 Here one needs to proceed cautiously, however, since it is sometimes said that Aristotle’s word “cause” (aitia) would be better translated as “explanation” (or “explanatory factor”, to avoid the implication that they are linguistic items, as opposed to things-in-the-world). Certainly modern philosophers tend to use “cause” in a narrower way, which approximates to Aristotle’s efficient cause. Aristotle’s idea is that there are four kinds of thing that need to be mentioned in order to give a full account of the nature of an object, each corresponding to a particular kind of question. We need to know what the thing is made of, and the answer to this question is the thing’s matter—bricks, in the case of a house; bodily organs in the case of a human being. Next we need to know what the thing is, or how it is defined, and the answer to this is the thing’s form or essence. We also need to know what made the thing come into existence, who or what created it, and this is the thing’s efficient or “moving” cause. Lastly, we need to know what the thing is for, what its purpose or function is—the final cause. Now Aristotle observes that, although these are all distinct questions, in the case of the last three very often the same thing will serve as the answer to all of them (Physics ii 7, 198a24–27). A house is defined as a shelter of a certain sort (De Anima i 1, 403b3–7; Metaphysics viii 3, 1043a29–36). That is what a house is, i.e., its formal cause, but it is also what a house is for, its final cause, since houses, like all artefacts are functionally defined. Similarly, a human being is defined as something which lives a certain kind of rationally-directed life. But, on Aristotle’s view, this is also what a human being is for. The human function is to live such a life (Nicomachean Ethics i 7, 1097b22–1098a20; cf. De Anima ii 1, 412a6–22). As for the efficient cause, it is qualitatively, although not numerically, identical with the formal cause, at least in the organism case, since human beings give birth to human beings, and the same goes for all other living things. Thus, even though Aristotle admits four different kinds of cause, in a sense it is only really matter and form that play any ineliminable explanatory role in his system.