Book 3 of Aristotle's Physics primarily concerns two important concepts for his theory of nature: change and infinity. Change is important because, in Book 2, he has defined nature - the subject-matter of the Physics - as an internal source of change. Much of his discussion is dedicated to showing that the change occurs in the patient which undergoes it, not in the agent which causes it. Thus Book 3 is an important step in clearing the way for Book 8's claims for a divine mover who causes change but in whom no change occurs.
The second half of Book 3 introduces Aristotle's...