III.

Aristotelian Physics of Galileo’s Time

Aristotelians made a sharp division between Earth and the heavens. In the heavens there could be no change except the recurring patterns produced by the circular motions of the perfectly spherical heavenly bodies. The sublunar world (the universe below the Moon) was the region of the four elements—earth, water, air, and fire—and was subject to its own distinct laws of natural motion. Fire, for instance, had lightness, which made it rise vertically, away from the center of Earth. Earthy objects fell naturally downward toward the center of Earth: the heavier the object, the faster its fall. “Natural” motions of the elements took them to their natural place, where they rested. Rest was the natural state of an element; it was motion that needed explaining, since every motion must have a cause. This common-sense physics held sway until Galileo began to undermine it.