 Thus, apart from direct literary influence, the nature of the philosophical and theological themes which were popular in the Middle Ages also led to an emphasis on causality. Writers studied the interrelationship of divine grace and natural processes, the role of the will in ethics, free will and determinism: all of these problems have an important causal component.  Similarly, they investigated the nature of the causal processes involved in human and animal perception.  These questions were often handled by methods which might seem to us to be extraordinarily naturalistic – naturalistic, of course, in the sense of the modes of natural investigation which were current at the time. It comes as no surprise to know that many medieval thinkers discussed the question of whether divine grace can increase: what is surprising is that many of the discussions use the technical tools of Aristotle’s physical and biological works, tools which were originally developed to discuss problems of continuity and change in the natural world.  What is even more surprising is the technical proficiency of many of these discussions: fourteenth-century work on this topic gave rise to very acute analyses of the variation of continuous quantities (see Murdoch 1975), and this study of continuous variation played an important role in the development of the calculus.