The Physics of Local Motion

Abstract

THE PHYSICS OF LOCAL MOTION I T HE science of local motion is fundamental in natural philosophyo Locomotion is the primary kind of motion; it is generated last of the three kinds of motion so that in order of being it is first, as the Philosopher has observedo1 Thus, owing to its primacy, a proper knowledge of the nature of local motion is necessary for a full understanding of the first of the five ways by which St. Thomas establishes by reasoning the existence of God as the Author of Nature, namely, the argument from motion in the world.2 In the philosophia perennis, unfolded by Plato, Aristotle, the later Greek and Roman philosophers, and the Jewish, Arab, and Christian doctors of succeeding ages, motion is recognized as a transition from a state of potentiality to a state of actuality; further, such a transition to actuality can only come about by the influence of something already in a state of actuality; hence we have the principle that whatever is in motion must be moved by anothero This principle illuminated the sciences of Nature and guided philosophers along sure paths during those ages in which the philosophia perennis was the inspiration of an but a few eccentric and misguided individuals. However, since the revolution in physical science in the seventeenth century and the appearance of sciences of Nature alien to the philosophia perennis, an entirely different doctrine concerning local motion has generally prevailed. Newton's first law of motion lays down that every body in the universe continues in its state of rest or of uniform motion in a straight line unless compelled by external force to change that state, On Newtonian 1 Aristotle, de Caelo, 310 b 33-35; Physic., 243 a 12; 260 a 26-29; etc. 2 Summa Theologiae, I, q. 2, a. 3. 145 146 GAVIN ARDLEY dynamics a freely moving body does not require an external mover; a mover is required only to change the motion of the body, i.e. to cause acceleration, not to continue its steady state of motion; the Newtonian theory of locomotion thus evokes the principle of the inertia or impetus or momentum possessed by bodies. Newton's doctrine is thus quite at variance with the doctrine of Aristotle and SL Thomas according to which a moving body must be moved continuously by another agent if its movement is not to cease. The Newtonian system was not wholly new; it was rather the definitive form of doctrines of motion which were propounded in the fourteenth century by Jean Buridan, Nicolas Oresme, and others of the University of Paris; doctrines which became ever more widely disseminated until their final triumph in the seventeenth century with Galileo, Descartes, Newton and hosts of others who participated in the elan of the times. These doctrines of motion are quite foreign to the philosophia perennis; with the triumph of the impetus doctrine in the seventeenth century came the general abandonment of the traditional doctrine of motion; the ancient science of nature with its rich patrimony of so many centuries henceforth found no hearing in the universities of Europe; the ancient learning was preserved in obscurity by a discriminating few, for the most part in the seclusion of the cloister, while the learned world followed new and seductive paths. Further, not only were the Aristotelian doctrines of motion abandoned in the seventeenth century, but the whole framework of the philosophia perennis was given up at the same time by the generality of philosophers, and indeed has :remained in general oblivion to this day. There are no doubt many reasons for this great seventeenth century tidal movement in philosophical opinion, but prominent among them is this: it was believed very widely that the experimental physics of Galileo and Newton had overthrown the Aristotelian physics, and it was further generally believed that this defeat in the field of physical science was of such a fundamental character that the THE PHYSICS OF LOCAL MOTION 147 whole Aristotelian system was thereby rendered untenable, and that other philosophies of the world must be sought. The science of motion was certainly not the prime mover in the philosophical revolution; for that we must...