Aristotle’s own account of change starts by noticing that the reality of one and the same change can be accounted for in three different ways. The process of coming to know how to play an instrument can be described as either
a.) a man becomes musical
b.) the non-musical becomes musical
c.) a non-musical man becomes a musical man.
The accounts divide according to the answers we get to the question “does something remain the same during a change?” In the first case, the answer to the question is an unqualified “yes”; in the second it is an unqualified “no” and in the third it is yes and no, both taken in a qualified sense. The problem is that no one way seems the correct way to describe the change to the exclusion of others, and so we are at a loss as to how to answer the question. Now any view of change will at least tacitly assume an answer. The first answer is the default since it’s the simplest. Change is just, say, the shuffling of elements and various parts from one place to another, and “the element” is imagined as being as real as a man. What is change? The element gets a new arrangement just as man becomes musical. Done. The difficulty with this barely needs to be said: to focus on simply the element that stays the same is to overlook something essential to the change. But then again, to include the part that is “not the same” as essential makes the essence of change “being both the same and not the same”. We might solve this with a distinction, but the distinction is trickier than it looks: it’s not as if the paradox is a facile as noticing that a pizza cutting wheel is both round and flat, or that a sword is both cuts and doesn’t (at the handle). This distinction can’t be made into physical parts (like a blade and the hilt) nor can it be made into logical parts (like “square” into “quadrilateral” and “equal sided”) or as relating to different categories (like a fire-hydrant into red and thirty inches high). Aristotle will explain all this by making a famous division of being into potency and act. But what does this mean?
While every becoming or change requires a subject, the very existence of this subject is derived from its term. To remove the term of a becoming immediately destroys the subject. If you drink the last coffee in the break room, you destroy the possibility of anyone coming down the hall to get some, irrespective of what anyone in the hall might think or do. Even if something is merely moving left to right, to remove anything on the right (whether we do this per impossible or by putting up a wall) is to destroy the subject of any such motion or coming to be. Now to destroy a subject isn’t necessarily to destroy something altogether, but only in the way it could come to be. Still, there is an odd dependence of a subject on its term, such that what changes must borrow its existence from the term, so that the existence of the subject is a kind of emanation or participation in the term. The subject, taken precisely as a subject, only exists in the way that a mirror has the shape and color of a face.
But while it is true that to do away with the term is to do away with the subject, isn’t it just as true to say that to do way with a subject would do away with the possibility of attaining the term? If we got rid of an airport, we would get rid of the possibility of any subject coming there, but we could do the same thing by hangaring or scrapping every airplane. While there is a dependence here, the dependence is not equal in both directions. Even if some term requires a subject, we can’t say that the term is simply a participation in the subject or an emanation from it. Terms aren’t mobile. It makes no sense to talk about them as though subjects were drawing them in, or as if they had some activity that completes in getting to a subject. The principle of relativity or the equivalence between motion and rest is not applicable when we distinguish motion into subject and term.