be fully developed. Aristotle frames his answer in terms of the human form, maintaining that a human form is fully realized at the end of the generative process. This process does not end at birth but continues until the human being reaches its acme. But this does not explain why it takes a human being to generate a human being. Note, however, that a fully developed human being is not only the final outcome of the generative process; it is also what initiates the entire process. For Aristotle, the ultimate moving principle responsible for the generation of a human being is a fully developed living creature of the same kind; that is, a human being who is formally the same as the final result of the generative process. (A final clarification is in order here: Aristotle is committed to a hylomorphic explanation of animal generation. His considered view is that the father supplies the form whereas the mother provides the relevant matter.) Thus, the student of nature is often left with three types of causes: the formal/final cause, the efficient cause, and the material cause. However, the view that there are in nature causes besides material and efficient causes was controversial in antiquity. According to Aristotle, most of his predecessors recognized only the material and the efficient cause. This explains why Aristotle cannot be content with saying that formal and final causes often coincide, but he also has to defend his thesis against an opponent who denies that final causality is a genuine mode of causality. 4. Final Causes Defended 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