 The word aitia (“cause” or, perhaps better, “explanation”), Aristotle tells us, is “said in many ways.” In one sense, a cause is “that out of which a thing comes to be, and which persists; e.g., bronze, silver, and the genus of these are causes of a statue or a bowl” (Physics 194b24). A cause in this sense has been traditionally called a material cause, although Aristotle himself did not use this label. In a second sense, a cause is “the form … the account of the essence” (194b27), traditionally called the formal cause. A third sense, traditionally called the efficient cause, is “the primary source of change or rest” (194b30). In this sense, Aristotle says, an adviser is the cause of an action, a father is the cause of his child, and in general the producer is the cause of the product. Fourth is what is traditionally called the final cause, which Aristotle characterizes as “the end (telos), that for which a thing is done” (194b33). In this sense, he says, health is the cause of walking, since we might explain a person’s walking by saying that he walks in order to be healthy—health is what the walking is for. Note that, as in this case, “things may be causes of one another—hard work of fitness, and fitness of hard work—although not in the same sense: fitness is what hard work is for, whereas hard work is principle of motion” (195a10). So hard work is the efficient cause of fitness, since one becomes fit by means of hard work, while fitness is the final cause of hard work, since one works hard in order to become fit.