 Significantly enough, there is no attempt to argue for the existence of four fundamental modes of causality in the first book of the Parts of Animals. Evidently, Aristotle expects his reader to be already familiar with his general account of the four causes as well as his defense of final causality. The problem that here concerns Aristotle is presented in the following way: since both the final and the efficient cause are involved in the explanation of natural generation, we have to establish what is first and what is second (PA 639 b 12–13). Aristotle argues that there is no other way to explain natural generation than by reference to what lies at the end of the process. This has explanatory priority over the principle that is responsible for initiating the process of generation. Aristotle relies on the analogy between artistic production and natural generation, and the teleological model that he has developed for the explanation of artistic production. Consider, for example, house-building. There is no other way to explain how a house is built, or is being built, than by reference to the final result of the process, the house. More directly, the bricks and the beams are put together in the particular way they are for the sake of achieving a certain end: the production of the house. This is true also in the case of natural generation. In this context Aristotle’ slogan is “generation is for the sake of substance, not substance for the sake of generation” (PA 640 a 18–19). This means that the proper way to explain the generation of an organism like an animal, or the formation of its parts, is by reference to the product that lies at the end of the process; that is to say, a substance of a certain type.