PHYSICS remained Aristotelian until the fourteenth century We may illustrate this by considering the concepts of motion, time, and causation Aristotle had deWned motion as ‘the actuality of what is in potentiality, in so far as it is in potentiality’.3 Arabic commentators struggled to relate this deWnition to the system of categories Avicenna placed motion in the category of passio: all changes in nature were due to the action of the heavenly intelligences, who as it were stirred the forms around in the broth of the natural world Averroes emphasized the variety of types of change covered by Aristotle’s term ‘motion’: there was local motion, which was change in place, growth, which was change in size, and there were qualitative changes of many kinds Any instance of motion belonged in the same category as its terminus: location, quantity, or quality So far from being the passive result of the operation of the heavenly intelligences, any change in a natural body, animate or inanimate, was the action of an internal agent (a motor conjunctus) Albert the Great, with support from Aristotelian texts, sought to combine the two Islamic accounts: a motion was simultaneously an action of an agent and a passio of a recipient: when a gardener turns the soil, the turning of the soil is at one and the same time an action of the gardener and an event that happens to the soil He agreed with Averroes that motion was an analogical term, which ranged across several categories; but he thought that Averroes had not fully grasped Aristotle’s distinction between perfect and imperfect actualities A movable body at point A has a potentiality to be at point B Arrival at B is the perfect actuality of this potentiality; but motion towards B is the imperfect actuality, when the moving body is not yet at B but only on the way to B Albert maintains that Aristotle’s broad deWnition of motion—the actuality of what is in potentiality in so far as it is in potentiality—can be applied, extending its analogical sense to generation (substantial change) and to creation (bringing into being out of nothing).4 For Aristotle time and motion are closely linked: time is the measure of motion, and time derives its continuity from the continuity of motion The question whether motion and time had a beginning was a subject of keen debate among Christian philosophers in the thirteenth century in connection with the provability of God’s existence Following al-Kindi and the See vol i, p 184 See J Weisheipl, ‘The Interpretation of Aristotle’s Physics’, in CHLMP 526–9 182