Descartes ardently opposed atomism. His natural philosophy was appealing in part because it offered a well-worked-out plenist alternative (see plenum) to the atomist philosophies that were in vogue in his day – one that could take advantage of the atomists’ style of explanation of phenomena in terms of the size, shape, and motions of microcorpuscles, but without the need for the atomists’ dangerous metaphysics, which rendered an immaterial God and soul superfluous. Nevertheless, there are certain aspects of Descartes’ views that have led readers to suspect that his opposition to atomism was not as absolute as his words suggested.

In his Principles of Philosophy (1644), Descartes explicitly rejects the two founding assumptions of atomism. First, he argues that it would be contradictory for there to be an absolute vacuum (AT VIIIA 50, CSM I 230–31); then he offers a demonstration of the impossibility of atoms, arguing that their existence would be contrary to the fact that matter is essentially divisible (see divisibility): “It is impossible that there should exist atoms, that is, pieces of matter that are by their very nature indivisible, as some philosophers have imagined. For … no matter how small we imagined them to be, they would necessarily have to be extended, and hence we could … recognize their divisibility” (AT VIIIA 51, CSM I 231). The fact that matter is necessarily extended precludes the kind of point atoms entertained by the Zenonists, whereas the divisibility of extension precludes something being both indivisible and extended, as were the atoms proposed by Descartes’ contemporaries Sebastian Basso, Daniel Sennert, Jean-Chrysostôme Magnen, and Pierre Gassendi. Descartes adds that even if God had made some particles “indivisible by any of his creatures,” he himself would still be able to divide them – a weak argument, to which Leibniz (2001, 25) responded, “This Gassendi would not have denied.”

Much more influential are Descartes’ arguments for the possibility of motion in a plenum. Against the atomists’ argument that a motion could not begin unless there were a void space for a body to move into, Descartes argues in the Principles that this objection is evaded if a ring of matter begins moving simultaneously; and against the objection that voids or condensations would be created by circulation of matter through unequal spaces, he argues that continuity can be preserved if the flow of matter through a space is swifter in inverse proportion to the width (i.e., cross-sectional area) of the space (AT VIIIA 58–59, CSM I 237–39).