Depending on how we view the purpose of the Summa Contra Gentiles, the natural theology in this work may come as a surprise to us. If we are more familiar with the other Summa, or if our only exposure to the Five Ways used by Aquinas to show God’s existence is in that more widely read work, then we may come to this book expecting Thomas to give us simply a larger version of those five arguments. That is because this is thought of as the work of apologetics—aimed at the “Gentiles,” meaning Jews and Muslims mostly at that time—though other scholars surmise that this was meant to be a manual of doctrine for Catholic missionaries in Spain.

Be that as it may, what we do find here is valuable to our study of natural theology in a different way. Instead of expanding on all five of those ways in one place, Thomas begins by offers the first two, much more explicitly in the mold of Aristotle. The form of the arguments themselves are about the same size. However, immediately afterwards (especially with regard to the first), Thomas then anticipates objections against each vulnerable premise.

Here we also find Thomas’ most succinct statement of how natural theology works.

“Yet, beginning with sensible things, our intellect is led to the point of knowing about God that He exists, and other such characteristics that must be attributed to the First Principle.”1

The “and” in that statement is all-important. Aquinas did not just view natural theology as tightly-contained arguments for God’s existence. Rather, natural theology is exactly what Paul’s words about natural revelation (in Romans 1:20) would tend to suggest. While the divine act (revelation) and the human response (theology) are distinct, yet it is precisely “the invisible things” of God that are made known through the things that have been made. Paul does not restrict this knowledge either to (a) latent principles untouched by discursive reasoning (contra all forms of fideism), nor to (b) formal demonstrations of God’s existence in apologetics.

As in the other Summa, so it is here. Natural theology extends its resources from establishing that God is to “other such characteristics,” or what God is. Those who read bits and pieces of Aquinas—likely only quotes taken out of context in modern books—and conclude that Thomas utterly compartmentalized faith reason, or that his via negativa implied no real knowledge of who God is, are simply mistaken.

A Schema of the First Way

One way to set the argument to simple logic is as follows:

1. All things in motion are moved by another.

2. Either this proceeds to infinity or an unmoved mover.

3. It is impossible that this proceeds to infinity.

∴ There is an unmoved mover. This is God.2

Now in the course of putting together these premises, we run into Aristotle’s lingo, in which he is much more ready to propose an either-or scenario. In the Physics (VIII, 5), Aristotle speaks of the First Mover under two “conditions,” namely that this mean “absolutely unmoved” or else “self-moved.” Obviously, Thomas is appropriating Aristotle here to drive to the first condition.

What would we immediately suppose were the objections against Aristotle’s version of the argument? The first is that it presupposes the eternality of the world of motion. The second is that it concludes in a First Mover that is that self-mover.  Yet of the second of these criticisms, Thomas' follow-up words should be noted:

“even Aristotle himself proposed this conclusion as a disjunction: it is necessary either to arrive immediately at an unmoved separate first mover, or to arrive at a self-moved mover from whom, in turn, an unmoved separate first mover is reached.”3

Defending the Premises

If the real contribution of the natural theology here is in the justification of the vulnerable premises, how does Thomas do that with premises 1 and 3 above? He need not have defended the second premise, as that is simply a straightforward disjunct of what is already given.

To justify premise 1, namely that everything in motion is moved by another, Thomas turns to three reasons given by Aristotle. 

First, “If something moves itself, it must have within itself the principle of its own motion.” Further, “it must be moved by reason of (all of) itself, and not by reason of a part of itself.” In other words, “self-moved” is still being moved, since the divisible depends on its parts. 

Second, by induction, those things moved by accident, by violence, by their nature even from within, heavy or light bodies through nature, all of these are moved by another.

Thirdly, the same thing cannot be in both act and potency at the same time and the same way. “But everything that is moved is, as such, in potency,” whereas whatever moves is in act. But nothing can be both moved and mover at the same time and in the same way.4 

As with the first premise, so with the third, he leans back on another three arguments from Aristotle:

First, if all such movers and moved proceed to infinity, all such would be bodies, since only the divisible is moved (as already shown). So also all such things (an infinite set) would be moved simultaneously and so in a finite time (an infinity in a finitude: another contradiction). 

Second, in any ordered series of movers, to remove the first mover is to cease all motion, as none other would be moved. But if movers proceed to infinity, then there is no first. 

Third, in any ordered series, if all causes are instrumental, and there is no efficient cause, then there will be no principal mover.5

Interestingly, Thomas also suggests a teleological reason that a self-mover cannot be the First Cause. Every being that moves itself is moved toward some end; but ends are always objects “higher, in the order of motion, than the mover desiring it.”6 Naturally the First Mover cannot desire an object outside of itself, let alone a greater object, since then it would be extraneously moved. Now this belongs to the section of Aristotelian arguments that Thomas replies to as those "that presuppose the eternity of motion”7

A Schema of the Second Way

I am content to sacrifice the ordinary numbering of premises with one general conclusion because I want to mark where an inference has already been made. Besides, very often in natural theology especially, people are impatient with any argument that is not in the three steps of a syllogism. In real life—if their conversations could wait to be mapped out by logicians, there would be far more enthymemes and other longer forms than we realize. That said, all we are doing here is summarizing as concisely as possible without sacrificing accuracy. Thomas reasons in this way:

1. The first cause is the case of all intermediates, and the end, in any ordered series of efficient causes.

2. When you suppress a cause, you suppress its effects.

∴ If you suppress the first cause, the intermediate causes cannot be a cause.

3. If there were an infinite regress among efficient causes, no cause would be first; and so with intermediate causes.

4. But this is manifestly false.

∴ There is a first efficient cause. This is God.8

For the second argument he does not bother to offer any lengthy defense of its premises.

Aquinas closes chapter 13 of the SCG with two seemingly random arguments to commend, without putting much effort at arguing for them. They strike the reader almost as “honorable mentions.” 

The first is also from Aristotle, and involves the fact that of any two falsehoods, one is more false than another; hence the other is more true than the other. Ergo, there is a spectrum from the more true to the more false, which parallels the scale of being. Hence, “what is most true is also most a being” (Metaphysics, Ia, 1).

The second was found in both John of Damascus and Averroes. It is the conclusion of a government of the world from the fact of contrary and discordant things always finding some unity. This is naturally a more difficult argument to make sense the harmonizing of that which is most opposed, such as true and false, good and evil, can only be sufficiently judged in the end.

The Third Way and Divine Eternity

The third way that Aquinas uses to show the existence of God in the Summa Theologiae is placed here in the chapter on God’s eternity (ch. 15). We could put it in the following form:

1. In the world are beings of generation and corruption.

2. Such beings can be or not be: i.e., they are contingent.

3. That which merely can be requires a cause.

4. But (as has already been shown) one cannot proceed to infinity in causes.

5. But even of beings more necessary, its necessity lies either in itself or in another.

6. But (for the same reason as 4) one cannot proceed to infinity in the causes of necessity.

∴ There is a necessary Being, whose necessity is through itself. This is God.9

One can debate why Thomas places this reasoning here rather than in a separate section on natural theology, as in the other Summa, or whether or not he ought to have done so. What cannot be debated is that Aquinas freely “intermixes” natural theology and theology proper; and, in fact, so relates them that I would even hesitate to call them an “intermixing.” The thought process seems rather inevitable and ordinary, even for non-theologians and non-philosophers. Consider three other ways that natural theology is related to eternity in this section.

He moves from the First Way to immutability to eternality, as follows: “Everything that begins to be or ceases to be does so through motion or change. Since, however, we have shown that God is absolutely immutable, He is eternal, lacking all beginning or end.”10 Or, again, taking directly from Aristotle: “Those beings alone are measured by time that are moved. For time, as is made clear in Physics IV, is 'the number of motion.’”11 Thomas works in the other direction as well: “if it were true that there was a time when He existed after not existing, then He must have been brought by someone from non-being to being. Not by Himself, since what does not exist cannot act. If by another, then this other prior to God. But we have shown that God is the first cause.”12

___________

1. Summa Contra Gentiles, I.2.3.

2. SCG, I.13.1-16.

3. SCG, I.13.32; cf. Physics, VIII, 5

4. SCG, I.13.5-9

5. SCG, I.13.11-15

6. SCG, I.13.28.

7. SCG, I.13.29.

8. SCG, I.13.33

9. SCG, I.15.5.

10. SCG, I.15.2.

11. SCG, I.15.3.

12. SCG, I.15.4.