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 21st 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.