It is not easy to recall the image of the universe which was taken for granted in the world tradition. In spite of its dramatic rendering (as in Dante), of the dialectical elaborations of Aristotle and St. Thomas, in spite of the fact that it held men's minds captive until the last three hundred years, and that its overthrow involved a religious upheaval, it is already dim, faded and remote. Even as a separate and abstract thing of theory it is not easy to recover. As something pervasive, interwoven with all the details of reflection and observation, with the plans and rules of behavior, it is impossible to call it back again. Yet, as best we can, we need to put before our minds a definitely enclosed universe, something which can be called a universe in a literal and visible sense, having the earth at its fixed and unchanging centre and at a fixed circumference the heavenly arch of fixed stars moving in an eternal round of divine ether, hemming in all things and keeping them forever at one and in order. The earth, though at the centre, is the coarsest, grossest, most material, least significant and good (or perfect) of the parts of this closed world. It is the scene of maximum fluctuation and vicissitude. It is the least rational, and therefore the least notable, or knowable; it offers the least to reward contemplation, provoke admiration and govern conduct. Between this grossly material centre and the immaterial, spiritual and eternal heavens lie a definite series of regions of moon, planets, sun, etc., each of which gains in rank, value, rationality and true being as it is farther from earth and nearer the heavens. Each of these regions is composed of its own appropriate stuff of earth, water, air, fire in its own dominant degree, until we reach the heavenly firmament which transcends all these principles, being constituted, as was just said, of that immaterial, inalterable energy called ether.