 Physics II 8 contains Aristotle’s most general defense of final causality. Here Aristotle establishes that explaining nature requires final causality by discussing a difficulty that may be advanced by an opponent who denies that there are final causes in nature. Aristotle shows that an opponent who claims that material and efficient causes alone suffice to explain natural change fails to account for their characteristic regularity. Before considering how the defense is attempted, however, it is important to clarify that this defense does not perform the function of a proof. By showing that an approach to the study of nature that ignores final causality cannot account for a crucial aspect of nature, Aristotle does not thereby prove that there are final causes in nature. Strictly speaking, the only way to prove that nature exhibits final causality is to establish it on independent grounds. But this is not what Aristotle does in Physics II 8. Final causality is here introduced as the best explanation for an aspect of nature which otherwise would remain unexplained.