Impetus

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Aristotle distinguished between two sorts of motion: natural and unnatural. Natural motions were those induced by the elemental constituents of things to seek their natural places—the earth at the center of the cosmos, fire under the periphery, water and air in their intermediate locations. Anything not in its natural place (i.e., not at that point in the stratification of things appropriate to its elemental composition) has an internal inclination to reach its natural place that will be exercised so long as nothing impedes it. Moreover, the speed of any body in natural motion (i.e., approaching its natural place) is a function of its heaviness in the case of downward motion, lightness in the case of upward, and an inverse function of the resistance of the medium through which it moves.

Not all motions, however, are natural; heavy objects can be hurled upwards, buoyant ones forcibly submerged. These unnatural motions are the result of force, yet Aristotle also notoriously held that in any change (including change of position) there must be a continuously acting agent of change; in the case of projectile motion, he supposed that the original action of the thrower endowed successive enveloping portions of the medium (air or water) with the ability both to receive and to transmit motive force; and so the projectile continues to move after it leaves the thrower's arm as a result of the continuing—albeit diminishing—power successively induced in the surrounding elastic medium. This explanation was often felt to be less than adequate; John Philoponus explicitly rejected it, supposing rather that the thrower imparted a certain quantum of force into the projectile, which it gradually exhausted in the course of its flight until it fell to earth.

Thus Philoponus crucially rejects the Aristotelian assumption that there must be continuous contact between a thing in motion and some external mover of it, speaking of an "induced power" (endotheisa dunamis) possessed—albeit temporarily and in this sense nonnaturally—by the moving body. This was to become the vis impressa of medieval theorists such as Jean Buridan, who attacked the reemerging Aristotelian orthodoxy with vigor (although perhaps without fully understanding it).

Impetus theory (the term is owed to Pierre Maruice Marie Duhem) thus maintains that all motion relies ultimately on the transmission of force from a mover, although not necessarily simultaneously with that motion, as Aristotle required. The theory also maintained that force gradually diminishes, although different versions of the impetus theory disagreed as to whether it does so simply because all movement requires force, and that force is like a fuel to be consumed, or (as some writers in the Arabic tradition such as Avicenna apparently held), only as a result of contact with a retarding medium (and hence that in a vacuum the impetus would continue for ever). Whereas both versions retain the ancient commitment to the view that all action requires a continuous active cause, the latter was a step in the direction of the inertial notions that would revolutionize physics. Galileo Galilei, in his early De Motu (1590) is still an impetus theorist, and gratefully acknowledges Philoponus and other predecessors for showing the way beyond Aristotle. By the time of the mature physics of the Discorsi (1638), impetus theory itself has been left decisively behind.

Aristotleburidan, Jean;; Galileo Galilei; Philoponus, John.

Wolff, Michael. "Philoponus and the Rise of Preclassical Dynamics." In Philoponus and the Rejection of Aristotelian Science, edited by R. Sorabji. London: Duckworth, 1987.