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.