sphere of the Moon, its soul, and a final intelligence. They considered that each sphere was moved continually by its soul, seeking to emulate the perfection of its intelligence. Avicenna maintained that besides an intelligence and its soul, each sphere was also moved by a natural inclination (mayl). An interpreter of Aristotle from Muslim Spain, al-Bitruji (d. c.1024), proposed a radical transformation of astronomy that did away with epicycles and eccentrics, in which the celestial spheres were driven by a single unmoved mover at the periphery of the universe. The spheres thus moved with a "natural nonviolent motion". The mover's power diminished with increasing distance from the periphery so that the lower spheres lagged behind in their daily motion around the Earth; this power reached even as far as the sphere of water, producing the tides. More influential for later Christian thinkers were the teachings of Averroes (1126–1198), who agreed with Avicenna that the intelligences and souls combine to move the spheres but rejected his concept of emanation. Considering how the soul acts, he maintained that the soul moves its sphere without effort, for the celestial material has no tendency to a contrary motion. Later in the century, the mutakallim Adud al-Din al-Iji (1281–1355) rejected the principle of uniform and circular motion, following the Ash'ari doctrine of atomism, which maintained that all physical effects were caused directly by God's will rather than by natural causes. He maintained that the celestial spheres were "imaginary things" and "more tenuous than a spider's web". His views were challenged by al-Jurjani (1339–1413), who argued that even if the celestial spheres "do not have an external reality, yet they are things that are correctly imagined and correspond to what [exists] in actuality". Medieval Western Europe In the Early Middle Ages, Plato's picture of the heavens was dominant among European philosophers, which led Christian thinkers to question the role and nature of the world-soul. With the recovery of Aristotle's works in the twelfth and thirteenth centuries, Aristotle's views supplanted the earlier Platonism, and a new set of questions regarding the relationships of the unmoved movers to the spheres and to God emerged. In the early phases of the Western recovery of Aristotle, Robert Grosseteste (c.1175–1253), influenced by medieval Platonism and by the astronomy of al-Bitruji, rejected the idea that the heavens are moved by either souls or intelligences. Adam Marsh's (c.1200–1259) treatise On the Ebb and