The Cambridge Descartes Lexicon

In the autumn of 1631, Descartes comments favorably on the content of a Memoir by Étienne de Villebressieu, which states that “the nature of these elements … which are called earth, water, air and fire consists only in the difference between the fragments, or small and large particles” of one and the same type of matter (AT I 216, CSMK 33). When he writes to Villebressieu, Descartes had already drafted chapter 5 of The World, which explains that three elements form “all the bodies of which the universe is composed.” The first one, which “may be called the element of fire,” consists of extremely fast and small particles that constitute the matter of the sun and of the fixed stars. Owing to their extreme subtlety and to their capacity to “change shape at every moment to accommodate themselves to the shape of the places they enter,” these particles are also able to fill the gaps among the particles of the other two elements. The second element, “which may be called air,” is made of “more or less round particles” of different size which constitute celestial vortices (see vortex); the third element, “namely that of earth,” which makes up the earth, the planets, and the comets, is composed of comparatively large parts “which have very little or no motion that might cause them to change position with respect to one another” (AT X 23–31, G 16–21). There is no qualitative difference between the elements: the two pairs of contraries (hot and cold, moist and dry), which characterize the four Aristotelian elements, are “themselves in need of explanation,” being like all other qualities the result of the “motion, size, shape and arrangement” of the particles of matter (AT X 26, G 18). In the Principles of Philosophy (1644), Descartes reiterates his conviction that only one type of matter exists and that “all the properties which we clearly perceive in it are reducible to its divisibility and consequent mobility in respect of its parts” (AT VIIIA 52–53, CSM I 232), but he does not associate his own elements with air, fire, and earth anymore.

Both in The World and in the Principles, Descartes offers a hypothetical reconstruction of the emergence of the three elements out of an undifferentiated first matter.