  Even the cursory discussion we have presented here should make a few things clear about the development of category theory in the Middle Ages. First, Aristotle's views had an extraordinary impact throughout the period, for his Categories were ever-present in medieval discussions of categories. Second, major changes occurred in thinking about this topic depending on the availability of various sources and the impact of different philosophical traditions. In the early Middle Ages, Neoplatonic works such as the Categoriae decem were very important, whereas later the influence of these works faded and a more Aristotelian approach became dominant. Third, in spite of Aristotle's influence, medieval authors charted new courses, which often went contrary to Aristotle's explicit views; we need only think of the number of categories, for example. They also raised questions that were not explicitly posed by Aristotle, such as the way that categories are established. Fourth, there was considerable development in the way in which the general topic of categories was discussed and the tools used to deal with it; late medieval logic played an increasingly important role in discussions, the terminology became more precise, the questions raised were made explicit, and the controversies were increasingly pointed. Finally, the relevance of categories for metaphysical issues became crucial, so that it was in the context of metaphysical works, such as Aquinas' Commentary on Aristotle's “Metaphysics” and Suárez's Metaphysical Disputations that we find some of the richest discussions.