 From Aristotle we learn that Empedocles explained the articulation of the human spine into vertebrae as the result of the twisting and turning that takes place when the fetus is in the womb of the mother. Aristotle finds this explanation unacceptable (PA 640 a 19–26). To begin with, the fetus must have the power to twist and turn in the way it does, and Empedocles does not have an explanation for this fact. Secondly, and more importantly, Empedocles overlooks the fact that it takes a human being to generate a human being. That is to say, the originating principle of the generation is a fully developed human being which is formally the same as the final outcome of the process of generation. It is only by looking at the fully developed human being that we can understand why our spine is articulated into vertebrae and why the vertebrae are arranged in the particular way they are. This amounts to finding the role that the spine has in the life of a fully developed human being. Moreover, it is only by looking at the fully developed human being that we can explain why the formation of the vertebrae takes place in the particular way it does. (For further information about the explanatory priority of the final over the efficient cause, see Code 1997: 127–143.)