Daniel Shields

James Dominic Rooney, O.P. has reviewed my book Nature and Nature’s God for the American Catholic Philosophical Quarterly. Fr. Rooney begins his review: “Could another book on Thomas Aquinas’s “five ways” for proving the existence of God add something that has not yet been said? Yes, indeed it can.” He states that “Shields’s book presents overall a comprehensive and engrossing account of how to defend Thomistic natural philosophy within the context of contemporary physics.” “The case is presented cleanly; the responses to objections are generally compelling and plausible. Shields does a particularly good job of illustrating and defending a turn to philosophy of science that can enhance the work being done in philosophy of religion. His book will be essential reading for those who want to defend arguments for the existence of God within our contemporary scientific context.”

I thank Fr. Rooney for his kind words.

Of course, Fr. Rooney also has criticisms of some aspects of my book. I would like to respond to those here.

Fr. Rooney seems to read my account as turning the unmoved mover argument into a natural scientific argument (e.g., p. 667 of the review.) He later says that it seems to him to be “fundamentally an abductive argument” (p. 669.) But in the book I said that the unmoved mover argument is a philosophical, not a scientific one; the science only confirms/corroborates the philosophical argument rather than refutes it, as commonly thought. The unmoved mover argument as I present it is not abductive but rather deductive and demonstrative. Yet the argument is not a metaphysical argument; it is a natural philosophical argument. The significance of this distinction is often missed by modern Thomists. Aquinas, however, describes metaphysics, natural philosophy, and natural science (what he called “middle science,” that is, applied math) as three formally distinct disciplines. Some of Fr. Rooney’s criticisms may be based on a misunderstanding here.

Fr. Rooney also states that “Aquinas’s argument has been thought by many prominent interpreters (e.g., Weisheipl) to rely on a problematic premise that everything ‘being moved’ requires a mover whose action is somehow simultaneously bringing about that motion, which Shields concedes would be false in light of inertial motion.” But Fr. Weisheipl is not a prominent interpreter of Aquinas’ First Way. As far as I know he never gave an account of the unmoved mover argument in print. What he does say suggests that he would interpret the argument as focusing on substantial change. Fr. Weisheipl does, however, read the mover principle as “everything being moved requires a mover”; he therefore exempts natural motion from the mover principle. I do see this as problematic and argue against it in the book. (The great merit of Fr. Weisheipl’s work, however, is that he shows that Aquinas does not think that every motion requires a continuously conjoined mover.) What I see as more problematic is that some interpreters, such as Fr. Garrigou-Lagrange, read the mover principle as “everything in motion is being moved by another” and then deny the existence of truly inertial motion, requiring God to carry bodies along so to speak. Others interpret the First Way as hinging on the act of existence (Owens, Knasas, and Kerr for example) and yet there is no reference to the act of existence at all in the text of the First Way. What I argue in the book is that the mover principle should be read “everything in motion is moved by something else” but that the mover may have acted in the past to cause present motion. This makes it possible to reconcile the unmoved mover argument with modern physics.

Fr. Rooney also says that I allege “that Aquinas’s argument only pertains to physical motion and that Aquinas only later establishes the act/potency distinction in a subsequent logical move (esp. 96–100). I fail to see, however, that the ‘first way,’ even as Shields presents it, can be defended without the act/potency distinction.” While it is true that I read the First Way as pertaining only to physical motion, the rest of this is a mischaracterization of my book. I discuss the act/potency distinction in chapter one in the course of explaining the mover principle. Certainly the act/potency distinction is necessary for understanding the First Way. What I say, however, is that the thesis that God is pure act is not part of the First Way. It is only established in q. 3, a. 1, and not by an appeal to the First Way but rather to the Fourth Way.

Fr. Rooney continues the criticism by saying that “these same claims [about the necessity for an immaterial mover for everlasting motion] end up ramifying into precisely the considerations about God lacking any potency at all, since the arguments require ultimately a mover that undergoes no change in entropy and has no potency for any change. So Shields cannot fault these other interpretations for incorporating into the proof other premises that Shields himself needs to defend.” There are several things wrong with this. First, Fr. Rooney seems to conflate the First Way in theSumma Theologiae with the second unmoved mover proof in the Summa contra Gentiles. I show in detail in the book how these two arguments are different. Only the latter proof shows, without further argumentation, that an immovable, everlasting mover is required. Secondly, saying that the mover must be exempt from potency for physical motion is not equivalent to saying that the mover is without potency altogether and thus pure act. Third, I do not deny that St. Thomas makes a further, metaphysical argument for the conclusion that God is pure act on the basis of the second unmoved mover argument in the Summa contra Gentiles, following Aristotle’s procedure in the Metaphysics. What I deny is that this argument is part of the unmoved mover argument, which is natural philosophical. Failure to observe this distinction turns the “more manifest” natural philosophical unmoved mover argument into a subtle metaphysical argument that only the wise can follow. This is contrary to Aquinas’ intention.

Fr. Rooney also objects to my claim that the origin of life requires an intelligent, supernatural cause. (Note that I make it clear in the book that once life exists it can evolve by purely natural causes. What is in question here is only the first production of a living organism on the early earth, roughly three and a half billion years ago.) He says that “even if we were to grant a principle of ‘proportionate causality,’ that alone does not allow us to conclude to intelligent creation. This is a non sequitur. For some medieval thinkers, too, the action of the celestial bodies could bring about spontaneous generation of living organisms from inanimate matter, without intelligent intervention.” (p. 670) The principle of “proportionate causality” is certainly a central principle in Aquinas’ own thought and it is a staple of contemporary Thomism, albeit one that is interpreted differently by different Thomists. But as far as the present issue is concerned, St. Thomas applies the principle of proportionate causality explicitly to the (putative) spontaneous generation of living organisms from non-living matter:

“As Augustine says, a living substance is superior to any non-living substance whatever. But a heavenly body is not a living substance, since it is inanimate. Therefore a sensitive soul, which is a principle of life, cannot be produced by its power. . . .

To the thirteenth objection it is to be said that even if a heavenly body is not living, nevertheless it acts by the power of the living substance by which it is moved, whether it be an angel or God.” Quaestiones Disputatae de Potentia Dei, q. 3, a. 11, obj. 13 & ad 13.

Aquinas holds that the heavenly bodies can only generate living organisms from putrifying matter because the heavenly bodies are being used instrumentally by their living, intelligent, immaterial movers. Otherwise they would be producing an effect disproportionate to their power, in violation of the principle of proportionate causality (which shows up all over Aquinas’ account here in De Potentia, q. 3.)

A modern Thomist has several options in dealing with this issue. He could hold, with Aquinas, that angels move heavenly bodies around and are responsible for orbital motion. I presume most of us Thomists don’t want to take this route. Another alternative is to water down the principle of proportionate causality so that it only requires causes with the power to produce the effect in question, not causes that equal or exceed their effect in ontological perfection. Then one could say that living organisms exist virtually in non-living causes. Several modern Thomists do take this route, but it has to be acknowledged that it is a serious departure from St. Thomas’ own thought. (That does not automatically make it false, but it will cause serious difficulty for Aquinas’ Treatise on the Divine Nature in the Summa Theologiae.) I find the principle of proportionate causality persuasive, however, on its classic interpretation as held by Aquinas himself. Causes must equal or exceed their effect.

That leaves two more options. The first is to say that God, by initiating the universe in a low-entropy condition fine-tuned for the production of life, has utilized the heavenly bodies as His instruments in the production of life. Non-living substances thus act in virtue of God’s power in bringing about life. The other option is to say that God or an angel intervened to initiate life on the early earth. I find this last option the more plausible one both in virtue of the scientific evidence and in itself. But the former option has merit as well.

This brings me to Fr. Rooney’s further point: “Shields’s own statement of the facts seems to indicate—as seems scientifically demonstrable—that at least some simple living organisms could be created simply by combining chemical constituents in the right way. Scientists have already successfully assembled single-celled organisms whose parts are mostly synthetic; there is no reason to believe there remains anything more than technical obstacles to the future achievement of organisms entirely synthetic.” I have no idea what Fr. Rooney is referring to by “Shields’s own statement of the facts seems to indicate . . .” The rest of his point here seems to me too facile. Does it rest on anything more than an act of scientific faith? Perhaps; Fr. Rooney clearly cannot, as he himself says, turn a review into a complete philosophical or scientific argument for a substantive claim. But it mischaracterizes matters to say that scientists have produced mostly synthetic organisms already. He may be referring the work of Craig Venter and his lab (although he does not say what he has in mind.) David Oderberg has already debunked the claims made in his regard: “Synthetic Life and the Bruteness of Immanent Causation,” in Aristotle on Metaphysics and Method, ed. Edward Feser, 206–35 (New York, NY: Palgrave Macmillan, 2013.) What Venter did was to stitch together small portions of a genome manually, then coax living cells to stitch together the small portions into a complete chromosome, and then replace the natural chromosome in another living cell with the quasi-artificial chromosome that his team put together. That is a far cry from a synthetic organism. It fits, rather, with the maxim omne vivens ex vivo. And does Fr. Rooney have anything to say in response to my thermodynamic account of living organisms? (To be clear: I do not claim that living organisms violate the second law of thermodynamics, but only that they utilize it to their advantage in a way that non-living things cannot.)

In fairness to Fr. Rooney, however, my discussion of living organisms and thermodynamics was not fully worked out in the book. I hope to develop it more fully in the future.

Once again, I thank Fr. Rooney for his overall appreciation of my work reconciling the unmoved mover argument and Thomistic natural philosophy in general with modern science.

Nova et Vetera has accepted my article “Five Ways, One Journey: Do Aquinas’ Five Ways Attempt to Prove the God of Classical Theism?” In the article I present a new reading of Aquinas’ Five Ways by attending carefully to each article in questions 2 & 3 of the Summa. The first two Ways only demonstrate that a god exists, whereas the Third, Fourth, and Fifth show what sort of thing a god is. The first three Ways constitute one extended, natural philosophical cosmological argument, even though each serves as a successful proof of a god’s existence all on its own as well. The conclusion of each Way is subsumed by the next Way, providing assurance that each concludes to the same being. Aquinas utilizes a weak, nominal definition of “god” as the criterion for a successful proof, namely “some being distinct from and superior to ordinary beings that governs as a first cause some realm in the universe.” He does not intend to prove the existence of the God of classical theism in his Five Ways nor does he intend to prove the existence of God as the self-subsistent act of existence, nor as pure act. He does not even intend to prove the existence of a single individual god. But the Fourth Way shows that only one kind of thing is a god; q. 3, a. 3 shows that there is not more than one individual god in a species; and q. 3, a. 5 shows that there is not more than one species of god. The God of classical theism is proved by the entirety of questions 2 & 3 taken together.

If you would like a copy, please email me.

Agustin Echavarria, in a recently published article in Theologica, defends classical theism from Joseph Schmid’s providential collapse objection (here and here.) He begins by commenting on my Thomistic response article (here and here). I appreciate his taking the time to read my work and for his respectful interaction with it. After an accurate presentation of the earlier parts of my paper, he has the following to say:

“Shields’ proposal is certainly suggestive. Moreover, if successful, it would end at the same time with both the problem of intentional collapse and the problem of providential collapse. One might even argue that it has more textual evidence in its favor in Aquinas’s texts than Shields himself provides. [He helpfully cites ST I q. 19 a. 2 ad 5; SCG I c. 79 n. 8; and SCG I c. 82 n. 12.] However, I have some doubts about whether this is the best way to approach the problem of providential collapse in particular. Mainly because this proposal seems to be contrary to what Tomaszewski calls the “modal indiscernibility thesis” (MIT). MIT is the idea that “if the doctrine of divine simplicity is true, then God in any possible world is really, intrinsically indiscernible from God in any other possible world” (Tomaszewski 2023, 238). To say that there are contingent relations in God established by His will implies that God establishes these relations of reason in some possible worlds and does not establish them in others; therefore, His will is not identical in every possible world with regard to these relations, which is contrary to MIT.” (Echavarria, p. 71)

The mentioned texts are certainly very helpful and I’m grateful that Echavarria points them out. In my paper, however, I maintain that “The one necessary act that [God] is identical with can contingently relate itself to the dog Fido as willing to create or as not willing to create, without any difference or contingency in God’s act.” (Shields, p. 86) “The act need not be different in itself to relate itself differently to creatures, as I have shown above.” (Shields, p. 87) “God does not need to be any different to have different relations to the same possible creature. . . . God’s one essence, with no difference in the act with which his essence is identical, can relate in different ways to possible creatures.” (Shields, p. 90)

I don’t think that my position violates Tomaszewski’s “Modal Indiscernability Thesis,” although I suppose that depends on precisely what Tomaszewski meant by it. But I claim that neither God nor His will (which are identical) would be any different intrinsically if He related differently to creatures, willing to create some that He has not in fact willed to create, and not willing to create some that He has in fact willed to create.

Echavarria seems to find this implausible; he has the following to say:

“This conclusion [that the Modal Indiscernability Thesis would be violated] is not avoided by saying, as Shields holds, that relations are not accidents or absolute ontological items in God, and so they are not intrinsic properties. As Shields himself admits, God’s contingent relations are different “ways” in which God’s will is determined with respect to certain possible creatures. Moreover, since they are relations “of reason,” their only possible ground must be in God’s will (rational appetite) itself, and not in something extrinsic to it. Hence, the existence of these relations implies by itself an intrinsic determination in God’s will, because they imply the intentional and intrinsic direction of the will toward certain possible creatures in particular, instead of others. Being contingent relations, this intrinsic determination of God’s will obtains in some possible worlds and not in others. Therefore, the very existence of these relations implies that the divine will is not intrinsically identical in every possible world.” (p. 71-72, emphasis added.)

I think that the italicized premise of the argument here is incorrect, and I gave an argument against it in my paper, p. 84-87. In the case of libertarian free-will, on pain of infinite regress, there has to be a first difference caused by the will, prior to which there is no other difference at all. A free will is such that without being any different intrinsically it can immediately produce different alternative effects. In God’s case, this very first difference is the relationship between His will and the possible creature. There need be no difference within God’s will causally prior to His relating differently to creatures, nor need any difference within God’s will accompany the difference in His relationship to creatures. As for intentionality, it is itself relational. God’s intentionality of creating this creature and not creating that creature just is the relation between Him and the creature. Echavarria did not choose to saying anything in response to this part of my argument, but in fairness to him his real target was Schmid and he spent the majority of his article developing his own response to Schmid.

Echavarria’s own response raises concerns for me. He makes some helpful points in response to Schmid, in particular that he overreached in claiming that the set of God’s possible effects would be utterly without restriction on the doctrine of divine simplicity. But Echevarria seems to deny God’s total, immediate, and particular providence. St. Thomas certainly thinks that God has such total, particular, and detailed providential control over everything in the universe. (See, for example, ST I, q. 22, a. 2 & 3; q. 23, a. 4.) I am worried that Echevarria’s defense of classical theism is not very classical. However, he indicates that the question of intentionality and intentional collapse is crucial here, and that he deals with God’s intentionality in a forthcoming article. It’s possible that this will clarify his position in such a way that it preserves the particular divine providence essential to classical theism. I am interested to see what he has to say in any case.

On November 14th, 2025, I delivered a public lecture at Wyoming Catholic College titled “The Heavens Declare the Glory of God: Modern Physics and an Ancient Cosmological Argument.” The lecture summarizes and extends the second part of my book, Nature and Nature’s God. I defend the Thomistic cosmological argument for an unmoved mover in light of modern physics. A recording of the lecture and Q & A is currently available on youtube.

Edward Feser has reviewed Nature and Nature’s God in the July 2025 issue of The Thomist. I am grateful for his many kind words. He states that “the analysis in [the] first part, no less than in the second, is detailed and highly illuminating, and the scholarship deployed is impressive throughout. The book is a major achievement, and will contribute mightily to the revival of Aristotelian-Thomistic natural theology and philosophy of nature. Even those who disagree with Shields will learn from, and profit from engaging with, this important work.” “Shields’s analyses not only of inertia and thermodynamics but of gravitation, teleology, and fine-tuning constitute a major contribution to Aristotelian-Thomistic philosophy of nature even apart from any application to natural theology. The book advances both fields, and deserves a wide audience.”

Feser finds the second part of the book—in which I defend Aquinas’ second motion proof from the Summa contra Gentiles in light of modern science and its history—“entirely successful.” Nevertheless, despite his admiration in certain respects, he finds my interpretation of Aquinas’ First Way in the Summa Theologiae in the first part of my book quite implausible overall, and he levels some serious criticisms. This is only fair, since I criticized his reading of the First Way in my book. In fact, his criticisms helped to reveal to me a real weakness in my presentation, namely that I moved too quickly from the First Way in ST to the arguments in Summa contra Gentiles, without sufficiently addressing the immediate context of the First Way itself.

In a spirit of friendly debate, I will respond to Feser’s criticisms here. I have learned a great deal from his many books over the years (including from his characterization of essentially and accidentally ordered causal series, so central to the first three Ways), and we are joined in common cause in defending theism in general and Thomism in particular. But it serves the interests of the community of Thomists if many different interpretations are presented clearly and ably defended by their proponents. Each interpreter will inevitably notice things that others have missed, and the process of disputation will clarify the truth over time.

I begin by noting that I contrast Aquinas’ First Way with the second motion proof in SCG—which for convenience’s sake I call G2 following Kretzmann. The First Way has a quite modest conclusion, not ruling out naturalism all by itself, whereas G2 has a more robust conclusion, demonstrating the existence of a transcendent being. As I read the Summa Theologiae, the Five Ways prove conclusions of increasing strength, but the demonstration of classical theism is not complete until the end of question 3. Feser seems inclined to read the First Way as an abbreviated form of G2. What he says about the second part of my book implies that he appreciates my analysis of G2, which sets the stage for my science-engaged defense of Aquinas’ motion proof, but he is unsympathetic to my analysis of the First Way as a much more modest argument.

Feser’s constructive criticism of part one begins with some clarifications about his own interpretation of the First Way. He calls the First Way metaphysical in his book Aquinas, whereas I see it as belonging to the discipline of natural philosophy. But Feser clarifies that he was using the term metaphysical “in the broad sense common in contemporary academic philosophy (which includes philosophy of nature as part of metaphysics), rather than in the narrower sense in which Thomists often use the term.” Feser also, quite rightly, points out that I was not sensitive to the fact that in his Five Proofs he presents the motion (“Aristotelian”) proof as his own, inspired by Aristotle and Aquinas but not an account of Aquinas’ own proof, so Five Proofs should not have been used as evidence of his own interpretation of the First Way.

Feser makes three further, encouraging clarifications about his own position. First, although he denies “that the First Way need be read in a way that requires any premises from natural science as that is understood today” he does “not deny that it could be read that way.” In other words, he thinks the project of the second part of my book is an important and worthwhile one. Second, he does not want to assimilate the First Way with the De Ente et Essentia proof; they are two separate ways to God. From my perspective, the more proofs for God the better. Third, Feser seems quite open to the non-simultaneity of some essentially ordered causal series, which I argue for in the book and is an important point.

Feser’s criticisms center on my claim that the First Way, by itself, only proves that there is one or more first, unmoved movers, and does not show that any of those movers is not a natural being. “Aquinas tells us the Five Ways are arguments by which ‘the existence of God can be proved.’ And of the first mover he arrives at in the First Way, he says ‘this everyone understands to be God.’ Why say these things if he did not intend the First Way to get us all the way to God?” My response is that Aquinas does not share 21^st century philosophy of religion’s understanding of what a proof for God’s existence is. Aquinas tells us himself what he means by the term deus, and thus what it means to demonstrate the existence dei: “This name “god” (hoc nomen deus) is the name of an operation, in regards to the origin of the name. For this name is imposed from universal providence over things, for all who speak about god intend by “god” to name that which has universal providence over things.” (ST I, q. 13, a. 8, c.). He says, further, that

There is a certain common and confused knowledge of god (dei cognitio) which almost everyone has. . . . For human beings, seeing that natural things run according to a certain order, perceive for the most part that there is some regulator (ordinatorem) of the things that we see, since there cannot be order without a regulator. Who, however, or what sort of thing, or whether only one thing is the regulator of nature is not yet immediately grasped by this common consideration. Similarly, when we see that a human being undergoes motion and performs other actions we perceive that there is a certain cause of these operations in him that is not in other things, and we name this cause “the soul,” without yet knowing what the soul is, whether it is a body, or how it effects the aforementioned operations. . . . Certain people have believed that there is no other regulator of the things of the world than the astronomical bodies and so they have said that the astronomical bodies are gods (deos esse). Certain people have even gone farther [and said] that the very elements and those things that are generated out of them [are gods], as if thinking that the motions and natural operations that they have are not due to another regulator, but that other things are ordered by them. Certain people, believing that human acts are not subject to the ordination of anyone other than humans, have said that humans who order others are gods (deos esse). (SCG III, c. 38.)

To prove the existence of god is not to do very much; it is only to prove that there is some one or more ultimate determiners of what goes on in the universe. If the planets or elements really were the ultimate determiners of what goes on, they would be gods; since they are not, they are not gods. Note that Aquinas did not in fact capitalize the word deus/“god,” in the Five Ways; capitalization was only used at section beginnings in medieval manuscripts. The manuscripts of the Five Ways leave deus uncapitalized.The capitalization of Deus in modern Latin editions of the Summa, and of God in English translations, is an implicitly interpretive move. The use of the capital in the Five Ways biases modern readers and scholars towards thinking that Aquinas had something stronger in mind than he actually did. That the word deus/god does not automatically imply incorporeality is clear even from the immediate context of the Five Ways. In objecting to Anselm’s proof in the Proslogion, Aquinas says “Perhaps he who hears the name ‘god’ does not understand it to signify something than which a greater cannot be thought, since certain people have believed that god is a body.” (ST I, q. 2, a. 1, ad 2.) Anselm’s argument is built around the principle of non-contradiction. Aquinas would not have used the fact that certain people believed god to be a body to criticize Anselm if it was a contradiction in terms for god to be a body. Otherwise, he would have been playing into Anselm’s hands.

When Aquinas ends each of the first three ways by saying that “this everyone calls god,” he is not saying that he has just proved the existence of a superlatively divine being, meeting the strictest standards that could be set for the name “god.” He means, rather, that he has just proved the existence of a being that satisfies the lowest common denominator description of “god,” namely the understanding of the term he sets forth in the passages quoted above. This is why he shifts in the Fourth and Fifth Ways to saying “this we call god.” The Fourth and Fifth Ways prove stronger conclusions than the first three Ways, something much more like what Christians conceive God to be. Many people who believed in the existence of god did not believe in something as impressive as the being proved to exist in the last two Ways.

Feser objects that “even if a planet or an emperor counts as a ‘god’ in the thin sense Aquinas allegedly had in mind, an animal soul surely would not.’” But in the book I argue only that if an animal soul were a first, unmoved mover, then it would be the ultimate determiner of some restricted region of the universe, and thus a little god. Of course it is not a first, unmoved mover and so is not a god. Its actions are determined by outside factors in many ways. But the idea that an individual soul could be thought of as a god is not foreign to Aquinas’ intellectual context, as evidence by his comment about human rulers quoted above. Indeed, ancient Greek pagans believed in many minor deities, governing quite restricted domains (such as a single river or grove of trees) within the larger universe. The First Way, by itself, is agnostic as to what sort of thing really fits the bill of a first, unmoved mover. That is why it couches its conclusion with the indefinite adjective aliquod: there exists “some first mover moved by nothing.” All it demonstrates is that there is some one or more first unmoved movers, so some one or more rulers of the universe exist, and thus a god exists, one way or another.

Feser objects that “Shields himself admits that versions of the argument from motion Aquinas develops in the Summa contra gentiles and the Compendium theologiae do aim to get us beyond a mundane cause to a purely actual first mover. Why, then, not take the First Way to be another version of that line of reasoning, truncated but with the same aim? Shields’s reason for not doing so is that he thinks there is no good indication in the text of the First Way itself that it has the same aim as those other version of the argument from motion.” Feser thinks I am inconsistent in using outside texts to help interpret the Fifth Way and not doing so for the First Way as well. But in fact I do use outside texts in both cases. I point out in the book that there are two motion proofs in Aristotle’s Physics, one in book VII and one in book VIII. Aquinas tells us that the Physics VII proof only shows that there is a first mover while the Physics VIII proof shows what sort of thing a first mover is. Aquinas’ own version of both proofs is presented in SCG, as two separate motion proofs. The First Way has the structure of the Physics VII proof, and thus shows only that there is a first mover, not what sort of thing it is. Thus I use SCG and Aquinas’ Commentary on Aristotle’s Physics to interpret the First Way.In the case of the Fifth Way, the argument has the very same structure as an argument in De Veritate, q. 5, a. 2, c., and so I use that text to help interpret the Fifth Way. (I may have overstated the centrality of biology in specific to the Fifth Way, so I might concede to Feser there. However, beneficial adaptation is central to the Fifth Way on my interpretation and not Feser’s.)

My reading of the First Way is confirmed by the immediately following article of the Summa (q. 3, a. 1). There Aquinas gives three independent arguments that god is not a body. The first appeals to the First Way for the premise that god is an unmoved mover (or “immovable”; the Latin word is ambiguous, but the former reading makes the following argument a more perfect syllogism, and fits the logic of the First Way itself better.) It then adds an additional but simple empirical premise not contained in the First Way: “no body moves except insofar as it is moved by another, as is clear from induction.” Therefore god is not a body. This takes us one small step past the conclusion of the First Way. But Aquinas does not intend even here to show that God is a non-natural being. For in the third argument in the respondeo of q. 3, a. 1, Aquinas uses the Fourth Way to argue that god is not a body because the soul of a body is better than the body, and god is the best being. The conclusion that god is not a body thus leaves open the possibility that god might be the soul of a body. The latter possibility is only ruled out seven articles later (q. 3, a. 8, c.) where Aquinas notes that many pagans thought that god was the soul of the world or of one of the astronomical bodies, and then argues against that view. Thus demonstrating that god is not a body is not the same a demonstrating that he is not part of the natural world. To do that, one needs one of the stronger arguments for god’s existence, such as G2, the Third Way, the Fourth Way, the Fifth Way, or the De Ente argument.

Only here in q. 3, a. 1 does Aquinas draw the conclusion that god is pure act. He then appeals to this article again and again in q. 3 (not to the First Way or to q. 2, a. 3 as Feser states in his review) when arguing for further conclusions on the basis of god’s pure actuality. But to establish the conclusion here that god is pure act he uses as premise the fact that “it has been shown above that god is the first being”; he does not use as a premise the fact that God is immovable, as Feser stated in his review. Aquinas is appealing to the Fourth Way (or perhaps to the Five Ways as a collective whole), not to the First Way. In fact, Aquinas uses the First Way only once in his entire discussion of divine simplicity in question 3, and only as one of three arguments to show that god is not a body. The First Way does very little work in Aquinas’ discussion.

Feser also appeals to the second objection in q. 2, a. 3, where the objector argues that natural causes are sufficient and no god is needed. Certainly Aquinas thinks that the Five Ways taken collectively have rebutted naturalism. But Aquinas responds to the objection in two parts. First, in regards to non-conscious natural phenomena, Aquinas appeals only to the Fifth Way, not to the First. In regards to the conscious actions of humans (and perhaps animals) Aquinas appeals to the Third Way and First Way together (or perhaps just to the Third Way, depending on how one reads the text.) The First Way cannot do the job by itself. I argue in the book that the Third Way is doing in ST the job that G2 does in SCG, warranting the stronger conclusion that naturalism is false.

Feser also argues that “interpreting an argument charitably requires giving it a stronger rather than weaker reading if that is realistic. But the First Way would be a stronger argument for God’s existence if we read it as an abbreviation of the Summa contra Gentiles argument than if we read it Shields’ way.” But giving a stronger reading of an argument means interpreting it in such a way that it is valid rather than invalid, sound rather than unsound. It does not mean giving it a stronger conclusion if that conclusion is unwarranted by the premises and logical structure of the argument. That would be to interpret the First Way uncharitably, because it sets it up for ridicule by its opponents. I contend in the book that by giving it too strong of a conclusion, the First Way’s defenders have made it vulnerable to atheist attacks in ways that it wouldn’t be if it was read as I read it.

Despite our disagreement about the First Way in particular, Feser and I are in broad agreement philosophically. I am encouraged by his praise of my natural philosophical project, and am indebted to him for his engagement with my work.

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.