The term science as we use it today came into use only in the nineteenth century. Prior to the Scientific Revolution, many different scholars and practitioners were involved in aspects of what came together to form science. One of the most important disciplines was natural philosophy, which focused on fundamental questions about the nature of the universe, its purpose, and how it functioned. In the early 1500s, natural philosophy was still based primarily on the ideas of Aristotle, the great Greek philosopher of the fourth century B.C.E. Medieval theologians brought Aristotelian philosophy into harmony with Christian doctrines. According to the revised Aristotelian view, a motionless earth was fixed at the center of the universe and was encompassed by ten separate concentric crystal spheres that revolved around it. Beyond the tenth sphere was Heaven, with the throne of God and the souls of the saved. Angels kept the spheres moving in perfect circles.

Aristotle’s cosmology made intellectual sense, but it could not account for the observed motions of the stars and planets and, in particular, provided no explanation for the apparent backward motion of the planets. The great second-century scholar Ptolemy offered a cunning solution to this dilemma. According to Ptolemy, the planets moved in small circles, called epicycles, each of which moved in turn along a larger circle, or deferent. Ptolemaic astronomy was less elegant than Aristotle’s neat nested circles and required complex calculations, but it provided a surprisingly accurate model for predicting planetary motion.

Aristotle’s views, revised by medieval philosophers, also dominated thinking about physics and motion on earth. Aristotle had distinguished sharply between the world of the celestial spheres and that of the earth — the sublunar world. The spheres consisted of a perfect, incorruptible “quintessence.” The sublunar world, however, was made up of four imperfect, changeable elements: air, fire, water, and earth. Aristotle and his followers also believed that a uniform force moved an object at a constant speed and that the object would stop as soon as that force was removed.