The modern view of causation can be traced back to the mechanistic science of Descartes, whose rejection of Aristotelian physics, with its concept of substantial forms, in favor of mechanical explanations was a turning-point in the history of philosophy. However the reasoning which led Descartes and other early moderns in this direction is not well understood. For the first time, this book traces Descartes' groundbreaking theory of scientific explanation back to the mathematical demonstrations of Aristotelian mechanics and interprets these advances in light of the available arguments for and against substantial forms. It also examines how Descartes' new theory led him to develop a metaphysical foundation for his science that could avoid skeptical objections. It will appeal to a wide range of readers interested in the philosophy and science of the early modern period.