 Aristotle rejected the possibility of motion in the void because the medium was essential to natural movement at finite speed (Phys. IV.8). John Philoponus had already expressed the view that the medium is not a necessary condition, but only provides resistance. The different velocities with which the stone passes through the air or the water is only caused by the different density of the medium; it is not connatural to the medium. As a proof that motion without any medium, namely, through a void, is possible, Avempace adduces the movement of the spheres: