Books 3 through 7 deal generally with motion and the process of change. By the eighth book he is prepared to make some loftier conclusions. He postulates the existence of a prime mover that accounts for all movement in the world. Moreover, he argues that the prime mover is itself unmoved, just as a builder need not be built. He goes on to argue that the mover must be eternal, since it produces eternal motion.On the Heavens deals with questions of the universe. Aristotle's picture placed the earth at the center, surrounded by two spheres. The outer sphere contained the fixed stars, and between this region and the earth was the sphere that contained the planets. As he attempts to find first principles, he argues that circular motion is primary, since it is complete (in that it does not need a reversal to return to its starting point). And since
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