  Since the claim that observable bodies are made up of particles is central to the corpuscular hypothesis, an immediate question for its proponents asks how the particles cohere into compound bodies. Plenists have some resources for answering that question, though those resources may introduce worse difficulties. Descartes, for instance, though he speaks in terms of particles, understands an individual body as an area of extension moving as one with respect to surrounding areas. It is not possible for any particle to move away into empty space, there being no such thing as empty space once matter is identified with extension; with every bit of matter pressed from all sides by other matter, there is no problem about cohesion per se, though there is certainly a problem about individuating bodies from one another. In the same vein, Malebranche can invoke the pressure of the air to explain the coherence of bodies, and then invoke the pressure of an aether to explain the coherence of air particles. Locke objects that this explanation fails because it leaves us with the question of what causes the particles of the aether to cohere (E II.xxiii.23, p, 308). The objection reveals Locke’s atomist sympathies, drawing its power from the presumption that there is such a thing as empty space into which the aether particles could move. The problem for atomist versions of the corpuscular hypothesis is that the restricted set of properties that they allow the particles —size, shape, and motion—provides no obvious resources for explaining how the particles cohere with one another to form compound bodies. In various forms, the problem about cohesion has dogged atomists since ancient times.