Descartes’ view on the vacuum, or void, in nature relied on metaphysical rather than physical considerations. The Scholastic-Aristotelian denial of the possibility of a void had itself rested fundamentally on issues of principle rather than physical contingency, and despite occasional medieval arguments based on God's omnipotence, there was in practice little serious Scholastic dispute on the question in Descartes’ time. For Aristotle, place and space were interdefinitional concepts: space was always the place of something, and where nothing was in a place, there was no space there to be spoken about (see place, external versus internal). Characteristically, Aristotle happily multiplied arguments against void space despite the apparent conclusiveness of this basic metaphysical one in his Physics (bks. 4, 7). In discussing the speed of a moving body, he associated a higher speed with a greater moving force, but also related it inversely (whether strictly or not) to the resistance to motion: less resistance, faster motion; greater resistance, slower motion, all other things being equal. This enabled the observation that, counterfactually, if a body moved in a void, which by definition exerted no resistance, the body once pushed would move with infinite speed to its terminus. Since the concept of motion was necessarily sequential, as the body moves from its starting point to its end, the idea of an instantaneous translation from start to finish was incoherent. Hence, a medium with no resistance – a void – could not exist.

Descartes’ position on this subject broadly resembles the Aristotelian, in that it relies on a metaphysical argument for the void's in-principle impossibility. He holds that matter itself is nothing other than spatial extension, on the grounds that this is the only clear and distinct characteristic of matter (see clarity and distinctness). If, then, the definition of matter is identical to that of spatial extension, space itself differs in no essential respect from matter; the two are identical. Consequently, a region of space devoid of matter – a void or vacuum – is an impossibility (AT VIIIA 49, CSM I 229–30). Empirical considerations are entirely subsidiary to this argument.