Judging by the senses, we might easily mistake the quantitative extension of a thing for its substance. In appearance, say they are continuous. As St. Thomas remarks, magnitude or concrete quantity is that which can be divided into continuous parts: quod est divisibile in partes continuo. Thus, a line is at least, the quantity and substance of an object are indistinguishable. This may be the reason why, as mentioned earlier, some philosophers like Descartes profess what amounts to a mere distinction of reason between these two modes of being. By this reckoning the substance of bodily things is really nothing more than their quantification or extension. Nevertheless, in the Aristotelian tradition as well as in Scholastic philosophy generally there is no room for anything less than a real distinction between corporeal substance and its quantity or extension.
The proof of this thesis as well as the refutation of Descartes' position belongs primarily to metaphysics and epistemology; hence our remarks should not be taken as the full answer. Among other things, however, it may be noted that the formal effects of these two modes of being are so different as to seem irreducible. By substance a thing has absolute or subsistent existence (esse simpliciter), and its unity, too, comes from substance. Quantity, on the other hand, orders the parts of substance and accounts for its divisibility. Functions so diverse can scarcely be the work of the same principle; two separate principles are clearly indicated, one really distinct from the other and the first presupposed by the second. Again, the quantity of a body may change without modification of its substance. Generally speaking, moreover, quantity belongs to the sensible order, whereas substance, in the strict meaning, is not accessible to sense but only to intellect.
Notwithstanding this real distinction, quantity enjoys a proximity with substance that is unique. Quantity is, in fact, the first and immediate disposition of substance. For this reason it has, also, a priority among accidents, so that other accidents presuppose quantity as their immediate subject but substance as their ultimate subject. This closer solidarity of substance and spatial dimensions bulks large in metaphysics as well, specifically in the individuation of corporeal substance. Here, too, dimensive quantity interposes and is a necessary principle, the determinant of matter. Matter, I mean, individuates, but matter that is signate or quantified.
Accordingly, in maintaining the distinction between quantity and substance we must not overlook the many ways in which quantity touches and impenetrates substance. Nor, in Aristotle's physics, must quantity be discounted in favor of quality. To be sure, Aristotle's physics is in large measure qualitative, at least by comparison with the quantitative physics of today. But to see only its qualitative texture is to get a grossly distorted view. Dimensive quantity, even in Aristotle's physics, is just as important as quality, nay more important; it, and not quality, is the primary, the radical disposition of the being of nature. Aristotle, in this particular, is far less removed from Descartes than is sometimes imagined.