The famous argument which Aristotle employs in that passage of his Physics known to the Schoolmen as Text 71 of Averroes' commentary on the fourth book, enjoys a long and distinguished history. Projected originally as a critique of the Atomists' most fundamental position — i.e., that reality is ultimately reducible to matter in motion through the void — it proved still later, as E. A. Moody and others have shown, to be equally potent as a germinal agent. The argument itself embodies, as basic, Aristole's belief that the speed with which a heavy body falls through corporeal media of diverse densities is in inverse proportion to the densities of the resistant media. Hence, Aristotle holds that the velocity of a body falling freely through a rarefied medium will be proportionally greater than if that body were passing through a more dense medium. It follows on this assumption that were a body to be let drop through a medium offering no resistance — a void — its speed would be infinitely great. As the traversal of a finite magnitude of totally evacuated space would thus, necessarily, be accomplished in no time at all, Aristotle concludes the impossibility of any motion taking place in a void.