Aquinas on Motion

Abstract

Motion is the central phenomenon that Aristotle's physics endeavors to explain, and the whole superstructure of his natural philosophy is bound to it. This was certainly understood by Thomas Aquinas, who produced a most careful and thorough account of the Aristotelian theory of motion. What is rarely recognized is that in so doing he developed and improved that theory in a number of respects. This dissertation is a study of the theory of physical motion in Aquinas. It has two principal results. The first regards the concept of motion itself. Aquinas accepts Aristotle's definition of motion, but gives his own explanation of it, one which employs non-Aristotelian ideas like participation, and places motion in a more general and cohesive system of relations between non-being and being. The second regards what is called natural motion, i.e., that which inanimate matter exhibits of its own accord. How Aquinas explains this has rarely been understood and has frequently been misunderstood. I provide a thorough, systematic treatment that sets forth the principles underlying Aquinas's theory of natural motion, explains in what sense such motion is said to be caused, and notes – what commentators are prone to overlook – the critical role played by analogy as an aid to grasp the causality of natural motion. Chapter 1 briefly introduces Aquinas as a medieval "physicist", i.e., natural philosopher, specifies the scope of the dissertation, and lays out the path to be followed in the succeeding chapters. Chapter 2 summarizes what Aristotle says about motion in his Physics. His definition of motion is presented, and the terms in which it is given are analyzed. As the definition is famously perplexing, and its meaning a matter of debate, the opinions of a number of modern commentators are reviewed. After having explained the primacy that Aristotle assigns to locomotion and the connection he establishes between motion and time, the chapter concludes with a section on the causes of motion. This covers Aristotle's arguments for the eternity of motion, his denial of pure self-motion, introduces his views on animal, natural, and violent motion, and concludes with the first mover and first motion reached at the end of the Physics. Chapter 3 is a short introduction to the tradition of Aristotelian commentary. It contains brief reviews of some of what Simplicius, John Philoponus, Averroes, and Avicenna say about the Aristotelian theory of motion, with especial attention given to those aspects of it with which they disagree or which they find troublesome. The next three chapters are devoted to Aquinas. Chapter 4 covers how Aquinas interprets Aristotle's definition of motion and how he integrates it into his own metaphysics. The chapter begins with two preparatory notes. First, it introduces Aquinas as a commentator, and stresses the importance of having the context of the commentary tradition in mind when reading Aquinas, because he belongs to it and approached Aristotle in much the same way as earlier commentators. This has the consequence that his own developments are somewhat concealed, since what he says in his commentary on the Physics is presented as straightforward exposition, without any suggestion of originality. Second, modern readers are cautioned not to import modern senses of "motion" unwittingly into Aquinas, as this leads to confusion. The point is made that his Latin usage is regular and well-defined. After this, Aquinas's interpretation of the definition of motion is presented and what he means by calling it "imperfect act" is explained. This involves a sophisticated theory of the order of act to act, and incorporates the Neoplatonic language of participation and perfection. The next section of the chapter presents some of the analyses that Aquinas makes of motion, including its mental character and categorization. It is noteworthy, yet in fact quite unnoticed, that Aquinas provides a subtle and inventive solution to a modern debate over the proper interpretation of the definition of motion (the so-called "process" vs. "non-process" debate) which reconciles the two sides. Finally, I present a visual analogy as an aid for grasping how motion fits into a larger Thomistic metaphysical scheme concerning the relationship between non-being and being. Chapter 5 treats Aquinas's account of natural motion. It begins by noting some of the problems involved with Aristotle's explanation of natural motion, including, critically, that of the cause of such motion. It then highlights the work of James Weisheipl, who rejected a motor coniunctus interpretation of natural motion and offered in its stead his own, one which has since become well-known. But a careful examination of what Aquinas says shows that Weisheipl's interpretation of him is incorrect, and must also be rejected. The chapter then lays out (1) the principles of motion and their schematic organization; (2) Aquinas's theory of efficient causality and how we are to understand his denial of the possibility of action at a distance; (3) his use of analogies to indicate how we are to understand the efficient cause of natural motion; and then finally (4) shows, first, how the multiplicity and variation of the analogies lead commentators to misread Aquinas, and, then, how they illuminate other aspects of his theory of motion. Chapter 6 treats, in turn, the much debated principle that "everything that is moved is moved by another", and then the arguments that Aquinas gives for the existence of an unmoved mover, which he takes to be God. With regard to the first, it is pointed out that the theory of inertia is not nearly so fatal to the principle as many historians of science assume. Indeed, understood in light of what was said about Aquinas's understanding of the efficient causality involved in natural motion in the preceding chapter, it is compatible with inertia. However, a review of the three arguments Aquinas takes from Aristotle purporting to prove the principle are all found to be subject to serious objections. Greatest attention is given to the argument drawn from the claim that a thing cannot simultaneously be in act and in potency in the same respect. With regard to the second, it is shown that what Aquinas has to say about the unmoved mover is perplexed and inconsistent. Aquinas accepts two series of arguments from Aristotle, one from the Physics that concludes to a first moving cause, another from the Metaphysics that concludes to a final cause. From the former, Aquinas constructs his own argument from motion, most famously presented as the prima via, but at times he attempts to combine the two Aristotelian series. It is shown that these attempts at harmonization involve Aquinas in inconsistencies. What the prima via revision concludes to is also discussed. An important observation is that the prima via is manifestly intended by Aquinas to be an argument from physical motion. Yet numerous commentators, perceiving its weakness, attempt to recast it in a non-physical form, such that "motion" no longer signifies what Aquinas intends. The chapter ends with some cautious remarks on the state of these highly controversial topics. Chapter 7 concludes the dissertation, first, with a brief recapitulation of some of its major points, and then with some speculation as to what use its results may have for future scholarship.