  Philoponus’ impetus theory ties in with other far-reaching criticisms of Aristotelian principles of physics. Aristotle had dismissed the notion of a void as a logical impossibility. Philoponus concedes that in nature empty spaces never become actual, but he insists that a clear conception of the void is not only coherent but also necessary if one wants to explain movement in a plenum. When bodies move and in consequence exchange places, this presupposes that somehow there is empty space available to be filled by them (In Phys. 693f.). Again, there are certain phenomena which clearly exhibit the force of the vacuum, for example handling a pipette (clepsudra), which allows one to raise small quantities of fluids, or the fact that one can suck up water through a pipe (In Phys. 571f). Philoponus’ elaborate defense of the void (In Phys. 675–94) is closely related to his conceptions of place and space (In Phys. 557–85). Aristotle defined the place of a body as the inner surface of the body or all the bodies taken together that contain it (Phys. IV 4); Philoponus replies that place ought instead to be conceived as the three-dimensional extension identical to the determinate size of the given body, i.e. its volume. This was also the theory of the Stoic Chrysippus, but there is no indication in the extant text of Philoponus (as there are in his later works) that he saw himself influenced here by the Stoics.