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 such as Thomas Aquinas 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. In the first eight spheres were embedded, in turn, the moon, the sun, the five known planets, and the fixed stars. Then followed two spheres added during the Middle Ages to account for slight changes in the positions of the stars over the centuries. 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 (which we now know occurs because planets closer to the sun periodically overtake the earth on their faster orbits). The great second-century scholar Ptolemy, a Hellenized Egyptian (see “Technology and the Rise of Exploration” in Chapter 14), 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,” or fifth essence. The sublunar world, however, was made up of four imperfect, changeable elements. The “light” elements (air and fire) naturally moved upward, while the “heavy” elements (water and earth) naturally moved downward. These natural directions of motion did not always prevail, however, for elements were often mixed together and could be affected by an outside force such as a human being. 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.

Natural philosophy was considered distinct from and superior to mathematics and mathematical disciplines like astronomy, optics, and mechanics, and Aristotle’s ideas about the cosmos were accepted, with revisions, for two thousand years. His views offered a commonsense explanation for what the eye actually saw. Aristotle’s science as interpreted by Christian theologians also fit neatly with Christian doctrines. It established a home for God and a place for Christian souls. It put human beings at the center of the universe and made them the critical link in a “great chain of being” that stretched from the throne of God to the lowliest insect on earth. This approach to the natural world was thus a branch of theology, and it reinforced religious thought.