I’m taking a short break from St. Thomas Aquinas to talk about the Aristotelian underpinnings of Thomas’ philosophy. You can relax, though. All I’m going to be doing is putting words to something that you know so clearly you probably don’t even think about it.
Newton’s physics is about the motion of bodies, and particularly of predicting how they move quantitatively: speed, acceleration, momentum and so forth. Aristotle’s physics has the same name, but Aristotle was asking a different question: how can things change while remaining the same?
Before Aristotle, Parmenides had argued that there can be no change. There is only the unchanging One; all change is an illusion. Heraclitus argued that in fact there is nothing but change: any stability is an illusion. Both had arguments from first principles on their side; and both were quite clearly wrong. Aristotle looked at these two views of the world, and looked at the world around him and said, “Hogwash! I see things changing but remaining the same.” First principles always need reality as a sanity check.
So Aristotle said to himself, “I see change all around me; and I see things staying more or less the same. How does change work? What’s required for change to happen?”
Modern texts will often say that Aristotle’s physics were wrong; it would be more correct to say that Aristotle’s physics have little application to technology. His questions are still relevant.
Aristotle distinguished between two kinds of change. First, things change while remaining themselves, as an apple ripens or a child grows. And then, things come to be and pass away: a new child is born, or an old man dies. Something new comes into the world, something old passes out of it.
But there are also ways that that apple cannot change: at least, while remaining itself. An apple will never gradually turn into an orange. Apple seeds will never produce an oak tree. An apple is a bundle of possibilities, but only these possibilities, not those possibilities. And these possibilities are its potentialities, its potencies. And the apple has these potentialities precisely in that it is actually an apple, not an orange, or a dog, or Toyota minivan.
Everything we see around us is a similar bundle of possibilities, a composite, as Aristotle put of it, of act and potency. That might seem obvious, but it has more implications than you might think; and it’s an essential part of understanding Thomas’ reasoning.