Now available is an updated version of my ongoing translation project of John of St. Thomas’s Cursus Philosophicus. The new edition adds Q. 1, A. 1, which considers whether mobile being is the formal object of Philosophy (i.e., natural philosophy). This article serves, in most respects, as an extended commentary on the truth of a single sentence in St. Thomas’s commentary on Aristotle’s Physics, namely, the Angelic Doctor’s claim that the subject of natural philosophy is “mobile being” (ens mobile) and not “mobile body”(corpus mobile).
What is most striking in Poinsot’s answer to this question is his emphasis upon the formal notion of “mobility” or motion itself as the intelligible light in which we understand physical being. This contrasts sharply with Descartes’s entry point into physics, namely, by positing “body” or “extension” (corpus) as what is most known. Newtonian physics and, by extension, classical physics begin with corpus as the most known element in the study of nature.
St. Thomas holds for reasons of the logical structure of scientific demonstration that ens mobile must be the subject of natural philosophy.
This, then, is the book, The Physics, which is also called On Physics, or Of the Natural to be Heard, because it was handed down to hearers by way of instruction. And its subject is mobile being simply. I do not, however, say mobile body, because the fact that every mobile being is a body is proven in this book, and no science proves its own subject. And thus in the very beginning of the De Caelo, which follows this book, we begin with the notion of body. (see In Phys., lib. 1, lect. 1, n. 4)
William Wallace, in his article “Newton’s Early Writings: Beginnings of a New Direction” (in this collection), transcribes a portion of a student notebook of Newton’s connected to a textbook written by John Magirus, the Physiologia Peripatetica. The young Newton’s student notes on the subject of physics read in such a way that you could not tell they came from the same hand that would later pen the great Principia.
Physics, which is the science of natural bodies and whose subject is the natural mobile body, consists in the contemplation of nature, which is the cause of motion and rest in that in which it is primarily, essentially, and not accidentally.
Aside the from diligent repetition of the classical definition of nature, Wallace notes the following about this passage:
Newton’s entry . . . to the effect that the subject of the physics is ‘the natural mobile body’ represents a controversial teaching. Thomas Aquinas had held, contrary to Albertus Magnus, that this subject was mobile being (ens mobile) on the ground that its being ‘a body’ could be demonstrated . . . . Magirus discusses these and other opinions in his commentary, and attributes his teaching to the recentiores (i.e., the moderns), for which he cites Zabarella. . . . Newton, typically, shows no awareness of the problem and merely states Magirus’s resolution of it.
This approach of the recentiores, which might have influenced the more mature Newton’s musings about the fundamental concepts of physics insofar as he defines corpus through “quantity of matter,” obscured considerations that only later came to light, e.g., the conservation of energy and the extremal action principles of Lagrangian mechanics (more to follow in a future post).
Here is a relevant excerpt of Poinsot’s text:
Physics treats of body and mobile substance, but not formally to the same degree, but as drawn from the notion of mobile being, that is, mobile quiddity. As to the “being” in the formal object, we have already said that it is not explained by body or substance, and that it is not that being is taken in the mode of some higher-level notion, but in the mode of explicating the mobile quiddity and not positing something numerical in addition to it. Furthermore, it is not true that “mobile” names an essential difference with respect to body or substance, but only a property grounded in body. Thus, “mobile” will express not how Physics treats of mobility itself, which is a property, nor of the body itself absolutely, but as grounding that property of mobility. One expresses the actual notion of the formal object in a more formal way in more purely by saying “mobile” or “mobile being” (which is the same as if to say the foundation of mobility), rather than by saying “mobile body,” where one denotes that the very notion of body also enters into the formality of the object of Physics as body, and not only as mobile. Lastly, it is more known to us that something is mobile than that it is a body (even if body is its genus), because one proves divisibility and the notion of body through motion. Therefore the formal notion should not be explicated through “body,” but only through “mobile” or through “mobile being,” because the formal notion must be the most known thing in a science.
While the ancient natural philosopher takes the intelligibility of mobility itself as the light in which he sees nature, the mathematical physicist takes the intelligibility of quantity as the defining character of what is “most known” about nature.