Flow of the Sea, which was formerly attributed to Grosseteste, maintained al-Bitruji's opinion that the celestial spheres and the seas are moved by a peripheral mover whose motion weakens with distance. Thomas Aquinas (c.1225–1274), following Avicenna, interpreted Aristotle to mean that there were two immaterial substances responsible for the motion of each celestial sphere, a soul that was an integral part of its sphere, and an intelligence that was separate from its sphere. The soul shares the motion of its sphere and causes the sphere to move through its love and desire for the unmoved separate intelligence. Avicenna, al-Ghazali, Moses Maimonides, and most Christian scholastic philosophers identified Aristotle's intelligences with the angels of revelation, thereby associating an angel with each of the spheres. Moreover, Aquinas rejected the idea that celestial bodies are moved by an internal nature, similar to the heaviness and lightness that moves terrestrial bodies. Attributing souls to the spheres was theologically controversial, as that could make them animals. After the Condemnations of 1277, most philosophers came to reject the idea that the celestial spheres had souls. Robert Kilwardby (c. 1215–1279) discussed three alternative explanations of the motions of the celestial spheres, rejecting the views that celestial bodies are animated and are moved by their own spirits or souls, or that the celestial bodies are moved by angelic spirits, which govern and move them. He maintained, instead, that "celestial bodies are moved by their own natural inclinations similar to weight". Just as heavy bodies are naturally moved by their own weight, which is an intrinsic active principle, so the celestial bodies are naturally moved by a similar intrinsic principle. Since the heavens are spherical, the only motion that could be natural to them is rotation. Kilwardby's idea had been earlier held by another Oxford scholar, John Blund (c. 1175–1248). In two slightly different discussions, John Buridan (c.1295–1358) suggested that when God created the celestial spheres, he began to move them, impressing in them a circular impetus that would be neither corrupted nor diminished, since there was neither an inclination to other movements nor any resistance in the celestial region. He noted that this would allow God to rest on the seventh day, but he left the matter to be resolved by the theologians. Nicole Oresme (c.1323-1382) explained the motion of the spheres in traditional terms of the action of intelligences but noted that, contrary to Aristotle, some intelligences