to be the end of the argument.[1] (And perhaps it is for a certain kind of design argument.) But, for the Thomist, it is by no means the end of teleological arguments for God’s existence. In this article, I will distinguish Cleanthes’ Design argument, a teleological argument for God based on efficient causation, from Aquinas’ Fifth Way, a teleological argument for God based on final causation (for an explanation of the differences between these two types of causation, click here). I will show that by appealing to final causation, Aquinas’ Fifth Way avoids one of the main difficulties for some design arguments as revealed in Hume’s critique, namely, the god of the gaps objection.

Cleanthes' Design Argument

In his letter to his friend Hermippus, Pamphilus, the young student of Cleanthes, relates a discussion of natural religion between Cleanthes, a theist, Demea, a deist, and Philo, a skeptic, which took place in Cleanthes’ library. In Part 2 of this account, Cleanthes offers what he says is his sole argument both for God’s existence and for God’s intelligent nature:[2]

Look round the world, contemplating the whole thing and every part of it; you’ll find that it is nothing but one big machine subdivided into an infinite number of smaller ones, which in their turn could be subdivided to a degree beyond what human senses and faculties can trace and explain. All these various machines, and even their most minute parts, are adjusted to each other so precisely that everyone who has ever contemplated them is filled with wonder. The intricate fitting of means to ends throughout all nature is just like (though more wonderful than) the fitting of means to ends in things that have been produced by us—products of human designs, thought, wisdom, and intelligence. Since the effects resemble each other, we are led to infer by all the rules of analogy that the causes are also alike, and that the author of nature is somewhat similar to the mind of man, though he has much larger faculties to go with the grandeur of the work he has carried out.

Later in the dialog, Cleanthes elaborates on his argument by saying:[3]

But is it such a slight resemblance between how means are fitted to ends in a house and how they are fitted in the universe? The way things are fitted to their purposes? The order, proportion, and arrangement of every part?

From these two statements, Cleanthes strategy becomes clear. His argument relies on an inference from the order with which things are made to a best explanation for that order, a super-intelligence. This focus on the order with which things are made is evident in his mention in both statements of the way things in the universe are fitted together for their ends or purposes.

In focusing on the intricate fitting together of things in the universe, Cleanthes directs our attention not to the purposes for which things are made, but to the physical complexity of the things made. This machine-like complexity, as he sees it, is evidence for the probable existence of a designing mind. His argument, in essence, is:

1. Some things in nature are like machines that are intricately fitted together for ends (designed).

2. When we see human machines that are intricately fitted together for ends, they have a human designer.

3. These intricately fitted together machines in nature must, like human machines, also have a Designer.

In emphasizing the machine-like way things are made, Cleanthes is arguing from efficient causation regarding a physical or mechanistic process. He is saying God is the best explanation for how something this complexly physically ordered came to be. And he admits that this explanation is at best probabilistic—God is probably the best explanation for this phenomenon. Because Cleanthes’ argument focuses on efficient causation, the way things are physically fitted together, it is subject to one particularly strong critique called a god of the gaps critique.

Philo’s God of the Gaps Critique

Philo the skeptic, most likely expressing Hume’s own view, responds critically to Cleanthes’ argument. Among the several criticisms he levels against the argument is one which might be called a god of the gaps criticism. In response to Cleanthes’ claim that the complexity of created things must be explained by God’s design, Philo argues that it is possible that what looks like design arising from an intelligent mind in intricately ordered physical objects could really be the result of an as-yet-unknown physical process. He says:[4]

Now, Demea, this method of reasoning leads to something that Cleanthes himself has tacitly admitted, namely: order, arrangement, or the suitability of things for various purposes (like the suitability of legs for walking) is not of itself any proof that a designer has been at work, except in cases where experience has shown us that such order, arrangement, etc. is due to a designer. For all we can know a priori, matter may have a source of order within it, just as mind does, having it inherently, basically, ·not acquired from somewhere else·.… When a number of elements come together in an exquisite arrangement, *you may think it harder to conceive that •they do this of their own accord than to conceive that •some designer put them into that arrangement. But that is too quick and careless. Think about what is involved in a designer’s arranging them: it means that he creates the arrangement in his mind, assembling in the appropriate way the ideas of the elements in question. But, then, how does that happen? I put it to you*, it is no harder to conceive that •the elements are caused to come together into this arrangement by some unknown cause that is internal to them, than it is to conceive that •the ideas of these elements come together in that arrangement in the great universal mind, being caused to do so by a similarly unknown cause that is internal to that mind.

This response is a textbook example of a god of the gaps critique—an argument which says that there is no need to posit a God to explain some gap in our scientific knowledge because it may be that what is not yet explained by science one day will be.

The effectiveness of this argument is evident. Design Arguments since the time of Bishop Butler, whose argument Cleanthes is advancing, have been met with just this rejoinder—perhaps what is as-yet-inexplicable by natural means will one day be explained naturally. Therefore, there is no need to resort to a theistic explanation. Some will cite Darwin’s theory of the origin of species via natural selection as a case in point. They will argue that what men once believed could only be the work of a god is now explained by a natural mechanism. The consequence of this natural explanation, they claim, is that they have also explained away the god that was once thought to be necessary. Given this clearly problematic weakness in Cleanthes’ Design argument, we might wonder whether teleological arguments have any value as a theistic proof. Here it is beneficial to consider a different species of teleological argument, one which relies on final causation instead of efficient causation.

The Advantage of Aquinas’ Fifth Way

Aquinas’ Fifth Way of proving God’s existence is, like Cleanthes’ argument, a teleological argument. It is concerned with order in the universe. Importantly, however, it is concerned with the final causes of that order. It focuses on the purposes toward which things are ordered rather than the physical complexity by which they are ordered to purposes. Aquinas says:[5]

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.

Notice, Aquinas is interested in the ends for which natural, unintelligent bodies act with regularity. Bodies, he notes, act always or almost always to obtain the best end. He then argues that unintelligent things cannot move toward ends unless some intelligent being is moving them toward those ends. An arrow moves toward a target because an intelligent being, an archer, shoots the arrow toward the target. According to Aquinas, in that we observe that things in nature are ordered to act toward ends with regularity, we can infer that an intelligent being, i.e., God, has ordered them toward these ends.

But why isn't Aquinas’ Fifth Way subject to the god of the gaps objection? Crucially, Aquinas’ argument focuses not on the way things come to be intricately fitted together, a physical question about a physical phenomenon, but on the purposes for which they come to be intricately fitted together, a metaphysical question pertaining to a mental phenomenon. As such, no physical explanation could possibly account for the existence of purpose. Ed Feser explains:[6]

Aquinas is not presenting a quasi-scientific explanation of some psychological phenomenon that we simply haven’t got enough empirical data to explain in a materialistic way. As with the Five Ways, he is attempting to provide a metaphysical demonstration. He is claiming that it is in principle impossible, conceptually impossible for the intellect to be accounted for in a materialistic way.

In asking about final causes, Aquinas wants to know why physical things have been fitted together in an ordered way for ends? Why is it that a heart pumps blood? Why is it that an acorn produces an oak tree? Why is it that male and female produce children by their union? Why is it that massive bodies are attracted to one another? Why do we observe order governing nature in these and many other ways?

The answer, as Aquinas would argue, is that all of these things are fitted to ends. They have purposes. Material things, however, cannot purpose. Therefore, if by the ordered regularity we see in nature, we can infer purpose, then by the inference of purpose, we can also infer a Purposer. Notice that this conclusion is not probabilistic, but absolute. And it is not subject to future material explanations. If there is a purpose, there must be a mind which purposed it.

Conclusion

In that Aquinas’ Fifth Way focuses on final causation instead of efficient causation, it is different from and superior to Cleanthes’ design Argument. Whereas Cleanthes’ argument in focusing on efficient causation is subject to a god of the gaps critique, Aquinas’ argument in focusing on final causation is immune to this critique. Therefore, while we may take Hume’s critique of the design argument to be a formidable one, we should not take it to be a refutation wholesale of all teleological reasoning or theistic proofs.

[1] Hume, David. “Dialogues Concerning Natural Religion - Early Modern Texts.” Accessed July 10, 2022. https://www.earlymoderntexts.com/assets/pdfs/hume1779.pdf. [2] Hume, “Dialogues,” 11. [3] Hume, “Dialogues,” 13. [4] Hume, “Dialogues,” 12. [5] Aquinas, Thomas. Summa theologiae FP Q[2] the existence of god. Accessed July 10, 2022. http://summa-theologiae.org/question/00203.htm. [6] Feser, Edward. Aquinas: A Beginner's Guide (Beginner's Guides) (p. 183). Oneworld Publications. Kindle Edition.