Physics

A treatise in eight books, originally lectures, on the principles of the natural world and of change. Book I argues that change requires three principles, matter, form and privation, and answers Parmenides' denial that anything comes to be. Book II defines nature as an inner source of movement and rest, sets out the four causes (material, formal, efficient and final), discusses chance, and defends purpose in nature against those who explain everything by necessity. Book III defines motion as the actualisation of what is potential, insofar as it is potential, and argues that the infinite exists only potentially. Book IV treats place, rejects the void, and defines time as the number of motion with respect to before and after. Books V and VI classify kinds of change and analyse continuity and divisibility, answering Zeno's paradoxes; Book VII argues that whatever is moved is moved by something. Book VIII argues that motion is eternal and depends on a first, unmoved mover.

It is the source of the four causes and of the vocabulary of potentiality, nature and purpose, and it shows how physics and philosophy were once a single inquiry.