  A longer and more detailed text is On Sleep and Dream (Abu Rida 1950, 293–311; also Ruffinengo 1997), which gives a naturalistic account of why prophetic dreams occur, and how they may be interpreted. Here al-Kindi’s chief source was Aristotle’s Parva Naturalia, which include the works On Sleep, On Dreams, and On Prophecy in Sleep. The extant Arabic version of these texts, which may well be related to the version used by al-Kindi, is importantly different from the Greek version, in that it admits that genuinely prophetic dreams can be sent from God (cf. Pines 1974). If al-Kindi knew this version then he follows it only in part: he embraces the idea of prophetic dreams, but does not claim that they come to us from God. To explain dreams al-Kindi invokes a faculty we have not yet discussed, namely imagination or phantasia. Following Aristotle, al-Kindi says that dreams occur when we are sleeping because the senses are no longer active, and the imagination has free rein to conjure up forms on its own. We are also given a physiological account of sleep, which departs from Aristotle by placing the imaginative faculty in the brain. Whereas Aristotle has some difficulty explaining, and is in fact rather skeptical about, the phenomenon of prophetic dreams, al-Kindi is enthusiastic about them. He even explains the various types of dream, with their accuracy determined by the physical state of the brain. But despite the physiological aspects of al-Kindi’s account, the fundamental explanatory work is done by the incorporeal soul, which “announces” its visions of the future to the imagination. Again, the rational soul grasps its objects by itself. Tellingly, al-Kindi thinks that sensation hinders this power of the soul, rather than contributing anything to it.