PHYSICS Philoponus says, is inconsistent not only with the Christian doctrine of creation, but also with Aristotle’s own opinion that nothing could traverse through more than a Wnite number of temporal periods For if the world had no beginning, then it must have endured through an inWnite number of years, and worse still, through 365 times an inWnite number of days (book 5, frag 132) In his commentary on Aristotle’s Physics (641 13 V.) Philoponus attacked the dynamics of natural and violent motion Aristotle encountered a diYculty in explaining the movement of projectiles If I throw a stone, what makes it move upwards and onwards when it leaves my hand? Its natural motion is downwards, and my hand is no longer in contact with it to impart its violent motion upwards Aristotle’s answer was that the stone was pushed on, at any particular point, by the air immediately behind it; an answer that Philoponus subjected to justiWed ridicule Philoponus’ own answer was that the continued motion was due to a force within the projectile itself—an immaterial kinetic force impressed upon it by the thrower, to which later physicists gave the technical term ‘impetus’ The theory of impetus remained inXuential until Galileo and Newton proposed the startling principle that no moving cause, external or internal, was needed to explain the continued motion of a moving body Philoponus applied his theory of impetus throughout the cosmos The heavenly bodies, for instance, travel in their orbits not because they have souls, but because God gave them the appropriate impetus when he created them Though the notion of impetus has been superannuated by the discovery of inertia, it was itself a great improvement on its Aristotelian predecessor It enabled Philoponus to dispense with the odd mixture of physics and psychology in Aristotle’s astronomy Natural Philosophy in the Thirteenth Century Nonetheless, Aristotle’s natural philosophy remained inXuential for centuries to come Both in Islamic and in Latin philosophy the study of nature was carried out within the framework of commentaries on Aristotle’s works, especially the Physics Individuals such as Robert Grosseteste and Albert the Great extended Aristotelian science with detailed studies of particular scientiWc topics; but the general conceptual framework 180