In the first book of Physics, which is said to belong to his early Academia period, Aristotle investigates the principles of change in general - matter, privation and form. The most important of his discoveries in that book is, it seems, the concept of matter analysed in terms of the underlying thing (substratum) of change; the thing underlying is the terminus a quo and the thing constituted is the terminus ad quem of change. The relation of both termini consists in the fact that matter is the proximate cause of the thing constituted, such as bronze becoming a statue and wood becoming a bed, so that an analogy is found in the relation between the matter qua terminus a quo and the thing constituted qua terminus ad quem as between bronze and statue, wood and bed, and so on. It follows that Aristotle devised at first the concept of matter in relation to the thing constituted, not in relation to the formal cause as seen in later writings, for matter is consistently. in Physics, A the proximate cause of the thing constituted, and not in such a way that prime matter is claimed to be the ultimate cause of all things as many commentators interpret the text.