For medieval scholars, philosophy was the path to true knowledge about the world, and its proofs consisted of the authority of ancients (as interpreted by Christian theologians) and their techniques of logical argumentation. Questions about the physical nature of the universe and how it functioned belonged to a minor branch of philosophy, called natural philosophy. Natural philosophy was based primarily on the ideas of Aristotle, the great Greek philosopher of the fourth century B.C.E. According to the Christianized version of Aristotle, a motionless earth stood at the center of the universe and was encompassed by ten separate concentric crystal spheres in which were embedded the moon, sun, planets, and stars. Beyond the spheres was Heaven with the throne of God and the souls of the saved.
Aristotle’s views also dominated thinking about physics and motion on earth. Aristotle had distinguished between the world of the celestial spheres and that of the earth — the sublunar world. The sublunar realm 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.
The work of the ancient Greek scholar Ptolemy provided the basic foundation of knowledge about the earth. Rediscovered around 1410, his Geography presented crucial advances on medieval cartography by representing a round earth divided into 360 degrees with the major latitude marks. However, Ptolemy’s map reflected the limits of ancient knowledge, showing only the continents of Europe, Africa, and Asia, with land covering three-