In his Metaphysics, Aristotle maintained that an individual unmoved mover would be required to insure each individual motion in the heavens. 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. In On the Heavens, Aristotle presented an alternate view of eternal circular motion as moving itself, in the manner of Plato's world-soul, which lent support to three principles of celestial motion: an internal soul, an external unmoved mover, and the celestial material (aether). Later Greek interpreters In his Planetary Hypotheses, Ptolemy (c.90–168) rejected the Aristotelian concept of an external prime mover, maintaining instead that the planets have souls and move themselves with a voluntary motion. Each planet sends out motive emissions that direct its own motion and the motions of the epicycle and deferent that make up its system, just as a bird sends out emissions to its nerves that direct the motions of its feet and wings. John Philoponus (490–570) considered that the heavens were made of fire, not of aether, yet maintained that circular motion is one of the two natural motions of fire. 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." This is interpreted as an application of the concept of impetus to the motion of the celestial spheres. In an earlier commentary on Aristotle's Physics, Philoponus compared the innate power or nature that accounts for the rotation of the heavens to the innate power or nature that accounts for the fall of rocks. Islamic interpreters 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. A first intelligence emanated from God, and from the first intelligence emanated a sphere, its soul, and a second intelligence. The process continued down through the celestial spheres until the