Daniel Shields. Nature and Nature’s God: A Philosophical and Scientific Defense of Aquinas’s Unmoved Mover Argument. Washington, D.C.: The Catholic University of America, 2023. Hardback, 305pp; $75.

Reviewed by John G. Brungardt

While book reviews are typically featured elsewhere, the length of this review places it in our “Essay” category.

Introduction

More philosophy students should be troubled by their teachers telling them that, for Hume, cutting-edge chemistry was still gasping for life in invisible clouds of phlogiston. Does this render irrelevant, limit, undercut, or of itself disprove Hume’s arguments against our prospects of discovering “the secret powers” of material things? If not, then should St. Thomas Aquinas’s natural philosophy—or that of any other historical figure—be subject to special scrutiny due to its historical conjunction with superannuated science while Hume’s causal skepticism about the course of nature is not? We insist upon the natural philosophical transparency of our own day’s scientific image at our peril.

St. Thomas Aquinas’s “First Way” to prove the existence of God from motion is without doubt the most controverted of his famous Five. Composed of less than 250 words, this “manifestior via” has inspired commentaries, explanations, responses, and attempted refutations thousands of times its length. What is manifest is that Daniel Shields has written an excellent and compelling account of the historical meaning and perennial relevance of Aquinas’s proof of the existence of God from the reality of motion. Currently, it is one of a kind, yet hopefully only the first of its kind.

The challenges faced by a contemporary defender of the First Way include those both historical and contemporary. They are historical, since modernizing commentaries might have obscured the original sense of Aquinas’s argument. Indeed, Shields argues in the first, historical part of his monograph, the First Way has been subjected to a variety of interpretive hedges due to its association with false physics. The challenges are contemporary, because, even should an interpreter recover a perennially relevant philosophical sense out of the First Way, it is not immediately clear how to wed such an interpretation with the contemporary natural sciences. Indeed, perhaps the attempts to establish such a harmony in turn influenced the very interpretation of the First Way, attempting to meet certain assumptions—real or perceived, scientific or philosophic—about modern physics.

Shields navigates these two challenges admirably. The stakes are high, especially in view of contemporary accounts of “natural theology” that insist it be a revealed “theology of nature” for fear that the Thomistic version of the discipline provides only “henotheistic” results, arriving at just one “god” among others (see McGrath, A Fine-Tuned Universe, 62–63), or in view of some Thomistic interpretations that transpose the argument into existentialist terms. Must the First Way be rescued even from its would-be defenders? In what follows, I review the book’s content and principal claims. Then, I raise some more particular points of constructive criticism by way of “vehement agreement.”

Book Overview

The book is divided into two parts. Part I, “Aquinas’s First Way in Its Historical Context,” provides an interpretation of the First Way that hews closely to that context and the wealth of parallel and supporting passages from the corpus Thomisticum. Part II argues that, when the context is updated by the current standards of the natural sciences, St. Thomas’s argument is sound.

A More Adequate Historical Interpretation?

The first part of the book is, to my knowledge, the most accurate and detailed historical interpretation of St. Thomas’s First Way in print. More specifically, Shields rearticulates a defense of the infamous “mover principle”—omne quod movetur ab alio movetur—in Chapter 1, while Chapter 2 defends the second premise of the First Way, that a series of moved movers cannot be infinite. Having defended the premises of the First Way, Shields argues it proves little—a minimal notion of a first mover or god—in Chapter 3. In order for the motion proof as contained in the First Way to prove adequately the existence of God it must be completed. This is done in two ways: first, in Chapter 4, by an argument that there must be a supra-cosmic first mover of a potentially eternal motion; second, in Chapter 5, by the argument of the Fifth Way. This “completion thesis,” among all others in Shields’s book, is its most unique interpretive feature, but, as we shall see, not wholly de novo in the Thomistic tradition.

Chapter 1, “The Mover Principle,” is a tour de force. Shields examines the motor causality principle (omne quod movetur ab alio movetur), translated as “everything in motion is moved by something else.” He (correctly) defends the middle voice reading of the first movetur and, for the sake of his own natural philosophical account, renders the second movetur in the simple passive present as opposed to the stronger “everything in motion is being moved by something else.” In the small morass of literature debating the translation of this line, it is helpful to have this stated clearly. After all, even Newton—who clearly did not intend an ontologically passive meaning—used verbs such as gyrare with a middle-voice sense but in a grammatically passive form. The philosophical argument Shields makes to defend the motor causality principle is focused around the inductive Aristotelian argument (based on Physics, VIII.4). This has the advantages, on the one hand, of attending more closely to common experience while avoiding more complicated textual interpretations and, on the other hand, of preparing for more contemporary inductive arguments. After this successful review of the inductive argument, Shields replies to various objections, such as those raised by Scotus and others.

Chapter 2, “The Impossibility of Infinite Regress,” explains the First Way’s second premise, that there cannot be an infinite series of moved movers. Primarily, this involves Shields in a nuanced clarification of what an essentially ordered causal series is, first in responses to objections raised by the likes of Flew, Kenny, Williams, or Hume, and second from conflations of Aquinas’s view with Scotus’s. Principally, Shields shows that Aquinas does not think that such a series must be temporally simultaneous (63ff). Shields carefully shows the textual basis for Aquinas’s view that causal contact does not involve continuous physical contact and that, even if agent causes are arrayed through history in an accidentally ordered series, such a series still requires a cause essentially responsible for it: “An infinite, successive, accidentally ordered causal series must be sustained by an enduring, finite, essentially ordered causal series” (80). Here, Shields is preparing the ground for discussions not only of inertial motion but also universe-wide entropic decay to thermal equilibrium.

Chapter 3, “The First Way: A Modest Interpretation,” Shields argues that the First Way as stated in ST “proves God’s existence, but only in a very minimal sense” (84). This means “that proving His existence in a robust sense requires extending the argument with the Third Way, the Fifth Way, and G2” (ibid.), where “G2” refers to the second defense of the motion proof in Summa contra Gentiles, nn. 17ff. These “completions” of the First Way are the topics of Chapters 4 and 5. What, then, does the First Way prove merely on its own? Shields contends against the interpretations of Garrigou-Lagrange, Owens, Knasas, and Feser that the proof remains solidly within the realm of cause of motion and that it is not correct to transpose the proof into the key of “cause of existence.” Nor are continuity and simultaneity of physical contact involved (as some maintain), nor (other interpreters such as Kerr or Wippel, among many others) does the proof of itself establish that the First Mover is pure act. In Shields’s view, the First Way is not sufficient without supplementation to show that the first mover is pure act or the first being (96–100). This chapter also contains an extensive exposition of Aquinas’s texts on the counterfactual case of what would happen if God stopped moving the universe (89–96), all to anticipate the need to defuse the view that “an unmoved mover is needed to keep everything in motion every instant” lest it “freeze instantly in place” (96). The success of the First Way is modest: some first mover exists “that is not moved by anything else and not itself in motion,” (102) but this does not exclude the possibility of that mover possessing the subjective potency of bodiliness, for instance. Hence the need for supplementation. The notion of the divine by which the intellect can contemplate God is minimal, enough to get the natural philosopher to the natural theological door: “The First Way can conclude to the existence of God by means of an implicit disjunction: one or more primary—in the sense of noninstrumental—movers exist. If any primary mover is not subject to any higher mover it would be a god. If it is moved by a higher mover, then the higher mover would be a god. Either way, one or more gods exist” (112).

Chapter 4 provides the completion of the First Way through the extended motion proof in the Summa contra Gentiles (a.k.a. G2), where the aim is to specify the notion of “god” to a first mover, moved in no way, not even accidentally, and wholly separate from bodily potency. That is, the First Way in the Summa Theologiae shows that there are first movers; the First Way (extended) in the Contra Gentiles shows of what sort the ultimate first mover is. This route is doubly disjunctive: either the first mover is an ensouled first mover or some supra-cosmic first mover, and motion in the universe is either temporally past-finite or temporally past-infinite. However, whichever pair of disjuncts is chosen, one must eventually reach a supra-cosmic first mover, viz., God. As such, this chapter is an exposition of an argument in the Contra Gentiles in which Aquinas is summarizing an even more extensive argument from Aristotle’s Physics, Book VIII. Through all of this discussion, the heavy engagement with Aquinas’s historical philosophy of nature and cosmology does prompt one to apt wonder about its replacement later in the book. On the flip side, “manifestior via” might appear more and more as an “immanifestior via.” Indeed, at the end of the chapter, in order to show that any intra-cosmic ensouled movers are still dependent upon a supra-cosmic mover, one must appeal to that mover as the object of desire:

The mover of a sphere is drawn by an end outside itself, something that it lacks or desires. The end that it seeks to achieve cannot be a benefit that it has freely and gratuitously chosen to bestow on its own initiative, because that end is beyond its power. The end is, rather, ordained by a higher beig whose power embraces the whole, and the immaterial mover of the sphere moves out of love for this higher being, desirig to participate in its life by fulfilling its generous plan. It is, therefore, moved metaphorically and is in a sense a moved mover. (136)

One may be concerned that it is incorrect to interpret this as a metaphorical “being moved” by God—this well-known interpretation of Aristotle’s discussion in Metaphysics XII has been criticized by Thomas De Koninck, for instance (see his “Aristotle on God as Thought Thinking Itself,” especially pp. 512–14). At any rate, on the one hand, this prong of the disjunction will be of less concern for most of those searching for the modernized details of the argument; on the other hand, for such sphere-souls to be movers by desire requires a discussion of finality, and so, Shields contends, the First Way cannot be completed without the Fifth Way.

This completion is the job of Chapter 5, “The Living God: Uniting the First and Fifth Ways.” It is the best chapter in the entire book and a real contribution to recent discussions of the Fifth Way (among which, the reader should recall the appendix to Fr. Andrew Younan’s book, reviewed here). Shields successfully intervenes—if not settles—a long-running debate over teleological reasoning in the Fifth Way between Marie George and Edward Feser, connects the immediate natural ends of individual things with their cosmological context found in the more extended “Fifth Way” types of reasoning in other Thomasian texts, and adds a discussion of the nature of life and intelligence to manifest the connection of the First and the Fifth Way. The “self-motion” or intelligent self-determination of the First Cause shows it is the deeper sense of first mover sought by the First Way. The chapter also anticipates discussions of entropy in the second part of the book and sketches out harmonizations with Darwinian evolution.

What emerges in the summary of the first part of Shields’s book is an astounding claim, one which I hope is true: “Together, the First Way, G2, and the Fifth Way constitute Aquinas’s natural philosophical path to God. This natural philosophical path is good and convincing in its historical context” (182, my emphases). That is, Shields would maintain that the philosopher of nature enjoys a much more commanding position at the outset of natural theology than is typically thought by not a few Thomists. (My own principal source of hesitancy is that it seems natural philosophy can discover that mind must be immaterial, but to treat of mind as such is the office of metaphysics; thus, at best, the Fifth Way would be reached by the natural philosopher in virtue of axioms already taken for granted by induction.) If Shields is right about this, then Aquinas’s philosophical way to God, as presented in his own day, has been frequently and severely misunderstood in our own.

Towards a Perennial Proof

Throughout the first part of the book, Shields hews closely to the details found in the natural philosophy of Aquinas’s own time. Even granting that the Completed First Way is an argument from natural philosophy, is it still true? In Part II, “Defending Aquinas’s Argument Today,” Chapter 6 parallels Chapter 1: Shields explores classical mechanics and provides his Thomistic account of inertia and gravity, since such principles might be thought to undermine the first premise of Aquinas’s argument. Chapter 7 parallels Chapter 4’s discussion of the eternity of motion and delves into the history of thermodynamics. Shields argues that the universe cannot be a closed system (in the thermodynamic sense), and that God is required to causally sustain its motion. Finally, in Chapter 8, Shields discusses the nature of life and provides a compelling—if brief—connection to information and cosmological fine tuning to parallel his equally compelling, albeit much more detailed, account of the Fifth Way. Before raising some points of detail, I would highlight the strengths of these chapters.

Chapter 6, “Classical Mechanics: Assessing the Motion Proof,” attempts to show the compatibility of Thomistic natural philosophy and some of the key ideas of classical mechanics, such as inertia, momentum, and gravity. The chapter begins by highlighting the potential historical connection of Aquinas’s argumentation: “Aquinas’s motion proof depends upon his view that motion cannot continue undiminished for an indefinite amount of time without an external agent” (185). He opens with a brief historical recapitulation of the fate of Aristotle’s distinction between the natural motion of heavy bodies and the violent motion of projectiles. The common principles behind both of these—unbeknownst to Aristotle, Aquinas, and even those ancients and medievals who rejected the Aristotelian account of projectile motion or the cosmological account of angelically-moved spheres—was inertia and gravitational force. Shields leans heavily into a defense of the reality of inertia against certain competing Thomistic interpretations, claiming that the reality of inertia is revealed to the senses (197; to which I return below). Yet the reality of inertia seems to threaten the motion proof: there is no need for a first unmoved mover to sustain a potentially sempiternal motion if all bodies move inertially. However, there is no conservation of an absolute quantity of motion among bodies in the cosmos: “Since no real-world bodies are perfectly elastic, some motion is lost in any collision” (206). As a result of this cosmic loss of the quantity of motion, Shields argues that Isaac Newton’s own defense of the necessity of God as a first mover (in Opticks, Query 31) corroborates Aquinas’s motion proof (207). Over a potentially endless sequence of motions in the universe, God is required to sustain a motion that would otherwise “wind down” (despite objections from Leibniz; 210–12). The chapter concludes with a sketch of a Thomistic theory of inertia and gravity (212–22; to which I return below). By decoupling Aquinas’s view, earlier in the book, from interpretations which require simultaneous and continuous causal contact of essentially subordinated moving causes, Shields has opened up the possibility that Aquinas’s historical position on these general principles of natural philosophy are compatible with an updated physics.

Chapter 7, “Thermodynamics and the Motion Proof,” reviews the development of classical thermodynamics, arguing that Aquinas’s motion proof is corroborated by its second law. The chapter opens with discussions of the development of the understanding in classical mechanics of energy (224–29) and entropy (230–42). Shields provides a very helpful, concise discussion for Thomists not familiar with the relevant history of physics. Analogous to the threat inertia poses to the motion proof, the law of the conservation of energy seems to undermine Aquinas’s empirical foundations. However, the second law comes to the rescue, for the energy available for work (the “free energy” of a system) ever decreases: “For this reason, no closed system can be self-sustaining, and if there is no God the universe is a closed system” (224). In other words, mimicking the second dilemma of the Extended First Way in the Contra Gentiles, a Thomistic natural philosophy with thermodynamics sees that the universe cannot be such a closed system. If the entropy of the universe tends to a maximum, then either the universe began a finite time ago or, in a universe with potentially sempiternal motion, a mover outside of the entire system of the universe is required to sustain it (242–43). (With some additional points, the disjunctive argument would work against Julian Barbour’s proposal of a bi-directional or “Janus-faced” past hypothesis just as well as it does against the unidirectional “past hypothesis” articulated by David Albert.) Shields closes the chapter by answering a slew of objections: that entropy is not valid when applied to the cosmos as a whole, that the cosmos might be infinite, or that entropy is merely a statistical principle and not a real, physical principle, and from certain very speculative ideas in contemporary cosmology such as eternal inflation.

Chapter 8, “Entropy, Information, and Fine-Tuning: One Living God,” concludes the series of corroborations which the modern and contemporary natural sciences provide to Aquinas’s motion proof by highlighting features of contemporary discussion as instances of the effects considered by the Fifth Way: that it is a property of living things to maintain a low-entropy condition which requires a physical analog of information processing as well as fine-tuning effects discussed by modern cosmology. Shields discusses contemporary speculations about life as completely auto-poetic systems or the possibility of a Maxwellian demon countering the second law. Still, while living things do not on balance act counter the second law, they do locally and intelligently order matter so as to sustain themselves: they maintain a nearly net-zero change in entropy. Shields argues that this physical analog of information processing as a form of order is a physical reflection of the order caused by intelligence as such (274). This section could have been helpfully supplemented by drawing on Feser’s extensive discussion of information in nature and its hylomorphic interpretation in Aristotle’s Revenge. Shields concludes the chapter with a brief review of some of the better-known, so-called “anthropic coincidences” or fine-tunings of physical constants such that the cosmos is bio- and intelligence-friendly.

The cumulative effect of Part II upon even a skeptical reader ought to be that modern science has not refuted Aquinas’s motion proof. Its opponents will have to dig deeper into natural philosophy if they wish to do so.

Critical Analysis and Questions

Now, I would join Shields in defending the First Way and would rather to hand the strongest possible defense. I would also join him—once I can fully understand it—in the claim that not only the conclusion of the First but also of the Fifth Way is the proper fruit of a natural philosophical habitus of mind. So, I propose three points of constructive criticism towards such ends. The first point is about Shields’s interpretation as an argument for God’s existence as such; the other two turn to some details of his accounting of natural philosophy—physical motion and natural place and the argument from Physics VII.1, nearly but not quite elided from the book.

Returning to Traditional Ways?

If nothing else, Shields’s book has eradicated any need to succumb to the metaphysical malaise afflicting too many interpretations of the motion proof. Thus, for instance, while I would follow Shields in his criticisms of an interpretation of the First Way as found in the work of John F. X. Knasas, I would hesitate to criticize as swiftly certain others whose specific interpretations of the First Way rely upon their own view of the natural philosophical details in question.

For instance, imagine someone not convinced that Shields is right to deny that the non-expanded First Way fails to lead to the conclusion that God is pure act. The Leonine text reads that the First Mover is “a nullo movetur” and one variant reads “nullo modo movetur”. Even Shields himself notes (127) that “all material agents are susceptible to change,” an inductive claim about subjective and not merely objective potency such that a bodily or material mover would itself be in motion in some way when causing change. That is, the First Way as it stands seems rather to imply—by relying upon the act-potency argument for the mover principle—that any movers susceptible to change will themselves fall under the dependency relationship highlighted in the first premise. The intensional scope of the First Way seems to suffice to lead mediately to the conclusion that the first mover is pure act. One reaches the ratio of “first mover qua in act” (a “first being,” in some way), even if not in all their intensional specificity. Granted, it is this specificity of “first mover moved in no way” that is brought to light by the further discussion of the First Way (in G2 or Physics VIII). Yet similar sorts of “incompleteness” objections could be raised against the other Ways, by the way—not all the details are spelled out in the text.

As a general point of agreement to this more minor criticism, Shields is right to return Thomistic interpretation of the proofs of God’s existence to the language and very idea of a path or way to the contemplation of this truth. The via causalitatis provides certain rationes for divine names, and these are grasped fully only by a contemplative discourse along the via resolutionis (see SBdT, q. 6, a. 1, c3). The unity of the ways and their causal rationes by which we speak of the existence of God are not immediately apparent to us. This has long been recognized by Thomists and should be more commonplace today. Consider Cajetan’s comments on this score, which I quote at length:

In the body [of the article] there is one conclusion, answering the question affirmatively, namely, God exists. And this is proposed for proof in five ways, about which [number] there is no need for ado, since they are found both here and in SCG I, QDP, QDV, and I Sent.
Concerning these arguments in common, it is to be noted diligently that they can lead to two results. In one way, to the conclusion that there is a being incorporeal, immaterial, eternal, most high, immutable, first, most perfect, etc., both what and how we maintain God to be. And thus these arguments meet with many disputes, because the first way, as is said in SCG I.13, does not lead to a mover more immobile than an intellectual soul; the second, however, as Averroes says, does not result in aught but a heavenly body and its mover; so too, the remaining [arguments] seem to lead to nothing higher. And for this purpose these reasons are not brought forward here, as will be evident below.
In another way [these arguments] are able to lead to the conclusion that certain of the aforementioned are to be found in the nature of things, which according to truth are proper to God, not attending to what way or how they are, etc. And [it is] for this purpose [that these arguments] are brought forward here—and [as such] there is practically no difficulty to be had [on their account] according to philosophy.
So as to better understand what we are saying, let us consider each [way] one by one. For the first way, on the part of motion, it is enough that it infers ‘therefore, there is a first immobile mover,’ not attending to whether that [mover] be the soul of a heaven or one of earth, for this is asked in the following question. So too, for the second way, on the part of the efficient cause, it is enough that it lead to a first efficient cause, not attending to whether it be a body or incorporeal, for this is asked in the following question. As for the third way, on the part of the necessary, it is enough that it lead to a first necessary [being] not from another, not attending to whether it be one or many, for this is asked in Question 11. So too, for the fourth way, from the grades of things, it is enough for it to lead to a maximum being, true, good, and noble from which all [all things] are participations. And similarly with the fifth way, from governance, it is enough for it to lead to a first governing intellect, whatsoever it be. For all these predicates—namely, an immobile mover, a first efficent [cause], a necessary [being] not from another, a maximum being, and first intelligent governor—are according to truth proper to God. Therefore, by concluding that these are found in the nature of things, one concludes directly [yet] by accident, as it were, that God exists—that is, God, not as God, but as having such a condition, exists—and consequently the very substratum, namely God as God, exists. (Cajetan, In ST, Ia, q. 2, a. 3; Leon.4:32)

Such thinking is endorsed by John of St. Thomas (see Cursus Theologicus, Vìves ed., t. 1, q. 2, n. 1, pp. 537–38), echoed briefly by others (such as Charles De Koninck, see The Cosmos, 274), and even implicit in Feser’s procedure in his Five Proofs. The speculative riches potentially graspable by the intentiones of the causal divine names is realized along the Ways, not at their outset. Taking this more circumspect pathway to the being of God could avoid misunderstanding and nurture a properly contemplative attitude.

The traditional Catholic arguments for the existence of God are not geometrical proofs derived from self-evident axioms, but are something more elevated and deal with a subject matter that is more elusive. They function primarily as intellectual discernments about the nature of reality as we perceive it all the time. They begin from things around us so as to perceive the necessity of a transcendent origin, God the creator, who remains hidden and hence not immediately subject to the constraints of our “clear and distinct ideas.” That is to say, thinking about God is realistic and philosophical, but it also seeks to acknowledge the of numinous character of our existence and the ways that our limited, finite being points toward something transcendent, necessary, and eternal, which is the cause of our existence. Thinking about God in this sense is difficult for the human mind, not because theology is soft-headed, but simply because the subject matter is so elevated and not intrinsically capturable in the way mathematical or empirical topics are. (Thomas Joseph White, O.P., The Light of Christ, 57–58)

A Science-Proofed Proof?

It is, of course, in all its fine and gory natural philosophical and scientific details that I find Shields’s book the most enjoyable, rewarding, and, at times, even a bit frustrating. For instance, we are told that there are “some other Thomists” who hold that only “three philosophical species” exist (172n70). Who are they? (De Koninck “held,” with qualifications, that there were four.) His discussion incorrectly runs together vacuum energy, dark energy, and the inflaton field (258; the one belongs to quantum theory, the second a postulated field in general relativity; the third—not even noticed—is a sheer hypothesis). Also, Shields claims both that “in Aquinas’s day there was absolutely no empirical evidence that the universe was not in motion in perfectly uniform circles” (11n30), and that Aquinas had a “greater understanding” of astronomy than Aristotle (133n33, citing In Meta., n. 2511), yet where the text cited refers only due to a discovery not yet made in Aristotle’s day—the precession of the equinoxes—itself evidence that even the fixed stars have an anomalous motion. That is, there are no perfectly uniform and regular circular motions observable over time in the sky, despite the initial appearances that had to be “saved.” To say that all such motions could be decomposed into such circular motions is, of course, not empirical evidence but a supposition in response to the evidence. Against this ruling supposition of pre-Copernican astronomy there were lacking any countervailing suppositions joined with a more coherent physical theory of matter (see In De Caelo, II.17, n. 451).

All such matters aside, Shields’s book attains a level where philosophers of science and scientists will be forced to contend with the natural philosophical details and the interpretations Shields proposes, instead of being able to take an easy escape via superficial engagement. This is a rarity among Thomistic contributions in this area. In what follows, I wish to highlight briefly two areas which I hope will remain open to further inquiry along the trail blazed by Shields in his book. The first is the reality of natural place and the second is the character of the agencies concomitant to natural motion throughout the cosmos.

Shields joins a relative few Thomists who defend the reality of natural place today; indeed, his book makes good use of the work of Thomas McLaughlin on energy, inertia, gravitation, and place (although Shields does not agree with McLaughlin on all points). A similar accounting to Shields’s of the original Aristotelian view of natural place and motion of the elements can be found in Sheldon Cohen’s essay:

What is natural to earth is not motion, but rest: to be, to stay, to remain in a certain place, i.e., down—not movement toward that place. The actualization of the heavy is not to move toward the center, but to be at the center, and once the impediment is removed that has been keeping a rock from being there, the rock's potentiality to be in that place is immediately actualized, or at least it would be were it not for the resistance of the medium The rock's natural potentiality to be in that place is straight-away actualized, just as the perceiver's capacity to see is straight-away actualized by the perceptible, but this actualization is not a motion: “the activity (energeia) of lightness consists in the light thing's being in a certain place, namely high up” (255b11). The actualization of the rock's intrinsic potentiality does not consist in its admittedly natural motion toward its natural place, but in the absence of motion when the rock is in that place, and perhaps in the rock's resistance to being moved from that place. ... The nature of the heavy is not to move towards some place, or in a certain direction, but to be in that place, and it actualizes that nature not by falling, but by being where fallen things end up. ... Perhaps this is the point of Aristotle's closing assertion, at 256a4, that rocks fall not because they move themselves, but because they have a principle not of causing motion, but of undergoing it. The rock suffers natural motion due to an external cause, and moves only that it may rest in peace. (Cohen, “Aristotle on Elemental Motion,” 158–59)

In particular one must note the key points that “actualization of the heavy is not to move toward the center, but to be at the center” and that the natural motion is something the heavy or light undergo so as “[to be] where fallen [or risen] things end up.” For his part, Shields emphasizes Aquinas’s view that “motion, in and of itself, can never be a natural consequence of a material form” (44). The form of a thing is not a conjoined mover, and, rather, “all motion, just as such, essentially involves a potency requiring the influence of an external mover for its reduction to act” (ibid.; emphases removed). This means that nature is not ordered to motion for the sake of motion, and that natural motion is “just the completion of the process of [inanimate bodies’s] generation” (43). As Shields puts it,

Since resting in its natural place, rather than motion, is the natural consequence of a body’s nature, in order to undergo natural motion two things are necessary for a body: a determination to a specific natural place, and a potency to being in that place due to the occupation of another place. No inanimate body determines for itself what its proper place will be, and no inanimate body determines itself to the actual occupation of an improper place. (45)

Now, on the one hand, the “generator of the elements” of Aristotelian cosmology does not exist. Rather, all “terrestrial” elements are generated in situ by “celestial” bodies which are subject to the same parameters of natural motion as all other material substances. On the other hand, as Carlo Rovelli shows (“Aristotle's Physics: A Physicist's Look”), Aristotelian physics is a limit case of Newtonian mechanics, just as Newton’s theory of gravity is a limit case of Einstein’s. The Aristotelian interpretation of spontaneous natural motion was proposed with only a vague grasp of the specific contributing components of gravity or levity: the gravitational field, fluid pressure, viscosity, friction, etc. That natural motion to a natural place spontaneously arises from the body due to its form as a principle quo might have better contemporary luck, then, by asserting that the body’s form is a necessary but insufficient principle (see ST, Ia, q. 77, a. 6, ad 3). The realization of such a “natural resultance” of being in a place requires other necessary principles, namely the agency of surrounding bodies and fields of force.

For his part, Shields notes that the natural resultance of a body “resting in its natural place, rather than [being in] motion” is “still in tension with the modern principle of inertia” (45n64). For a light body such as fire, “its natural motion is a direct consequence of its generation [outside of its proper place], rather than of its mere existence as a body of light nature” (45). Without a replacement for the medieval doctrine of natural place, Shields’s recapitulation and corroboration of Aquinas’s motion proof in modern terms is going nowhere. Following a brief suggestion from Pierre Duhem—and portions of McLaughlin’s work—Shields notes that “In contemporary physics and chemistry, any system spontaneously heads towards an equilibrium condition—different systems at different rates—and then remains in that equilibrium condition until outside influences disturb it. Hence ‘equilibrium’ is the physicist’s word for what a Thomist calls ‘natural final cause,’ or ‘natural place,’ etc., at the lowest level of the inanimate” (115n1; see also 162n56 and 221). However, what exactly Shields means by “equilibrium” for inertial motion must be examined.

As we do so, it seems to me that Shields’s corroboration of the motion proof is a type of conservative minimum—provided his interpretations of inertia, gravity, energy, and force are sound. There remains the possibility of a more maximal or stronger corroboration. This brings me to my final point.

For all of the exquisite and exhaustive detail of Shields’s presentation of Aquinas’s motion proof, along with its parallels and extensions in the Contra Gentiles, he does not engage with Aquinas’s first argument in the latter work, the argument which St. Thomas learns from Aristotle’s Physics, VII.1. (This text receives excellent treatment by Richard Hassing in “Thomas Aquinas on Physics VII.1 and the Aristotelian Science of the Physical Continuum,” as well as in Marcus Berquist’s article “The Proof of the First Mover in Physics VII, 1.”) Overall, this argument is identical to the First Way: whatever is in motion is put in motion by another, and what is put in motion by another must be moved by a first mover. However, the defense of the first premise rests upon the argument that whatever is in motion must be divisible, and, because of this divisibility, it cannot be in motion of itself first and essentially (primo et per se), and so must be put in motion by some other cause (see St. Thomas, In Phys., lib. VII, lect. 1). The defense of the second premise is by reductio. If every thing in motion is extended and moved by another, which mover is itself in motion, this series either comes to some first mover or it does not. If not, an infinite series of movers results. This infinite mobile’s motion would occur over a finite time (see Aristotle, Physics, VI.7, 238a20-238b17). However, an infinite mobile cannot be moved in a finite time. Therefore, “it is clear that one thing being moved by another does not proceed to infinity, but rather will come to a stand at some point, and there will be a first mobile which is moved by some other immobile [mover]” (St. Thomas, In Phys., lib. VII, lect. 2, n. 894).

Noting that the “continuum” defense of the first premise is “the most neglected of Aquinas’s arguments because it is so difficult to understand and seems at first glance less promising” (7n22), Shields raises various “drawbacks” with the defense of the second premise, concluding: “I am not convinced that it cannot be made to work, but neither am I convinced that it can” (50n5). It is not my purpose here to convince Shields either way. Rather, the question worth pondering is what if the argument or some revised version of it could work today?

Here is its thumbnail sketch: Physics VII.1’s proof of the mover principle and the finitude of dependent cosmic movers amounts to treating mobiles under the abstraction of physically continuous systems in motion. The argument proceeds, even in Aquinas’s version, in a certain quasi-abstraction from the specific natures of the bodies similar to a mathematical or merely quantitative treatment of their motion, but distinct from it by conceiving of the continuum as a true physical continuum subject to Aristotelian hylomorphism. The inability of a given system to move itself primo et per se parallels the inability of a modern physical system to sustain a perpetual motion—free energy must at some point be supplied from without. The continuum of the entire conjunct of such moving systems would also be reflected in the ubiquity of the cosmological gravitational field—the apparent fact that there exists no force-free bit of matter in the cosmos—which gravitational energy is an ever-present reservoir of free energy, itself dependent upon the behavior of physical space.

All of this would require that one attend carefully to the specific, empirical determinationes of principles such as “form” and “natural place,” as well as to the empirical inquiries which support them, and the concepts crucial to any revivification of Aquinas’s motion proof: inertia, the force of gravity, energy, the laws of thermodynamics, and the principle of conservation of mass-energy, among ohers. Additionally, it requires us to attend to the way in which we conceptualize the principles and causes in nature.

The core of Shields’s recovery of the motion proof’s mover principle is harmonizing it with his interpretation of inertia and the clear position of Aquinas that a conjoined, physically-in-contact moving cause is not required for all motions, and yet, motions in the cosmos would not “freeze” in place if the First Mover ceased to act (see especially 42–46, and 89–96). Rather, “Aquinas conceives of the natural motion of inanimate bodies as just the completion of the process of their generation” (43; discussed above), and if, per impossibile, God ceased moving the cosmos, “the universe will not be able to continue in motion forever without [God’s] sustaining influence. It will wind down to equilibrium in a finite time” (92). This Thomistic version of the “heat death” of the universe anticipates the connections Shields makes between Aquinas and both inertia and entropy, and leaves the “conventional interpretation” of the First Way, which “holds that natural beings must be continually moved by an external mover as long as they are in motion” in conflict with both Aquinas and, apparently, modern physics and cosmology.

However, Shields must first show a harmonious connection with the inertia principle, gravity, and entropy (for the sake of space, I pass over questions I have regarding his interpretations of energy and conservation principles). First, in regard to inertia. He emphasizes the reality of inertia. This emphasis upon its reality is in contrast to views which interpret inertia as a mere mathematical device, a principle only counterfactually true, or some sort of limit concept (among them: Weisheipl, Wallace, Hanson, and one could add others). It seems that Shields offers three arguments for his side (197–98): first, that it is evident to our senses that bodies tend to move inertially; second, that to claim otherwise than an intertial tendency for an object moving in deep space is ad hoc; third, that even false or impossible conditionals can be true.

Shields’s first two arguments fall short of supporting the reality of inertia. As Weisheipl points out on the pages referenced by Shields, it is precisely not evident to our senses that inertial motion exists (see Nature and Motion in the Middle Ages, 27, 269). Rather, we observe motions ever under acceleration. Much less could the “senses reveal” that motions have a tendency to be inertial, for what a body can do is only evident through what it does do. Nor ought Shields’s questions about deep-space probes “[moving] nearly uniformly for unimaginably vast periods of time” move one, since all such objects are still moving within the local frame of the galaxy, and galaxies in their clusters move in curves. Rather, the truth about inertia is found only in a network of other principles; even its status as a derivative principle suggests this abstract-and-yet-real status (the principle of inertia can be derived from the principle of least action—and this relies upon energy as a more fundamental physical reality—or from principles of general relativity, or from principles quantum physics).

It is, of course, true that conditional statements can be true as a whole even if their antecedents are false or impossible: “If a man is an ass, then he is irrational” is Aquinas‘s example (ScG, I.13, n. 8). However, if Shields grants this, then his contentions against those who interpret the principle as an abstraction lose their edge. After all, he himself has to manifest the tendency via limit-cases and counterfactual reasoning. Indeed, it seems impossible to do otherwise: John Henry Newman in his Grammar of Assent (Pt. II, ch. 8, §2, 322), uses the realization of the principle of inertial motion as an example of the illative sense at work, an inference to a limit concept (see ibid., 321). What one reaches is true, but only under a certain abstraction. Perhaps this accounts for the reticence of a Wallace or Weisheipl, seeking to assign the valence of inertia within mathematical physics. For the mode of conceptualization is the key: inertia is a real principle, but one which is ever joined to other principles and is separable only in thought. In order to discern inertia as a principle that is a virtual component of complete natures, one must utilize limiting notions and counterfactual reasoning (see McLaughlin, “Nature and Inertia,” 259; Feser, Aristotle’s Revenge, 227–29; Hassing, “Physical Continuum,” 146–49). Indeed, the counterfactual reasoning necessary to have the insight to inertia relies upon a quasi-abstraction that, it would seem, Aquinas rules out as falsifying reality (see SBdT, q. 5, a. 3, c.). That is, if we think of a mobile body only as to its quantity and a certain rectilinear mobility and prescind from all other qualitative features, action and passion, and attendant sensible matter, this is to abstract a part from its whole. (If this is correct, it would make sense why inertia is conceivable as a counterfactual but true conditional, and it would shore up how it is real—as a part removable only in thought.)

Thus, the reality of inertia is much more limited than Shields seems to maintain, and he is forced to these limitations when providing a teleological interpretation of inertia (212–20). He proposes this along the way to answering two objections: first, that a Thomistic view of motion denies that motions can continue ad infinitum, and, second, that inertial motion would seem to denies the distinction between natural and violent motion (213). In his response, we learn that inertia, along with gravity, are “imperfect principles” (215) and “inertia must be understood in context,” namely, in the context of gravitational motion—and, I would add, the entire cosmos. This being said, Shields’s basic response to the second objection is unobjectionable: there are various contributing components to the natural motion of substances of various kinds, such that “‘natural’ and ‘violent’ are relative to specific natures” (216). What he must claim in order to answer the objection, however, is that inertial motion as such has a telos. This cannot be done without the fallacy of division: just because complete natures have a telos, and inertia is a component of a complete nature, it does not follow that inertial motion has a telos. No body is purely inertial in nature as such.

The lack of a real telos to inertia becomes clearer when we note that motion can only arise within a cosmos—what to do with a counterfactual cosmos of only one body is precisely the impossible scenario discussed by Hanson (and even John of St. Thomas). This is because the cosmos itself, the real context of the principle of inertia, is not infinite. So, “can [Thomistic] natural philosophy accept the infinite character of inertial motion? Can a body really tend toward infinity? Can a finite force produce an infinite result, e.g., an imaginary super-booster launching a satellite off into intergalactic space so as to move forever?” (218). His response:

Well, why not? Aquinas himself says that in the infinite future time an infinite number of thoughts and affections will be produced by our finite, immortal souls. This is a case of a finite power producing a quantitatively infinite result. But what is infinite quantitatively is still finite essentially; even if it does not have a quantitative limit, it is limited in its essence (it does not possess every aspect of being.) The same is the case with inertial motion: a finite force produces a quantitatively infinite result, namely unending motion. Yet this result is not infinite essentially, but quite limited in its mode of being. Moreover, the infinity of inertial motion is spread out over time, and does not exist all at once. At no point will a body, moving inertially, occupy an infinite amount of space (just as, at no point, will a soul have an infinite number of thoughts.) At any time, it will be in one place, not another, and it will be one kind of body, and not another kind. (218)

Shields’s response fails to convince, and, with it, his case for the reality of inertia. First, since the question concerns a physical principle, what is true of an immaterial power is irrelevant—what is true of one species is not necessarily true of another species, even granting inertia and intellect are species within some relatively proximate genus of powers or capacities. Second, the response answers the wrong question, for the objector was not asking about the infinity of an essence but the infinite distance towards which a body must travel if inertia has a real telos. However, Shields has not established that inertial motion is a separable reality such that it has its own telos, and his reply above does not prove this possibility. That the motion happens part by part over time does not help, for this does not distinguish inertia from other finite motions with a telos. Nor does his contention that inertially moving bodies infinitely far apart tending (yet always tending!) towards equilibrium as a condition of rest sustain the idea of a real telos (as opposed to a mathematical or abstractly conceived telos), because this depends upon a counterfactual cosmology: a finite amount of matter in an infinite space (as Shields notes at 218n81). To return to the point that counterfactual conditionals can yield true results is fine, but this does not take one any closer to proving that inertia has a real, non-counterfactual telos that is separable from the natural bodies in which it is a real principle or component.

What about Shields’s Thomistic account of the context of inertia: a cosmos of gravitationally moving physical substances subject to the second law of thermodynamics? This is where the stronger corroboration of the motion proof emerges more clearly.

Shields sketches three possible Thomistic accounts of gravity (221–22): the one defended by Thomas McLaughlin (see “A Defense of Natural Place”), in which the gravitational field actuates a natural passive principle for gravitational motion; one in which “gravitation is just inertia looked at from another frame of reference, and thus gravitational motion need involve no impressed force at all”; and a third, Shields’s own, in which “the contact between massive bodies, through the gravitational field, merely triggers the release of a heavy body’s natural active principle for motion, directing it a certain way and also determining the intensity of its acceleration” (222).

Which one of these is the correct view is not something I can determine here; neither does Shields need to pick, as his point is that gravitational motion can be accomodated to Thomist natural philosophy “provided one is not rigidly attached to the details of Aquinas’s view but only seeks to preserve his most fundamental principles” (222). Rather, all three involve some form of continuous causal contact, thanks to an updated physics. This is, in part, precisely what the argument in Physics VII.1 seemed to require. Thus one ought to insist that Aquinas was simply wrong as to his guesses (in Resp. ad 43 Art., aa. 19–23) about what would happen if God (and the angels) ceased to move the cosmos via the celestial spheres, and thus his views there do not provide philosophical evidence that act-potency or physical continuum arguments for the continuity of causal contact are false. The new inductive argument to defend the principle omne quod movetur, ab alio movetur seems to promise a seamlessly causal universe.

Even Shields’s discussion of thermodynamics and entropy open the way to this stronger corroboration of Aquinas’s motion proof. He argues:

Any identifiable process in which a closed system of bodies heads from a non-equilibrium condition to a state of equilibrium constitutes an essentially ordered causal series. When an agent places a system of bodies in a non-equilibrium condition (as the generator of a heavy body, when it generates in an upward region, does in St. Thomas’ medieval physics), it causes the whole, predictable process by which that system heads towards its equilibrium condition (towards its natural place, quantity, and quality in St. Thomas’ physics.) Free energy is not self-explanatory; possessing free energy (being out of equilibrium) is not the natural condition of a body or system of bodies, and so requires a causal explanation. The free energy present at a given point in the process has to come from somewhere, either immediately from an external agent or from a prior state of the system in which more free energy was present. The series of physical interactions that constitute the steps of the process by which the system heads towards equilibrium thus constitutes an essentially ordered causal series.
In tracing the essentially ordered series of causes backwards, we either come to a point where the free energy comes immediately from an external agent, or we continue backwards in time from one state of the system to a previous one with more free energy, without ever coming to an external agent. (250)

As Shields notes of the first disjunct, the free energy coming from outside the system is really asserting the existence of a First Mover, “since the system in question is the universe as a whole” (250). As for the second disjunct, this posits an infinite, essentially ordered series of moved movers (sources of free energy) which cannot exist. When one adds the fact that any mobile system in the universe, insofar as its energy-conditions are affected by the local gravitational field (which is in turn affected through the gravitational field of the entire cosmos), one seems to be sketching, in modern terminology, the concatenation of moved-movers and moving systems which Aristotle envisions in Physics VII.1 and which Aquinas follows without qualm in his Physics commentary and himself uses in the Contra Gentiles. Whether such a maximal version of the motion proof is sound, however, I have yet to determine.

Conclusion

For too long has Aquinas’s motion proof languished in the gaol of a contemporary Thomistic metaphysics unwilling to fully countenance the debt which Aquinas’s metaphysics owes to Aristotelian natural philosophy and unable to recapture the ground taken by materialist, naturalist, or positivist accounts of the cosmos. Shields’s book represents a real jail-break and counterattack. The rich achievements and possibilities of his Thomistic engagement with the natural sciences avoid an overarching methodological critique one might lodge. That is, one might think that the historical part of the book is surreptitiously doing the work of an unwritten philosophical part of the book that in turn reads the history of science in a spirit of “facile concordism” (see Maritain, Degrees of Knowledge, 60). This is not the case. The careful examination of what Aquinas actually meant and upon what empirical basis he propounds natural philosophical principles are included in both parts of the work and subjected to scrupulous examination. If anything, the risk readers run is that Shields’s minimalist recovery of the motion proof with extended completion—despite his Thomistic interpretation of Newtonian inertia—might close off their inquiry into a richer understanding of fluids, fields, energy, and forces as real principles for the natural motions in the cosmos. However, this is clearly not his intent, as his analysis does indeed have “the potential to breathe new life into Thomistic philosophy of nature and [open] avenues for an attractive integration of Thomism with contemporary science” (286).

Essays

Recovering Aquinas’s Motion Proof