Ancient, medieval and Renaissance astronomers and philosophers developed many different theories about the dynamics of the celestial spheres. They explained the motions of the various nested spheres in terms of the materials of which they were made, external movers such as celestial intelligences, and internal movers such as motive souls or impressed forces. Most of these models were qualitative, although a few of them incorporated quantitative analyses that related speed, motive force and resistance. The celestial material and its natural motions In considering the physics of the celestial spheres, scholars followed two different views about the material composition of the celestial spheres. For Plato, the celestial regions were made "mostly out of fire" on account of fire's mobility. Later Platonists, such as Plotinus, maintained that although fire moves naturally upward in a straight line toward its natural place at the periphery of the universe, when it arrived there, it would either rest or move naturally in a circle. This account was compatible with Aristotle's meteorology of a fiery region in the upper air, dragged along underneath the circular motion of the lunar sphere. For Aristotle, however, the spheres themselves were made entirely of a special fifth element, Aether (Αἰθήρ), the bright, untainted upper atmosphere in which the gods dwell, as distinct from the dense lower atmosphere, Aer (Ἀήρ). 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. Earth and water had a natural heaviness (gravitas), which they expressed by moving downward toward the center of the universe. Fire and air had a natural lightness (levitas), such that they moved upward, away from the center. Aether, being neither heavy nor light, moved naturally around the center. The causes of celestial motion As early as Plato, philosophers considered the heavens to be moved by immaterial agents. Plato believed the cause to be a world-soul, created according to mathematical principles, which governed the daily motion of the heavens (the motion of the Same) and the opposed motions of the planets along the zodiac (the motion of the Different). Aristotle proposed the existence of divine unmoved movers which act as final causes; the celestial spheres mimic the movers, as best they could, by moving with uniform circular motion.