Nature and Nature's God: A Philosophical and Scientific Defense of Aquinas's Unmoved Mover Argument. By DanielShields. Washington, D.C.: Catholic University of America Press, 2023. Pp. 328. $75.00.

Nature and Nature's God is a study of Aquinas's proof of God from motion, and undertakes to present an interpretation of that proof as drawn from philosophy of nature and a defence thereof in light of challenges drawn from modern science. Part I (Chapters 1–5) deals with the motion proof in Aquinas's historical context. Part II deals with the motion proof in light of contemporary science. Chapter 1 deals with the mover principle: omne quod movetur ab alio movetur (Shields's translation: everything in motion is moved by something else). As Shields understands this principle it states that whatever is in motion has an efficient cause of its motion, but that the efficient cause need not be simultaneous or in current contact with the thing in motion. Shields defends the mover principle by appeal to a much-neglected argument from SCG Lib I, Cap. 13, wherein it is defended by inductive means. The argument takes different ways in which things are in motion and, through an appeal to act and potency, it concludes that in each of them whatever is in motion is moved by something else. Chapter 2 concerns the denial of an infinite regress. Shields takes us through Aquinas's well-known distinction between essentially ordered and accidentally ordered causal series, and how, by appeal to the former, Aquinas can deny an infinite series of moved movers. A per se ordered series is such that the members of the series do not have the causality of that series in virtue of what they are but in virtue of something else. Hence, unless such secondary causes in the series participate in some primary cause, those causes themselves will be without their causality. What is interesting to note is that Shields maintains that on Aquinas's account (as opposed to Scotus's) the members of per se ordered series need not act simultaneously, but can act successively. Shields has made an important contribution with this latter point insofar as interested non-specialists (and some specialists) generally attribute to Aquinas the view that simultaneity is necessary for per se ordered series, when this is a feature of Scotus's characterisation of per se ordered series. Chapter 3 deals primarily with the conclusion of the first way. Shields contends that the conventional interpretation of the first way is mistaken. This interpretation follows the work of Owens, Twetten, Knasas, et al, to the effect that what is at stake in the first way is actuality per se, and so the conclusion to an unmoved mover is a conclusion to a source of actuality that does not derive its actuality from anything else. Against this metaphysical interpretation, Shields contends that an implication of this view is that if God ceased to act as primary cause of motion, then all motion would cease; and Shields shows the falsity of the consequence by drawing on a number of texts from Aquinas. Shields further argues against the metaphysical interpretation by pointing out that the first way does not establish the unmoved mover as pure act, and so metaphysical actuality cannot be at stake therein. The remainder of this chapter focuses on the logic of the first way and its minimal conclusion that some primary mover(s) exists. It is only when this argumentation is augmented by proofs that Thomas offers elsewhere that we have a more robust conclusion to a transcendent God. Chapter 4 takes up the augmentation of the first way by consideration of further proofs in the Summa Contra Gentiles and the third way. Chapter 5 then connects the first way with the fifth way. Shields's main conclusion in these chapters is that by connecting the motion proof up with these other arguments, Aquinas can conclude to the existence of a unique, living God to whom all things are subject. Thus ends part I. Part II (chapters 6–8) defends the motion proof against the demands of modern science. Chapter 6 concerns the principle of inertia and how, given this, motion does not require a sustaining cause. Chapter 7 considers thermodynamics and the conservation of energy, according to which energy never increases or decreases but is preserved, in which case there is no need for God as a sustaining cause. Finally, chapter 8 connects issues pertaining to entropy, information, life, and intelligence, confirming Aquinas's view that the first mover is intelligent and alive. These chapters go into some considerable detail in defending Aquinas's philosophy of nature and motion proof; indeed, these chapters could be read by themselves, and in my opinion they were the most interesting chapters in the book. In reaction to Shields's interpretation and defence of the motion proof, I would like to advance two general criticisms pertaining to: (i) the scientific context and (ii) the manifestness of the argument from motion. To begin with the scientific context, Shields takes the first way to be an argument drawn from physics or philosophy of nature. He does not take it to be an argument from metaphysics. But at no point does he consider Aquinas's thought on the scientific context within which demonstrations of God's existence are established. In various places but most notably in his commentary on the De Trinitate of Boethius, Aquinas sets aside metaphysics as having its own subject matter beyond that of physics and prior to that of theology; and it is in metaphysics that Thomas takes God's existence to be established. A defensible reading of these texts suggests that it is only in metaphysics that God's existence is established (this is at least a reading that I and others have advanced). Shields does not consider this issue and so we are left with a defensible reading of Thomas's texts in this regard that stands counter to Shields's position. This brings me to the second point, and it is that Thomas takes the first way to be the more manifest way. Frequently Shields maintains that if the proof from motion were the more manifest, then it could not be a metaphysical proof; for the latter is not so manifest and requires a lot of metaphysical reasoning that our contemporaries would not accept. However, the same can be put to Shields's physical reading of the proof. It requires subtle moves in the clarification of the motion principle, the nature of per se ordered series, not to mention modification given the developments of modern science. If the need for clarification of metaphysical issues would render the proof from motion less manifest on the metaphysical interpretation, then all the clarifications included in Shields's book would render it just as less manifest. Thomas himself was surely aware that labelling a proof the more manifest does not entail that delicate philosophical moves do not need to be made and at times clarified. So it cannot be that the manifestness of the proof means that it is plain and simple throughout or, even worse, that it is something that all would accept. Rather, it must mean that the proof can take us from an initial starting point that is obvious to all and that, if we follow the reasoning (which includes the various clarifications), we can see the force of the demonstration in the conclusion. In this way the proof from motion is more manifest since it begins with a reality of which we are all aware, but this does not exclude a metaphysical understanding of that reality and metaphysical considerations that take us from that reality to God. Hence appeals to the more manifest nature of the proof from motion are not sufficient to justify the physical reading. These critical remarks are par for the course in the consideration of any major philosopher. There is much with which I agree in Shields's presentation and in his criticism of some contemporary Thomists with whom he or others might think I share an affinity; indeed, I was very pleased to see that Shields and I agree that simultaneity is not a condition for per se ordered series. Overall, I think that this 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. If I may return to the manifest nature of the proof from motion, Shields shows that he is concerned with preserving the proof as the more manifest. This highlights a concern that people can follow the proof, not just that he (Shields) is right. In Nature and Nature's God we have a first-rate philosopher who robustly defends the claims that he makes but does not overlook the task of presenting such claims in a way that he can bring others with him. Even if he has not brought this reader with him, my understanding of the thought of Aquinas on the proof from motion has been deepened through my engagement with his book.