 All the four (kinds of) causes may enter in the explanation of something. Consider the production of an artifact like a bronze statue. The bronze enters in the explanation of the production of the statue as the material cause. Note that the bronze is not only the material out of which the statue is made; it is also the subject of change, that is, the thing that undergoes the change and results in a statue. The bronze is melted and poured in order to acquire a new shape, the shape of the statue. This shape enters in the explanation of the production of the statue as the formal cause. However, an adequate explanation of the production of a statue requires also a reference to the efficient cause or the principle that produces the statue. For Aristotle, this principle is the art of bronze-casting the statue (Phys. 195 a 6–8. Cf. Metaph. 1013 b 6–9).