Dynamics of the celestial spheres

[4] This account was compatible with Aristotle's meteorology[5] of a fiery region in the upper air, dragged along underneath the circular motion of the lunar sphere.

[8] While the four terrestrial elements (earth, water, air and fire) gave rise to the generation and corruption of natural substances by their mutual transformations, aether was unchanging, moving always with a uniform circular motion that was uniquely suited to the celestial spheres, which were eternal.

While stipulating that the number of spheres, and thus gods, is subject to revision by astronomers, he estimated the total as 47 or 55, depending on whether one followed the model of Eudoxus or Callippus.

[17][18] In his Planetary Hypotheses, Ptolemy (c. 90 – c. 168) rejected the Aristotelian concept of an external prime mover, maintaining instead that the planets have souls and move themselves with a voluntary motion.

[22] In a theological work, On the Creation of the World (De opificio mundi), he denied that the heavens are moved by either a soul or by angels, proposing that "it is not impossible that God, who created all these things, imparted a motive force to the Moon, the Sun, and other stars – just as the inclination to heavy and light bodies, and the movements due to the internal soul to all living beings – in order that the angels do not move them by force.

[29] The Islamic philosophers al-Farabi (c. 872 – c. 950) and Avicenna (c. 980–1037), following Plotinus, maintained that Aristotle's movers, called intelligences, came into being through a series of emanations beginning with God.

[32] 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.

[34][35] 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.

[39] 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.

"[39] 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.

[41] 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.

"[58] According to Grant, except for Oresme, scholastic thinkers did not consider the force-resistance model to be properly applicable to the motion of celestial bodies, although some, such as Bartholomeus Amicus, thought analogically in terms of force and resistance.

[59] By the end of the Middle Ages it was the common opinion among philosophers that the celestial bodies were moved by external intelligences, or angels, and not by some kind of an internal mover.