 One thing to be appreciated about Aristotle’s reply is that the final cause enters in the explanation of the formation of the parts of an organism like an animal as something that is good either for the existence or the flourishing of the animal. In the first case, something is good for the animal because the animal cannot survive without it; in the second case, something is good for the animal because the animal is better off with it. This helps us to understand why in introducing the concept of end (telos) that is relevant to the study of natural processes Aristotle insists on its goodness: “not everything that is last claims to be an end (telos), but only that which is best” (Phys. 194 a 32–33).