The article discusses the decline of Aristotelian physics at the University of Paris in the seventeenth and early eighteenth centuries. A course of physics remained essentially Aristotelian until the final decade of the seventeenth century, when it came under the influence of Descartes. But the history of physics teaching over this period cannot be properly appreciated if it is simply seen in terms of the replacement of one physical philosophy by another. Long before the 1690s, the traditional Aristotelianism of the Schools had been forced to come to terms with the New Science to some degree, while the Cartesianism of the early eighteenth century was always alive to the challenges to Descartes's particular physical theories. Except in the early seventeenth century the physics course at Paris was always in a state of change. The replacement of Aristotelian by Cartesian physics too involved the development of a novel epistemology. Although both Aristotelian and Cartesian professors believed that natural philosophy was a science of causes based upon a priori principles, the latter had a far more probabilist conception of physics.