 Aristotle goes on by adding a specification on his doctrine of the four causes: the form and the end often coincide, and they are formally the same as that which produces the change (Phys. 198 a 23–26). This is one of the several times where Aristotle offers the slogan “it takes a human being to generate a human being” (for example, Phys. 194 b 13; Metaph. 1032 a 25, 1033 b 32, 1049 b 25, 1070 a 8, 1092 a 16). This slogan is designed to point at the fundamental fact that the generation of a human being can be understood only in the light of the end of the process; that is to say, the fully developed human being. The question thus arises as to what it takes for a human being to 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 generation. 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 end of generation; 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 end of generation. (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 matter.) 