two species of entities, matter and soul. The essence of matter is spatial extension; the essence of soul is its cogitation, in the full sense which Descartes assigns to the word ‘cogitare’ For example, in Section Fifty-three of Part I of his Principles of Philosophy, he enunciates: “That of every substance there is one principal attribute, as thinking of the mind, extension of the body.” In the earlier. Fifty-first Section, Descartes states: “By substance we can conceive nothing else than a thing which exists in such a way as to stand in need of nothing beyond itself in order to its existence.” Furthermore, later on, Descartes says: “For example, because any substance which ceases to endure ceases also to exist, duration is not distinct from substance except in thought; . . .” Thus we conclude that, for Descartes, minds and bodies exist in such a way as to stand in need of nothing beyond themselves individually (God only excepted, as being the foundation of all things); that both minds and bodies endure, because without endurance they would cease to exist; that spatial extension is the essential attribute of bodies; and that cogitation is the essential attribute of minds.
It is difficult to praise too highly the genius exhibited by Descartes in the complete sections of his Principles which deal with these questions. It is worthy of the century in which he writes, and of the clearness of the French intellect. Descartes in his distinction between time and duration, and in his way of grounding time upon motion, and in his close relation between matter and extension, anticipates, as far as it was possible at his epoch, modern notions suggested by the doctrine of relativity, or by some aspects of
