As we have seen, Aristotle opens the Generation of Animals by announcing that he has already dealt with three of the four causes of animals and their parts (formal, final, and material) and that it remains to discuss the efficient cause. For ‘to inquire into this and to inquire into the manner of generation for each thing is, in a way, the same thing’ (GA I 1, 715a1–18). But here we immediately encounter a puzzle. On the one hand, the GA routinely identifies the male principle as the primary efficient cause of the animal and its parts, which is housed in another individual of the same species, namely the father. This would suggest that generation has an external efficient cause (external to the thing that comes to be from it).