 In the Categories, individual substances (a man, a horse) were treated as fundamental subjects of predication. They were also understood, indirectly, as subjects of change. (“A substance, one and the same in number, can receive contraries. An individual man, for example, being one and the same, becomes now pale and now dark, now hot and now cold, now bad and now good” 4a17–20.) These are changes in which substances move, or alter, or grow. What the Categories did not explore, however, are changes in which substances are generated or destroyed. But the theory of change Aristotle develops in the Physics requires some other subject for changes such as these—a subject of which substance is predicated—and it identifies matter as the fundamental subject of change (192a31–32). Change is seen in the Physics as a process in which matter either takes on or loses form.