Philoponus Against Aristotle on the Void

Many of Aristotle’s predecessors (notably, the Atomists) had argued that motion was impossible without void. In Physics IV.8, Aristotle turns the argument on its head and claims that the existence of a void would make motion impossible. His arguments are grounded in two basic assumptions. First, we observe bodies moving at unequal speeds. Second, the speed of a moving body is a function of the density of the medium and the weight of the body. Therefore, a difference in speed must be due either to a difference in the density of the medium or to a difference in the bodies themselves (an ‘‘excess of heaviness or lightness’’). Aristotle’s most famous argument against the void runs as follows. If the same body moves through a void, its journey must take time. And that time must bear some proportionate relation to the times for corporeal media. But since the density of the void is zero, there cannot be any ratio between the void and those other media (e. g., air cannot be ‘‘twice as dense’’ as void). Therefore, motion through a void is impossible. In effect, Aristotle is asserting that motion through a void would take no time, that a body would move through the void with infinite speed (215b20-3), which is absurd.

In Corollary on Void (a discussion in his commentary on Aristotle’s Physics Books V to VIII) Philoponus attacks the claim that the density of the media varies in direct proportion to the time taken (684,10-20). According to Philoponus, motion through a void must take some time t (684,23-5), which is determined by the body’s inner rhope (its downward impulse). What the medium adds to this is extra time, which is the time needed to part the medium. What should be compared, then, are the additional times taken to part the medium: the density of the medium is proportional to that additional time (684,32-685,10). For the void, this additional time will be zero since it offers no resistance, and so there will be no ratio of extra time through the void to extra time through a bodily medium. In effect, Philoponus thinks Aristotle is wrong to assume that there is some calculable relation that holds between the time taken to move through different media and the densities of those media. The only proportional relation that holds is between density and extra time (Sorabji 2005a:333).

Philoponus’ rejection of Aristotle’s arguments against the void is connected with three substantial revisions of Aristotelian science: Aristotle’s conception of dynamics, his conception of prime matter, and his conception of place (or space).