 Al-Rāzī’s conception of matter is unusual among falāsifa, that is, thinkers of the Islamic world who respond primarily to the Greek philosophical tradition. More or less all of them, from al-Kindī (d. after 870) to al-Fārābī (d. 950), Ibn Sīnā (Avicenna, d. 1037), and Ibn Rushd (Averroes, d. 1198), endorse a broadly Aristotelian understanding of bodies as potentially subject to indefinite division. By contrast, al-Rāzī is an atomist (Pines 1936 [1997], Baffioni 1982, Langermann 2009). He believes that the four elements (or possibly five: our sources are not clear as to whether he thinks the heavens are constituted from a distinct fifth element) differ in their properties because of variations in the ratio of atoms to void. Thus earth, which has little or no void in it, is dark, cold, dense, and moves downward; fire is luminous, hot, subtle, and moves upward. Actually these are not strictly “elements” but only the most basic compounds of atoms, which are truly fundamental. Though contemporary Islamic theologians were also atomists (Dhanani 1994), al-Rāzī’s theory differs from theirs in some respects, and we know that he debated the topic of matter with a theologian named al-Miṣmaʿī.