In agreement with the organic theory of nature which I have been tentatively putting forward in these lectures, I shall for my own purposes construe James as denying exactly what Descartes asserts in his _Discourse_ and his _Meditations_. Descartes discriminates 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 Bergson’s doctrine of the generation of things. But the fundamental principles are so set out as to presuppose independently existing substances with simple location in a community of temporal durations, and, in the case of bodies, with simple location in the community of spatial extensions. Those principles lead straight to the theory of a materialistic, mechanistic nature, surveyed by cogitating minds. After the close of the seventeenth century, science took charge of the materialistic nature, and philosophy took charge of the cogitating minds. Some schools of philosophy admitted an ultimate dualism; and the various idealistic schools claimed that nature was merely the chief example of the cogitations of minds. But all schools admitted the Cartesian analysis of the ultimate elements of nature. I am excluding Spinoza and Leibniz from these statements as to the main stream of modern philosophy, as derivative from Descartes; though of course they were influenced by him, and in their turn