 In the Physics, Aristotle builds on his general account of the four causes in order to provide the student of nature with the explanatory resources indispensable for a successful investigation of the natural world. However, the Physics does not provide all the explanatory resources for all natural investigations. Aristotle returns to the topic of causality in the first book of the Parts of Animals. This is a relatively independent and self-contained treatise entirely devoted to developing the explanatory resources required for a successful study of animals and animal life. Here Aristotle completes his theory of causality by arguing for the explanatory priority of the final cause over the efficient cause.