Motion in the Void and the Principle of Inertia in the Middle Ages

A AN INTEGRAL part of his repudiation of an actually existent void space, Aristotle in his Physics describes the properties which motions would possess in void space (Bk. IV, Chs. 6-9). Two of these properties will be of particular interest here in the course of summarizing significant reactions of a few important scholastics to the possibility of motion in the void. Intimately associated with this problem is the nature of a resistant medium, and of resistance to motion in general. Hence some discussion of this is indispensable in dealing with the major theme. The fundamental notion of void as formulated by Aristotle is a " place deprived of body . . . either unseparated or separated " (IV. 7. 214a. 17-20) . Among the absurdities which would follow from the existence of a void, two are especially relevant to the objectives of this paper. The first, paradoxically, is an enunciation of inertial motion deriving from an application of the principle of sufficient reason to conditions obtaining in a void. Since a material medium and natural places are lacking in a void, " no one could say why a thing once set in motion should stop anywhere; for why should it stop here rather than here? So that a thing will either be at rest or must be moved ad infinitum, unless something more powerful get in its way" (IV. 8. 215a. 19-21).2 This important statement evoked almost no critical