We must note that Aristotle himself observes in his Physica (Book IV, Part 8), that "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", a very good description of the concept of inertia in vacuum. He does, however, proceed to dismiss this idea on the grounds that a vacuum cannot exist.