The French philosopher René Descartes (1596-1650) made an even more radical break with Aristotelian physics than did Galileo. He abandoned the notion of the four elements (earth, water, air and fire) with their associated qualities (hot, cold, wet and dry) and their natural places (at the center or periphery of the cosmos). Instead, Descartes posited that all matter was made up of particles in motion. He divided matter into three elements: fire, air and earth. The smallest, fastest-moving particles were the element fire. Air particles were larger and slower than fire particles. And earth particles were the largest and slowest of all. The entire cosmos was made of these three elements. Further, the entire cosmos was filled with matter; there was no empty or void space. All particles of all the elements were constantly in motion. As one particle moved, it displaced another particle, which in turn displaced another particle, which displaced another, and so forth. Eventually, somewhere in this chain, a particle would have to take the place of the first particle. This meant that all motion was effectively circular motion, although this was certainly not the uniform circles of the Aristotelian ether.
Descartes posited that the sun and stars were made of fire. The heavens were made almost entirely of air, although they contained a small amount of the element fire. The earth, the other planets, and comets were made of earth. Each star was the center of its own heaven, meaning that the sun was the center of our heaven, as Copernicus had argued, and also that there were an indefinite number of other heavens, possibly with planets and inhabitants like our own. The second element, air, swirled around the central fire, carrying with it the planets. The motion of the air not only moved the planets around the sun, but also caused them to rotate on their own axes. The air closest to the sun moved fastest, while that at the periphery of the heavens moved slowest, so that the planet closest to the sun, Mercury, moved fastest, and the most distant, Saturn, moved slowest. Comets were earthy objects that moved between different heavens, hence their far more irregular appearance and path. Finally, mixed bodies, that is, those composed of all three elements, were only found on the surface of the earth, and they were produced by the agitation and mixing of the matter of the surrounding heavens. By the surface of the earth, Descartes meant the crust, extending some way underground. Mixed objects included metals and minerals. In sum, all terrestrial and celestial phenomena were produced by particles of matter colliding with each other, pushing each other and resisting each other. Descartes’ physics had the advantage of being considerably more comprehensive than Galileo’s, and of accommodating phenomena like comets and the rotation of the earth on its axis in more logically consistent ways. For a time, Cartesian physics achieved considerable prominence and inspired other natural philosophers eager to replace Aristotelian physics.
René Descartes, The World and Other Writings, trans. and ed. Stephen Gaukroger (Cambridge: Cambridge University Press, 1998).