Aristotle had explained how it all works, both in the heavens and on Earth, about three hundred years before the birth of Christ. For nearly two thousand years everyone found his scheme satisfactory. All earthly objects were formed from earth, air, fire, and water. The heavens were composed of a fifth element or essence, the quintessence, a pure, eternal substance, and it was only in that perfect, heavenly domain that mathematical law prevailed. Why do everyday, earthly objects move? Because everything has a home where it belongs and where it returns at the first opportunity. Rocks and other
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