The notion of substance played a key role in the metaphysical thought of the Middle Ages.
But if the idea of an independent being is construed in the strongest possible sense, as something whose existence is entirely self-sufficient, then one might conclude that the term "substance" should strictly speaking be reserved for God alone, since according to standard Christian doctrine He alone is the eternal source of all being, and the existence of everything else is dependent on Him.
This is precisely the line taken several centuries later by Rene Descartes in his Principles of Philosophy. God, according to Descartes, is the sole substance in the strict sense. Created things can count as substances only in a secondary sense. But in his account of created things Descartes makes a striking departure from the framework for understanding reality, which Aristotle had offered. The Cartesian framework for explaining the physical world offers a radically new ontology.
Aristotle grouped individual substances together as belonging to natural kinds (species and genera) and among scholastic medieval philosophers (those who followed a broadly Aristotelian approach), a great deal of energy was spent on classifying natural phenomena and explaining the way things behaved in terms of essential characteristics of the natural kind to which they were taken to belong.
Ushering in the scientific revolution of the seventeenth century Descartes argues, however, that to understand natural phenomena we need instead to adopt a mathematical approach. What matters for explanation in physics are not "differences in kind" but a quantitative analysis, expressible in terms of strict mathematical laws.
Consequently, we find in place of the traditional plurality of individual substances belonging to various natural kinds, just one essential kind of matter. The whole universe is composed of a single "extended stuff". All phenomena are to be explained quantitatively in terms of the size, shape and motion of its particles.
Descartes' account of the world conceives of matter as a single extended body, indefinitely modifiable as to its dimensions and dependent only on the supreme substance, God, for its existence and the movement of its parts.
To complete the picture, there are, in addition to the creator and the physical world, created minds or souls. These are individual centers of consciousness whose existence, Descartes maintains, does not require anything material.
Descartes' ontology, therefore, gives us three categories of substance. First substance in the strict sense, the independent, self sufficient creator, God; second, extended substance, or matter; and third thinking substance, the category to which created minds belong.
