The first unquestionably big idea in the history of philosophy was the idea of form. The idea of course belonged to Plato, and was then domesticated at the hands of Aristotle, who paired form with matter as the two chief principles of his metaphysics and natural philosophy. In the medieval period, it was Aristotle’s conception of form and matter that generally dominated. This was true for both the Islamic and the Christian tradition, once the entire Aristotelian corpus became available. For this reason, although there is much to say about the fate of Platonic Forms in medieval thought, the present chapter will focus on the Aristotelian tradition.

Aristotelian commentators have been puzzled by form and matter for as long as there have been Aristotelian commentators. Indeed, it would not be too much to say that these are topics about which Aristotelians have never formed a very clear conception, and that their failure to do so was the principal reason why Aristotelianism ceased to be a flourishing research program from the seventeenth century onward. For those who aspire to a modern revival of Aristotelianism, the concepts of form and matter can easily take on the aspect of a kind of Holy Grail, such that if only we could get these ideas clearly in focus, we could see our way forward on any number of philosophical fronts, such as the union of mind and body, the coherence and endurance of substances, the nature of causality, and so on. The historical record, however, suggests that this hope is a snare and a delusion, insofar as there has never been any such thing as the theory of form and matter. Although medieval philosophers of all kinds used this terminology incessantly, it had no more of a fixed meaning than does the ubiquitous contemporary philosophical talk of “properties.”