Aristotle vs. Galileo

Aristotle (384–322 B.C.) believed that a stone fell toward the earth because the stone and the earth were both in the “Earth” category among the four basic elements, Earth, Air, Fire, and Water. According to this reasoning, smoke, which consists of air and fire, wants to be closer to the sky (air) and further from an unlike element (earth). Hence, earthly objects move naturally toward the earth while “airy” objects move naturally upward. The assumed fifth element, the heavenly substance he called the Quintessence along with an assumed Prime Mover accounted for the “perfect” circular and uniform motion of the heavens. Finally, Aristotle asserted that motion of any object required a continuing force on that object as long as motion persisted.

As if to simplify the discussion, St. Thomas Aquinas (1225–1274) later declared that something is in motion when it is both what it is already and something else that it is not yet. (Apparently, new explanations do not always lead to clarity.)

In a complete departure from the convoluted explanations of Aristotle and Aquinas, Galileo Galilei (1564–1642) claimed that a force was not necessary to maintain constant, straight-line motion. This property is in evidence even today where it can be observed that an object in space will drift along forever without external forces.