  A central part of On Rays explains that the stars and planets bring about events in the sublunary world by means of rays emitted from the heavenly bodies to points on the earth’s surface. This differs from an account found in several other cosmological treatises by al-Kindi, where he follows Alexander of Aphrodisias in holding that the heavenly bodies literally heat up the lower world by means of friction as they pass over it. In either case, however, the account given is intended to explain the efficacy of the science of astrology. Al-Kindi wrote numerous works on this subject, and his associate Abu Ma‘shar was the greatest figure in Arabic astrology. Both of them saw astrology as a rational science, undergirded by a well-worked out theory of physical causes (see further Burnett 1993, Adamson 2002a).