Between Chapters 7 and 8 of Physics Book IV, Aristotle seems to commit a non sequitur. From a previous discussion about the necessity for void as a precondition for change, Aristotle abruptly turns to criticize an argument that the void is an aition of motion, specifically of motion upward or downward: he replies that void cannot be the aition if simple bodies have upward and downward motion by nature. The difficulty is to know how to understand the term aition at 214b15: Aristotle is often read as attacking the claim that void is no mere condition but the explanation of motion.1 Ross (1936, pp. 587-8) states this interpretation succinctly:

Aristotle’s argument here is not convincing. The supporters of the void put it forward as an a‡tion (necessary condition) of locomotion (213b4). He replies that it cannot be an a‡tion (determining cause) of locomotion in any particular direction. The ambiguity in the meaning of a‡tion makes the argument worthless.