are moved; for example, the intelligence that moves the Moon's epicycle shares the motion of the lunar orb in which the epicycle is embedded. He related the spheres' motions to the proportion of motive power to resistance that was impressed in each sphere when God created the heavens. In discussing the relation of the moving power of the intelligence, the resistance of the sphere, and the circular velocity, he said "this ratio ought not to be called a ratio of force to resistance except by analogy, because an intelligence moves by will alone ... and the heavens do not resist it." 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. 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. The movers and Copernicanism Although Nicolaus Copernicus (1473–1543) transformed Ptolemaic astronomy and Aristotelian cosmology by moving the Earth from the center of the universe, he retained both the traditional model of the celestial spheres and the medieval Aristotelian views of the causes of its motion. Copernicus follows Aristotle to maintain that circular motion is natural to the form of a sphere. However, he also appears to have accepted the traditional philosophical belief that the spheres are moved by an external mover. Johannes Kepler's (1571–1630) cosmology eliminated the celestial spheres, but he held that the planets were moved both by an external motive power, which he located in the Sun, and a motive soul associated with each planet. In an early manuscript discussing the motion of Mars, Kepler considered the Sun to cause the circular motion of the planet. He then attributed the inward and outward motion of the planet, which transforms its overall motion from circular to oval, to a moving soul in the planet since the motion is "not a natural motion, but more of an animate one". In various writings, Kepler often attributed a kind of intelligence to the inborn motive faculties associated with the stars. In the aftermath of Copernicanism the planets came to be seen as bodies moving freely through a very subtle aethereal medium. Although many scholastics continued to maintain that intelligences