Physics

Aristotle takes 'physics' to mean the study of things that have within themselves a principle of change. Across eight books he works through what nature is, the four causes, the nature of change itself, the infinite, place, void, time, and motion. The Physics is not 'physics' in the modern sense but the philosophical foundation underneath it: a sustained analysis of what it means for anything to come into being, persist, and pass away. Aquinas read it as the gateway to the Metaphysics; Newton wrestled with it; every subsequent theory of time or space has answered to it.