Daniel Shields

Dominic Ryan has reviewed the first two chapters of my book Nature and Nature’s God for New Blackfriars. In regards to the first chapter, which is devoted to interpreting Aquinas’ principle “omne quod movetur ab alio movetur,” Ryan states that I put “forward two main arguments in support of [my] contention” that this should be translated as “everything in motion is put in motion by another.” Ryan, however, does not attend to the text I allot the most space to, namely that from Aquinas’ commentary on Physics VIII.4 (p. 27-30, 34). That text is crucial to the whole argument.

What Ryan has to say about the text I cited from SCG I, c. 13 unfortunately misses the point. There Aquinas says: “Omne quod movetur ab alio movetur. That some] things are in motion (moveri)-for example, the sun-is evident from sense. Therefore it is movetur by some other mover.” Now we must translate movetur/moveri in such a way as to form a valid syllogism. But the minor premise that appeals to sensation cannot be interpreted as saying “that some things are moved (passively) is evident from sense.” It does not appear to the senses that the sun is moved passively, although it does appear to the senses that the sun is in motion. (Aquinas thought it was absolutely in motion, we might say that it is only in relative motion.) We certainly do not see a chariot pulling the sun, nor an angel pushing it, nor do we see some other massive body pulling it by gravity. (Is gravity a force? Both Aquinas and General Relativity say that it is not, albeit in very different ways. But even if gravity is a force it is invisible to the senses.) So we should translate Aquinas’ argument as follows: “Everything in motion is moved by something else. That some things are in motion-for example, the sun-is evident from sense. Therefore it is moved by some other mover.”

Ryan asks “What would be apparent to the senses were such ‘motion’ [of the sun] to occur? Surely what one would sense was that the sun appeared constantly to be changing place. But if the sun or indeed any other celestial body was constantly changing place would it not have to be moved in order to do so? Motion, after all, is not an end in itself.” But Ryan is saying nothing I disagree with, and nothing that is problematic for my interpretation. I take no issue with the obvious interpretation of the conclusion of Aquinas’ argument from SCG I, c. 13. He is certainly concluding that the sun is being moved by another, and for the reason that Ryan states, namely that motion is not an end in itself (see p. 91-92 of my book, from chapter 3.) My contention concerns the interpretation of the minor premise and thus of the middle term of the syllogism: “that something is in motion-for example the sun-is evident to the senses.” Interpreting the minor premise this way then requires the translation of the major premise I have given: “everything in motion is moved by another.” Otherwise the syllogism is invalid; it would commit the fallacy of having four terms.

Ryan continues: “Further and more fundamentally, it is not clear that sense knowledge alone can solve this problem. . . . motion . . . is, like any common sensible, something about which the senses can be in error.” To settle the question, Ryan tells us, we must use intellectual knowledge. Of course I don’t disagree, but this is besides the point. No one would claim that Aquinas had any doubts that the sun was itself in motion, living as he did long before Copernicus. He accepted the evidence of his senses in the most straightforward way. No one will read Aquinas as asserting “that something is passively moved-for example, the sun-is evident from sense”? Not even Pegis translates the text this way. But having gone on to translate the mover principle as “everything that is moved is moved by another,” Pegis makes Aquinas commit the fallacy of a syllogism in four terms.

Ryan takes issue with my characterization of the form of a natural body as the generator’s instrument by which it moves the body even when it is no longer in contact with it. The most helpful contribution of his review is to point out something that Aquinas holds about instrumental causes. Aquinas holds that instrumental causes have a proper effect in virtue of their own form and an instrumental effect in virtue of the principal agent’s form. (Ryan points to ST III, q. 62, a. 1, ad 2, where Aquinas is discussing the instrumental causality of the Sacraments; the same point is made in parallel texts from other periods of Aquinas’ career.) But Ryan argues that if the form of the element is an instrument in moving the element to its proper place–as I claim–then it will have no proper activity of its own.

Ryan does not want to accept the explicit meaning of Aquinas’ text from De Pot., q. 3, a. 11, ad 5: “An instrument is understood to be moved by a principal agent as long as it retains the power (virtutem) impressed by the principal agent. For this reason an arrow is moved by the projector as long as the force of the impulse of the projector remains. In this way also that which is generated is moved by the generator in regard to heavy and light things as long as it retains the form given to it by the generator. Whence also semen is understood to be moved by the soul of the generator as long as the power (virtus) impressed by the soul remains there, although it be physically divided. It is necessary, however, for the mover and the moved to be together (simul) in regard to the beginning of motion, not, however, in regard to the whole motion, as appears in projectiles.”

The distinction Ryan points to between an instrument’s instrumental activity and its proper activity is brought forward by Aquinas in a discussion concerning instrumental efficient causality. But I argued in the book that the form of a heavy body is not the mover’s instrument as an efficient cause; the form is not a mover. Rather, it exercises instrumental formal causality, and so Aquinas’ point in the text Ryan cites need not apply. The proper effect of the form of the heavy body may be instrumental to the external efficient cause. Aquinas states that the form is not a mover, but that by which (quo) the body is moved. (De Ver., q. 22, a. 3, c.). He also states that “an instrument is compared to the action more as that by which (quo) it is performed than as that which performs it.” (De Ver., q. 27, a. 4, ad 8.)

Furthermore, motion towards the element’s natural place is not actually the most proper effect of the form; rather, the truly proper effect of the form is rest in the natural place. (As Ryan himself says, motion is not an end in itself.) The elemental body only undergoes motion towards its natural place because it is out of its natural place. The form is not responsible for the body being out of its proper place, and thus not fully responsible for its motion towards its proper place.

It is also worth pointing out that whether or not we use the word “instrument” to describe the internal form by which the external mover moves the patient is irrelevant, as long as we accurately understand the picture Aquinas is painting. Nothing Ryan has said casts any doubt on my claim that the mover need not be in contact with the body undergoing motion throughout the entirety of the motion, and that the mover principle should be translated as “everything in motion is put in motion by another.”

In regards to the second chapter where I discuss Aquinas’ case against infinite regresses, Ryan states: “Whether Aquinas had intended this denial [of infinite regresses] to pertain to a moved mover’s being moved or its being a mover is not clear. Shields, however, opts for being a mover and therefore renders the proof from motion a more restricted version of the second way (p. 47). One might question this though.” I do not understand what point Ryan is making in the first sentence, but the second sentence requires a clarification: I claim on p. 47 that the premise in the first way that an infinite regress of movers is impossible is a more restricted version of the premise in the second way that an infinite regress of efficient causes is impossible. I make no claims about whether or not the first way as a whole is a more restricted version of the second way as a whole.

Ryan objects to my claim that in essentially ordered series of causes the secondary causes fail to explain their effect. This is quite strange, as it is standard fare among Thomists. He says I must argue “either that (i) per se causes cannot be essentially ordered, or (ii) that if they can, that they fail to explain their effect.” I have no desire to argue for (i), nor does anything in my text suggest I do. The second is not controversial among Thomists, and I provide an argument in the book for non-Thomists. But since Ryan’s concern is with my interpretation of Aquinas, I will provide an example here from Thomistic physical theory: a child sheep is a per se effect of a parent sheep, but on Aquinas’ view the parent sheep fails to explain the child sheep. The sun is also necessary to explain the effect. But the sheep is also a per se effect of the sun, not an accidental effect. Furthermore, the sun and the parent sheep together fail to explain the child sheep, according to Aquinas. The angel that moves the sun is also necessary to explain the child sheep. But the child sheep is a per se effect of the angel, not an accidental effect. Finally, the angel is not sufficient to explain the effect. God is needed, and the child sheep is a per se effect of God. Thus we have an essentially ordered series of causes, each of which is a per se cause, and none of the secondary causes explain their effect, despite being per se causes.

I thank Ryan for taking the time to review my book.

At the end of my book Nature and Nature’s God I point out that the history of science provides inductive confirmation of the philosophical truth that motion cannot continue in any meaningful sense on its own without beginning and end. I point to the repeated process in which new developments in sense were taken by some to provide support for beginningless and endless motion, only for further developments in science to clarify that this was impossible. There are many pairs of developments in which this is exemplified. In the book I discuss inertia and the subsequent analysis of collisions as less than perfectly elastic; the conservation of energy and the discovery of the second law of thermodynamics; statistical mechanics and the radiation of energy as photons that then undergo cosmological redshift. Big bang cosmology is another episode, but I did not note in the book another aspect of its development that repeats this pattern.

Einstein was initially excited that general relativity would show that the universe exists in a steady state, without beginning or end. But in his debates with De Sitter it soon became clear that general relativity won’t allow for a beginningless and endless universe. At best the universe can be in an extremely unstable equilibrium; even quantum fluctuations will push it out of equilibrium, and a gravitational runaway effect will cause the universe to eventually collapse or expand without end, diluting its contents into effectively nothing. The fact that GR requires a universe with a beginning is helpful to note. I skipped over this and went right to inflationary cosmology and the BVG theorem.

It is very illuminating to study the history of science and to note this pattern. Those who hope to get a beginningless universe (without divine sustaining action) out of the developments of science have been continually disappointed. They are chasing after a pie in the sky: a perpetual motion machine.

My paper “Formal and Proper: Substantial Form and Essential Accidents in Thomas Aquinas” has been accepted by the American Catholic Philosophical Quarterly. I take the same position about formal causality and essential accidents/properties as David Oderberg does. He does not make a historical case for the position, but I show that our shared view was also that of St. Thomas Aquinas. The abstract is below. If you would like a copy, please email me.

Abstract: Aquinas, following Aristotle, distinguishes between substances and their accidents, that is, between things and their attributes. He also distinguishes between proper accidents (also known as properties in Scholastic terminology) and accidental accidents, that is, between accidents that belong to a substance in virtue of what it is, and accidents that belong to it due to extrinsic factors. Aquinas says that a thing’s proper accidents are caused by the thing’s own principles. John Wippel interprets Aquinas as holding that a substance efficiently causes its own proper accidents. I argue that Aquinas is more plausibly read as holding that a substance formally causes its own proper accidents. Formal causality extends to more than just the informing of matter. Formal causality is a principle of determination: being a certain substance involves the determination of having certain proper accidents.

Two reviews of my book, Nature and Nature’s God, recently appeared, one by Gaven Kerr and the other by Glenn B. Siniscalchi. Siniscalchi (writing in the Journal of Theological Studies) appreciates my approach of bringing Thomistic philosophy together with modern science:

Shields’s nuanced interpretations of Aquinas’s first proof is a bold step forward in contemporary Thomistic studies. It strengthens the reasonableness of classical theistic approaches to the God question. Although Aquinas and the scholastic theologians have been accused of presupposing a dubious scientific view of the world, Shields subtly argues that modern scientific paradigms can enhance the soundness of the medieval proofs. Nature and Nature’s God is highly recommended for graduates students, fundamental theologians, and philosophers with a comprehensive background in Thomistic philosophy and modern physics.

I am grateful for Siniscalchi’s kind words.

I am also grateful for Gaven Kerr’s review in the Heythrop Journal and his kind words about my work. He says that my “book is a worthy addition to the library of any individual who takes the thought of St Thomas Aquinas seriously or who seeks to think seriously about the existence of God.” I am encouraged that Kerr agrees with my claim that essentially ordered causal series need not be simultaneous. He does put forward two criticisms of my claim that the First Way is a natural philosophical proof, rather than a metaphysical one. I wish to respond to these here in a spirit of friendly debate.

Kerr’s first criticism is that I did not address the texts in which, as he and others such as Knasas claim, Aquinas restricts demonstrations of God’s existence to the science of metaphysics. That is not completely true, but I only addressed them briefly in one footnote in the introduction. Here is what I say in footnote 17 on p. 5-6, with the most telling texts now bolded:

This understanding [that the motion proof is natural philosophical, preceding and paving the way for metaphysics] is shared by Fr. Benedict Ashley, O.P., The Way toward Wisdom: An Interdisciplinary and Intercultural Introduction to Metaphysics (Notre Dame: University of Notre Dame Press, 2006). Mark Johnson also shares a similar understanding: “Immateriality and the Domain of Thomistic Natural Philosophy,” The Modern Schoolman 67, no. 4 (1990): 285–304. The primary opponent of the natural philosophical reading of Aquinas’ motion proofs is John F. X. Knasas; see Thomistic Existentialism and Cosmological Reasoning (Washington, DC: The Catholic University of America Press, 2019), ch. 6. Knasas consistently fails to provide a plausible reading of a key text put forward by Johnson (In De Trin., q. 5, a. 2, obj. 3 & ad 3.) The celestial spheres fall squarely within the consideration of natural philosophy, not at its boundary as non-natural principles of natural motion, as Knasas would have it. Moreover, since Physics VIII explicitly proves the existence of a “wholly incorporeal” being and ends with God (In VIII Phys., l. 21, n. 1141 & l. 23, n. 1172), Knasas is forced to read Physics VIII, implausibly, as belonging for Aquinas to the science of metaphysics. Aquinas himself, however, explicitly includes it within natural philosophy (see, e.g., In III Phys., l. 1, n. 275, In VII Phys., l. 1, n. 884; In I Phys., l. 1, n. 1–4; l. 2, n. 12.) Knasas’ own favored texts—which do clearly assign the understanding of immaterial beings to the science of metaphysics—do not count against the natural philosophy reading of Aquinas’ motion proofs. Natural philosophy can only just reach the existence of at least one immaterial being, and it can know very little about it. A true understanding and developed science of God and angels belongs exclusively to metaphysics, and goes far beyond anything found even in Aristotle’s Metaphysics, as Aquinas indicates in In III De An., l. 12, n. 785 (cited by Knasas, Thomistic Existentialism, 182–83.) In speaking of such a science, Aquinas seems to have in mind something like what is found in Proclus’ Elements of Theology, or the Book of Causes. Furthermore, as I explain in this book, metaphysics has its own, alternative demonstrations for God’s existence, and these have a more robust conclusion than the motion proofs, but are correspondingly less accessible and manifest. Examples are the Fourth Way in ST and the proof from the De Ente, which I favor highly.

Given the importance of this issue, perhaps it would have been better to devote a section in the body of the text to developing these points. I will try to make the case briefly here. The central text of Aristotelian natural philosophy is Aristotle’s Physics. Book VIII of the Physics is devoted to a proof of the existence of a first, unmoved mover. At the very end of his commentary on Physics VIII Aquinas writes (my translation):

And so it is manifest that the first mover is indivisible and that it has no parts, as a point is indivisible, and also that it is entirely without magnitude, as if existing outside the genus of magnitudes. And thus the Philosopher terminates the common consideration of natural things in the first principle of the entire natural world, who is God over all, blessed forever. Amen. (In VIII Phys., l. 23, n. 1172 [9])

Thus we see that natural philosophy does contain a proof for God’s existence, and that it is not only the science of metaphysics that demonstrates God’s existence.

What then do we make of those texts which Kerr refers to, which seem to reserve the science about God to metaphysics? The answer is clear from Aquinas In De Trin., q. 5, a. 2, obj. 3 & ad 3:

Objection 3: The first mover is treated of in natural science [natural philosophy], as is clear in Physics VIII. But he is free of all matter. Therefore natural science [natural philosophy] is not only about those things which exist in matter.

Response to objection 3: The first mover is not treated of in natural science as the subject [of the science] or as part of the subject, but as the terminus at which natural science arrives. The terminus, however, is not of the [same] nature [as] the thing whose terminus it is, but rather has some relationship to that thing, as the terminus of a line is not a line but has some relationship to it. Just so the first mover also is of another nature than natural things, but nevertheless has some relationship to them insofar as it pours motion into them. And in this way it falls under the consideration of the natural [philosopher], not, namely, in itself but insofar as it is mover.

There is a difference between a science considering something as the object of its study, and a science considering something tangentially, that is, considering it as having an important effect on the object of study. For human reason to make God an object of study, without the aid of divine revelation, i.e., for reason to develop a science of the divine, it must use the full resources of human reason; it must undertake the study of metaphysics. Thus the science of the divine is part of metaphysics. But that does not preclude natural philosophy, whose object of study is not God but the natural world, from proving the existence of God as the source of the natural world. But natural philosophy can say very little about God. To say much, metaphysics is needed.

Kerr’s second criticism is in response to my claim that the First Way would not be called “the more manifest way” by Aquinas if it was a metaphysical argument based on the act of existence. He says that the same point could be made about the argument as I present it: my book is 328 pages long, and I have to say quite a lot to explicate and defend my understanding of the proof. “More manifest” only means that the proof’s point of departure, motion, is something readily accessible to everyone.

But this is to conflate the simplicity of an argument with the simplicity–or lack thereof–of the objections against it and the replies to those objections. I can explain the natural philosophical argument in short order, in readily accessible terms. Anyone can grasp it. But there is a long history of mistaken interpretations and sophisticated objections to this simple argument, and the length of the book is due to the necessity of responding to and untangling these. By contrast, the metaphysical distinction of essence and existence, as well as the necessity for an efficient cause of the act of existence of things, upon which Kerr bases his interpretation of Aquinas’ First Way, are difficult concepts and require subtle insight. Thus it is the argument itself, as opposed to just the objections and responses, that is less manifest on his interpretation.

Kerr has made many important contributions to philosophy through his analysis of the proof for God’s existence in Aquinas’ De Ente et Essentia, based on the real distinction between essence and existence, as well as his analyses of infinite regress. The view that Aquinas has other unrelated arguments for God’s existence does not detract from the value of that work. Once again, I am grateful for his kind words about my book, including his helpful criticisms.

In his summary of my book Nature and Nature’s God for the Review of Metaphysics, Caleb Estep writes that I hold that in infinite time all possibilities will be realized. This is not quite what I claim. As I explain in the book, what I hold is that all possibilities that approach arbitrarily close to 1 over time will be realized. In infinite time even something with a small probability of occurring, in any given finite duration, may become nearly certain to occur. This will be the case unless the probability of its occurring diminishes continually such that the sum-total of its probability over infinite time (technically the integral of its probability density from t=zero to t=infinity) is still significantly less than one.

For example, if there is a 1/2 probability of getting heads on a first coin flip, then there is only a 1/4 chance of not getting heads at all on two flips. After three flips there is only a 1/8 chance of not getting heads at all, and so on. However, in some cases the odds decrease so rapidly that there is very little chance of ever getting the desired result. For example, if there is a 1/100 chance of getting result X on the first go, and then a 1/10,000 chance of getting it on the next go, and then a 1/1,000,000 chance of getting it on the third go, and so on, then the odds are against ever getting the result.

We would colloquially say about a composite substance that it is “actually water, not potentially water,” yet it contains within itself both the act of water (the substantial form) and the potency for water (prime matter). The prime matter has not lost the potency for being water by having the form, just as I have not lost the power/potency (potentia) to see when I am actually seeing. When I am not asleep I both potentially see and actually see. When I am asleep I only potentially see.

In book 1 of the Physics, (see chapters 7-9, especially 9) Aristotle is at pains to distinguish matter’s being per se a potential principle from its having per accidens the privation of the form. The matter loses its privation of water-form when it changes into water, but it does not lose its potency to be water. Objective potency always involves a privation, involves not being in some condition. In some cases it is merely a privation, as in the objective potency of the universe to exist “before” God makes it out of nothing. In most cases it is a underlying principle together with a privation.

Aquinas’ statement in the First Way that a thing can’t both be in potency and act at the same time and in the same respect concerns objective potency. Aristotle’s definition of motion (the act of the potential insofar as it is potential) has reference to what Aristotle calls “imperfect potency” as opposed to “perfect potency” (Physics III.2; the distinction is elucidated in the Metaphysics and De Anima). The imperfect potency involved in motion implies privation; that is why it is called imperfect. The power of sight is an example of perfect potency. Hence the context of motion explains Aquinas’ statement. The potency to undergo a change is always an objective potency, because a thing can’t undergo a change to a condition once it is in that condition. But it must retain the subjective potency to be in that condition once it has changed to that condition, otherwise it could not be in that condition. The potency required for motion is a potency joined to a privation.

Prime matter is a subjective potency for substantial forms, and it retains the potency for a certain form when it possesses that form, for it is such a potency essentially, and not accidentally. So too essence is a subjective potency for the act of existence; it retains the potency for that existence when it exists. In Thomism there is a real composition of potency and act.

In his reply to Graham Oppy’s criticisms of Thomistic cosmological arguments, Ed Feser denies that creatures exist both actually and potentially at the same time. (“Oppy on Thomistic Cosmological Arguments,” Religious Studies 57 [2021], 503–522, at 509.) He states that the constituents of an object such as a cup of water are potentially a cup of water in the abstract, but in the concrete they are only actually a cup of water. The constituents in question, protons, neutrons, and electrons, are, considered in themselves, potentially a cup of water, but as possessing the form of water they are actually a cup of water and no longer potentially a cup of water.

This doesn’t seem right to me. The real distinction of essence and existence makes Aquinas’ position clear. Without existence an essence is precisely nothing, not any kind of potency at all. But in an actually existing finite being the essence is really distinct from its existence, and is related to its existence as potency to act, as Aquinas argues in De Ente et Essentia. Now the essence is in potency to existence, and not in potency to anything else at all. It is in potency to only one kind of substantial existence, and it is not in potency to non-existence. So either the essence has no potency at all, or it bears the potency to exist when it is actually existing through its composition with the act of existence. In other words, essence is what later scholastics called a subjective potency (the kind of potency that is composed with its act and thus coexists with it) rather than an objective potency (the kind of potency that is opposed to act and ceases to be when its act arrives.) Note that “subjective” does not here mean “relative to someone’s perception” but rather “subject, that is, receiver of act.” “Objective” does not mean “real,” but something like “the objective of a productive activity.”

The same must be true of the relationship between prime matter and substantial form. Certainly the case is less clear here: one could be tempted to think that prime matter, as a principle of potency, was only a potency to receive other substantial forms. But this is not the case. Prime matter’s potency does not undergo any change (see Aristotle, Physics I.9, 192a26–29); it is always a potency for all substantial forms. So when it is actualized by the substantial form of water, it retains the subjective potency to the substantial form of water, as well as potencies for all other substantial forms. Water is a composite of potency and act.

In teaching Aristotle and Thomas one often encounters the word “art.” This is generally translating techne in Greek or ars in Latin. I always remark to students that this word is being used more broadly than the way in which the English word “art” is commonly used. It does not just mean the fine arts such as painting, sculpture, music, or poetry, although it does include the latter. The word “art” as used by Thomas and Aristotle should be taken in connection with the English words “artificial,” “artifact,” and “artisan.”

While I think this is true, I’ve realized lately that more must be said; in a way, but only in a way, the word techne is opposed to the word “art” used in reference to the fine arts. For techne or ars means a know-how, a knowledge of how to produce or do something, and it characteristically gives a person the ability to produce or do this thing over and over again. The carpenter, through the art of carpentry, can make many tables with the same characteristics.

But although know-how is certainly present and active in the case of the fine arts, what characterizes them is a kind of moment of genius that is not reproducible or entirely under the control of the artist. Depending on the level of genius, an artist may be able to make many masterpieces, or just one in a lifetime. But in neither case is there a formula or process that can be followed to automatically guarantee the production of a new masterpiece.

What word do the Greeks have for this? One’s first thought is poiesis, the root word for the English word “poetry,” but this can’t be right. Aristotle uses poiein as one of his ten categories, and gives as examples “to lance” and “to cauterize,” definitely not specifically poetic productions. Poiein in philosophical Greek just means “to make or do.” The word one is looking for is mousike, an art in the sense of something metaphorically inspired by the muses. This is what we call “fine art” in English, something that may make use of techne/technique, but which is a non-reproducible product of genius.

My book Nature and Nature’s God is on sale until the end of the year. If you order from the CUA Press website and use the discount code CREL23 you will receive 35% off and free domestic shipping.

Last week Alex Pruss, Rob Koons, and Ed Houser graciously joined me for a panel discussion of my book Nature and Nature’s God. I enjoyed everyone’s comments and objections, and found the event productive. What follows is the precis I delivered to start the discussion.

In my book Nature and Nature’s God I try to do two things: to set the historical record straight about Aquinas’ unmoved mover argument, and to defend a contemporary version of the argument in light of the developments of empirical science. Over the course of two decades I have felt that most presentations of Aquinas’ First Way have read much into the argument that just isn’t in Aquinas’ texts, and at the same time have drawn too much conclusion out of too little argumentation. Although St. Thomas certainly isn’t infallible, he was a brilliant man, and I take him seriously enough to think that understanding him on his own terms is worth my time.

My project on Aquinas’ First Way began when I learned something interesting about his philosophy of nature from Fr. Weisheipl, namely, that on his view heavy bodies fall of their own accord, with nothing impelling them downwards. I knew that Aquinas builds his First Way around the premise that everything in motion is moved by something else, which I thought meant that everything in motion must be currently impelled by some mover. If that mover was in motion, it too was currently impelled by some other mover, until one gets to an unmoved mover that impels them all: God. I found the argument, understood this way, unconvincing. But now it was obvious that this account of the First Way did not square with Aquinas’ own understanding of natural motion. Perhaps a much more interesting unmoved mover argument was hiding underneath a conventional interpretive cover.

Through a careful reading of Aquinas’ more elaborate presentation of the case for an unmoved mover in the Summa contra Gentiles, as well as books seven and eight of Aristotle’s Physics and Aquinas’ commentary on those books, an exciting physical as opposed to metaphysical argument for God’s existence emerged. Aquinas’ argument is clearly rooted in Aristotle’s, but Aristotle’s argument is quite foreign to us. We are used to cosmological arguments that first try to prove that the universe has a beginning, and then that God must exist to get it started. But Aristotle begins his whole argument by attempting to prove that the universe cannot possibly have a beginning. That proposition then becomes the key premise on which his entire argument for God’s existence is based: only an infinite power could cause a motion of infinite duration, and no physical being can have infinite power. Thus an immaterial being exists.

Aristotle’s argument proved troublesome for Jewish, Christian, and Muslim Aristotelians. It is a central tenet of traditional western monotheism that the universe had a beginning in time, a moment of creation. But the culminating piece of Aristotle’s philosophical corpus was built around a denial of this point of religious doctrine. Moses Maimonides solved the problem, and Aquinas adopts his solution in the Summa contra Gentiles. Aristotle’s proof is placed inside a disjunctive wrapper: although we can’t prove that the universe has a beginning, clearly it either did or it didn’t. If it did, then since nothing brings itself into being God must exist to get the universe started. If it didn’t have a beginning, then it has been in motion for an infinite duration, and then Aristotle’s argument applies: God must exist to sustain the universe forever in motion.

But why must a motion of infinite duration be sustained by an immaterial mover? Motion can either flow from a thing’s nature, or be imposed by an external agent. In Aquinas’ view motion that flows from a thing’s nature is directed, as to an end, to the thing’s natural place and condition. When the body reaches its natural place it naturally comes to rest and does not undergo any further motion unless acted on by an external agent. Hence natural motion is of finite duration. But external agents can only act insofar as they are moved or energized by the motion of a heavenly body. Otherwise the would-be agent and would-be patient simply undergo motion to their natural conditions and rest in equilibrium with each other. Only a voluntary, rational agent can continue to move other things in a cycle indefinitely, and such an agent, if physical, will be of finite power and wear down over time.

According to St. Thomas, all things on earth would move towards their natural place, rest there, cease to act on one another, and remain forever motionless if it were not for the fact that the cyclical motions of the heavenly bodies continued to affect terrestrial things and change them out of their natural conditions, stirring the pot so to speak. But the motions of the heavenly bodies cannot themselves flow from the nature of the heavenly bodies, since such motion is cyclical and does not head towards a natural place of rest. Thus the heavenly bodies themselves are moved by one or more immaterial agents. Since their motions are choreographed to maintain the natural cycles and teleological harmony here on earth, they must ultimately be directed by one single, overarching immaterial being who is directed by no other. This is God.

But if this is Aquinas’ argument, of what interest is it to us? We surely do not believe in the harmony of the spheres, celestial influences, or angels moving planets around. We also believe in inertia, indefinite “natural” motion that never comes to rest, and the conservation of energy as a physical law. Yet a closer look at modern physics reveals that Aquinas’ proof, in its essentials, is just as valid as ever. Allow me to explain.

Isaac Newton, the father of modern physics, stated that “The force of inertia is a passive principle by which bodies persist in their motion or rest. . . . By this principle alone there never could have been any motion in the world. Some other principle was necessary for putting bodies into motion; and now [that] they are in motion, some other principle is necessary for conserving the motion. . . . By reason of the tenacity of fluids, and attrition of their parts, and the weakness of elasticity in solids, motion is much more apt to be lost than got, and is always upon the decay.” No collision is perfectly elastic, and so bodies rebound with less total motion than they had before. Also, the viscosity of fluids and the friction of surfaces diminishes the motion of bodies moving through them or over them.

But the first law of thermodynamics states that no physical process can increase or decrease the amount of energy in a closed system. The energy of motion (“kinetic energy”) lost in collisions and through friction is converted to the energy of heat. Heat, in turn, can be converted back into kinetic energy by various processes, for example in steam locomotives. It might seem, then, that Aquinas’ argument cannot work: no immaterial mover is needed to sustain motion in the universe forever; the energy conservation law guarantees perpetual motion.

Yet scientists are agreed that a perpetual motion machine cannot exist. Why? Because the first law of thermodynamics is complemented by the second law of thermodynamics, according to which the energy in a closed system can only decrease in its availability, and never increase. “Entropy” is a measure of the degree to which energy has become unavailable to do work, and thus the second law is often expressed thus: the entropy of a closed system can only increase, and never decrease. As energy goes through various changes, it tends to find its way into the form of heat, and thermal energy can only be converted back into other forms of energy if it exists in two bodies at different temperatures. But heat tends to flow directly from a hotter body to a cooler body without doing any work, and thus more and more of a system’s energy tends to get locked away into the unusable form of thermal energy at equilibrium. In a sense Aquinas was right: all bodies head towards the natural condition of equilibrium, and once they reach it they never change again unless acted on by an external agent.

Rudolf Clausius, the pioneer of thermodynamics who coined the term entropy, applied the two laws of thermodynamics to the universe at large and expressed them thus: “1. The energy of the universe is constant. 2. The entropy of the universe tends to a maximum.” Modern physics and cosmology conclude that the universe must inevitably head towards the equilibrium condition known as the “heat death,” in which all energy is dissipated about the universe at even temperature, and nothing interesting ever happens again.

All bodies radiate their energy away in the form of photons; they shine at a frequency proportional to their temperature. This is why night-vision goggles work. But many of these photons inevitably escape into space where they undergo a cosmological redshift as the universe expands. As each photon’s frequency drops, the amount of energy it can deliver upon impact with a body diminishes. Thus energy is spread thinner and thinner throughout space, entropy rises, and the universe gets ever closer to the heat death.

If the universe had no beginning and had already existed for an infinite time, it would already have reached heat death. The fact that it has not indicates that the universe had a beginning a finite duration in the past. Alternatively—and this is where Aquinas’ argument differs from that the kalam argument—the universe has existed forever but God has either continuously or periodically converted some of its high-entropy, unavailable energy back into usable energy. In that way it could still exist out of equilibrium as it does today, without having a beginning in time.

Thus Aquinas’ unmoved mover argument is still valid, and serves as a welcome, more accessible addition to his other arguments, particularly to the argument based on the act of existence in De Ente et Essentia. The De Ente argument is a powerful one, but very subtle and not very accessible to the empirically minded. Aquinas’ more physical unmoved mover argument is a good first step for such people.