ABSTRACT
At the boundaries of metaphysics and natural philosophy lies a fascinating medieval dispute over the way qualitative change takes place. A qualitative change is a change to quality, as opposed to a change to quantity or substance. When a thing becomes larger or heavier, or moves faster, it undergoes a change in quantity. When the thing itself— a dog, a tree, some water— goes out of existence, and is succeeded by something else, then there is a change in substance. When the thing endures, but becomes hotter, moister, or greener, it undergoes a change in quality. On the succession view habits turn out to be every bit as fragile as sensible qualities. A virtue is supposed to be a quality that is acquired slowly, over a protracted period of time, as one’s virtuous activity steadily strengthens the virtue. Heat becomes particles in motion, color becomes light waves, and the virtues become neural networks.
