 The second set of arguments includes his explanation about velocity and the motion of bodies in the medium (Phys. IV.8, 215a29–216a21). Velocity depends on the heaviness or lightness of a falling body, but also on the density of the medium, water or air, for instance. The same body will fall at different velocities in accordance with the ratio of the various media. The role of the medium is to oppose resistance to any natural or violent motion, but Aristotle does not see it as a hindrance but as necessarily complementing the action of the moving agent. The void has no density and since there is no ratio in which it stands to a body, motion would happen within any imaginable time.