Principles of Philosophy

Descartes's attempt at a complete system, written in short numbered articles so that it could serve as a university textbook in place of the Aristotelian ones. It was dedicated to Princess Elisabeth of Bohemia. Part I restates his metaphysics: methodical doubt, the cogito, the existence of God, the causes of error in the will outrunning the intellect, and the doctrine that there are two kinds of created substance, thinking and extended. Part II turns to the principles of material things. Matter is nothing but extension, so there is no vacuum, and Descartes states three laws of nature, including an early form of the principle of inertia, and rules for collisions. Part III builds a cosmology of swirling vortices of matter carrying the planets round the sun, framed so that the earth is strictly at rest in its own vortex. Part IV explains the earth, from heavy bodies and tides to magnetism. The 1647 French edition adds a preface comparing philosophy to a tree.

It shows how Descartes's metaphysics was meant to ground a whole mechanical science, and it was the physics that Newton's Principia later set out to replace.