The Thomist story of prime matter starts with Aristotle. For Aristotle being was essence, or what a thing is. Each thing, a stone or a flower, had a certain kind of whatness, but Aristotle noticed that there was a radical kind of change in which one thing lost its essence and became something else. A piece of wood, for example, was thrown into the fire and turned into ashes. This posed a serious problem for him. The wood was certainly not the ashes. Genuine deep change did take place. But the wood became the ashes. There was a continuity in this change, and thus, he reasoned, some kind of substrate to it. It is here that Aristotle’s view of being got him into trouble. If being is essence, and in this radical change one essence becomes another, then the substrate of this change could not be essence or being at all. Therefore it had to be a certain kind of non-being which would have none of the positive attributes of being. If essence was act, then the substrate had to be a sort of pure potency. This was the origin of the notion of prime matter that we have been encountering.

     Then came Thomas Aquinas who took up Aristotle’s philosophy, including the notion of prime matter, but in doing so he profoundly altered it. In his hands Aristotle’s idea of being as essence underwent a revolutionary change in which essence was no longer the supreme principle of being. We can try to understand it like this. We live in a world of many different things. We look around and can say, here is a bird, or there is a tree, and these simple assertions are the starting points of metaphysics. We have no doubt that a bird is a bird and not a giraffe, and a tree is a tree and not a hippopotamus. At the same time we are equally convinced that each of these things is, that each of them exists. But how do we reconcile these two equally certain assertions, that is, that the tree is not a bird, yet both of them exist? Thomas realized that the only way to do this was to see by means of a deep metaphysical intuition that essence was not really the supreme principle of being. Essences differed from each other, and so could not have given rise to each other. What made a what, or essence, to be what it is was not another what, but some the very isness or existence of things, and essence stood in relationship to existence as potency to act. Or to put it in a more graphic way, an essence, or a what, was a certain capacity for existence. We can imagine essences like different shapes and sizes of crystal glasses in which we pour the water of existence. Existence is then limited by the particular size and shape of the glass in which it is poured, but flows into each glass and fills it.13

     This new vision was such a departure from Aristotle that those who followed Thomas often didn’t grasp how different it was. Still less did they see that it had a profound impact on Aristotle’s prime matter. If for Aristotle being equaled essence, then the substrate of change had to be non-being, and this kind of non-being was really quite inconceivable, and though Thomists wrestled with prime matter long and hard, they could never get a handle on it. It remained a mysterious kind of non-being that was continually taking on a life of its own. With Thomas, however, there was another possibility. Certainly essences change in radical or substantial ways, but why not say that the substrate to change is existence. Both the wood and the ashes are forms of existence, and so in radical change we could say that wood existence becomes ashes existence, and the change from one essence to another takes place within existence, itself.