elements of the sublunar world tend to move in straight lines: earth downward, fire upward, water and air falling in between. Aether, on the other hand, naturally moves in circles. He further maintained that everything that is moving has to be set in motion by something else, and thus in order to avoid an infinite regress, he posited a first mover. Aristotle's descriptions of such a "prime mover" demonstrate how he mixes physics with metaphysics. In De Caelo, Aristotle equated the prime mover of all things with the sphere of the fixed stars, which was itself moving with unceasing motion. (De Caelo, Book I, chapter 9) In the Metaphysics, however, he placed an unmoved prime mover "behind" the fixed stars. He describes this transcendent first mover as eternal and without magnitude; he says that it causes circular movement, and that is the kind of movement that is most perfect, since it has no beginning or end; he states that it is good, and its activity is the highest form of joy. It seems that at one point Aristotle thought of the prime mover as somehow an integral part of the universe itself, and at another as existing outside space and time. These differences may mirror different objectives that Aristotle had at various points in his career. Aristotle's hierarchical model of the universe had a profound influence on medieval scholars, who modified it to correspond with Christian theology. Saint Thomas Aquinas, for example, re-interpreted the prime movers as angels. Backed up by religious authority, Aristotle's model lasted for centuries. Unfortunately, this had the effect of restraining the progress of science, as few people dared to challenge the authority of the church. Nevertheless, we can say of Aristotle that he made a contribution to astronomy simply by starting to ask certain questions about the universe, thereby stimulating other minds to do the same....
