  Like place, space as a whole is the indeterminate three-dimensional extension everywhere devoid of body, though it is not actually infinite – so much Philoponus concedes to Aristotle. Philoponus’ discussion of matter builds upon this conception of space. In the Physics commentary (687f.) he argues in a similar vein to Aristotle in Metaphysics VII 3: In abstracting all qualities and other determinations from body Aristotle arrived at a conception of characterless, undetermined matter, an idea of matter like the “prime matter” that the Neoplatonists later defined as formless and incorporeal (because it was no actual body, but only the necessary underlying condition for bodies).  Philoponus, in contrast, arrives at something he calls corporeal extension (sômatikón diástêma), which is a composite of Stoic-Neoplatonic prime matter and indeterminate quantity and must not be confused with Philoponian space. His argument here may still be regarded as an elaboration on or correction of Aristotle. However, in Book XI of the polemical treatise against Proclus (see below, 3.1) he jettisons the Neoplatonic conception of prime matter and posits as the most fundamental level of his ontology ‘the three-dimensional’, as he calls it, i.e. indeterminately extended mass. In this he claims to be following the Stoics (Aet. 414), and it has been pointed out that this ontological level is reminiscent to us of the Cartesian res extensa, although Descartes would not allow Philoponus’ distinction between space and corporeal extension.  In order to rebuff the likely objection that ’the three-dimensional’ cannot be the most fundamental level of being because extension, belonging to the Aristotelian category of quantity, is an accident and requires the assumption of a distinct underlying subject, Philoponus argues that extension is in fact not an accident, but an essential and inseparable differentia of ‘the three-dimensional’, just like heat in fire or whiteness in snow. Thus quantity (corporeal extension) is constitutive of body as such. This amounts to a promotion of one sort of quantity to the category of substance.  There are indications that Philoponus would accordingly have modified Aristotle’s scheme of the Categories if he had revised his early commentary on that treatise.