The paradigm of substance is the human self since we can verify its existence immediately. After Descartes, however, this self is understood as immaterial, thus making the material world insubstantial.* The material world became perfectly homogeneous since there was no longer any foundation in it by which things might differ. All distinction was introduced into it from outside, and thus from immaterial substance. But the introduction of distinction into homogeneity gives us first of all numbers and shapes. Whammo – physics becomes math.
What can we say about this? It’s true that if we are the paradigm of substance that we have to decide whether we are more like or more dissimilar from the other things of sense. Is the self-reflection of human thought an insight into all natural things, or something that is better taken as dividing persons from nature?
*The Cartesian hypothesis of animals and plants as sort of machines is a logical extension of this, for they must perform complex operations without being substances.
All this is a different perspective on the modern critique of Scholastic occult ideas, the chief of which is substance. Substance, in the Cartesian view, seems to be a way of taking about what is unique to the immaterial, and therefore has no place in physics. Given human nature, we can expect the insubstantiality of nature to be accepted long after the theoretical basis of this (the immateriality of substance) is forgotten.