4. Local Motion
a) Its nature. - Local motion is a fact of experience. Yet in the history of philosophy there have not been wanting those to question this experience. Despite the evidence of sense Achilles can never overtake the tortoise. This was Zeno's cry; but it harbored a fallacy, which lay in thinking that motion is composed of actually divided parts, though indivisible in themselves, when in fact motion is only potentially divisible. With this distinction no valid argument against local motion can be raised; logic and experience bear each other out.16
Experience also gives us the basis for the definition of local motion. When we see a thing passing from one
place to another, we say it has changed its place or locus (hence local motion). Local motion is therefore a change of place, or the transit from one place to another. More metaphysically, and in Scholastic phrase, it is the "act of the transitive as transitive,"
actus transeuntis ut transeuntis.
b) The cause of local motion. - Assuming for the present that whatever is moved is moved by another, then whatever moves locally is moved by another; the cause of its motion is extrinsic. Aristotle speaks of this motive causality being exercised in two ways, in the natural motion of bodies, and in the oblique motion of projectiles, things hurled or thrown.
Some bodies, being heavy, move naturally down; others, naturally light, move up. To explain this natural motion Aristotle invokes what he calls natural place (locus naturalis), to which its own nature inclines a thing. Accordingly, the motion of heavy bodies toward the center of the earth and the upward motion of light bodies away from the center are explained by these bodies seeking their natural place.
Natural place alone, however, cannot account for the oblique motion of projectiles. When a thing is uninterruptedly borne or pushed by a discernible agent, the cause of its translation is obvious. The ragpicker pushes his cart; he is a perceptible mover in unbroken contact with his cart. But far different is the motion of a thing which, after launched, seems to follow its path by itself, as for example a stone hurled in the air. Such motion was a real puzzle to the ancients, to whom the concept of energy and the law of inertia were unknown. Aristotle firmly believed that even here there was a mover in constant contact with the object, namely the surrounding air, which, stirred by the impinging object, became in turn the mover of the object.
The motion of projectiles was to become a central point in the development of modern physical theories. Already in the sixth century the Aristotelian commentator John Philoponus abandoned the theory that a projectile's motion was by constant push from the surrounding air. What he proposed instead was an impetus or thrust from within the projectile itself. This suggestion was later borrowed and turned to account by a leading professor at the University of Paris, John Buridan (fourteenth century). The conclusions he drew from it were nothing short of revolutionary, scientifically speaking. If, as John Buridan declared, the movement of celestial bodies comes from an internal thrust, the circular motion of celestial spheres can be explained without recourse to intelligent movers. Thus, by one stroke celestial mechanics is assimilated to the mechanics of sublunary bodies and the unification, or at least the integration, of all physical sciences is on the point of being realized.17
What John Philoponus had scarcely more than surmised was in modern times to find scientific expression. Descartes with his quantification of motion and Leibnitz with his concept of energy made great strides in this direction. When, not long after, Newton propounded the law of universal gravitation, the scientific obsolescence of Aristotle's explanation of local motion was complete.18 But the vicissitudes of scientific thought are inexorable. With the advent of Einstein and the atomic age, Newtonian physics itself was marked for eclipse.