A salient issue in the first chapter of Book II is whether nature is principally matter or form. The answer will help to decidethe burden of chapter 2what the natural philosopher should study, form or matter, or both; and if both, which should command his inquiry.
Aristotle's predecessors had tended to identify nature with the material elements, water, air, fire, and the like. Aristotle grants some justification to this opinion. After all, the elements and, more generally, matter are integral parts of nature. Nevertheless, nature is more than matter; it is also, in fact mainly, form or the principle of perfection in a thing. What distinguishes one thing from another, and causes its activities, is primarily its form. The implication for the natural philosopher is clear. He must study matter, yes; but even more must he study form. Writes Aristotle: "Since nature has two meanings, form and matter, we must search nature in the same way as we would the essence of snub-nosedness. In other words, such things neither exist without matter nor can be considered from their material aspect alone." 2 Form, in short, is the primary consideration in the study of nature.
In taking this position Aristotle was announcing the basic trend and character of his whole physical philosophy. Doubtless, a being of nature is composed of both matter and form, and Aristotle's analysis of its compositeness is second to none. But his finest and most satisfying answers are about its form or formal structure and consequently about end or final causality. I say "consequently," because form and end have coincident meanings and what is form in one respect is also end from another, as we shall see in the next chapter, on causes. All told, Aristotle's physics is centered on formal and final cause, and this, to be sure, sets it off from the mechanistic interpretation of nature, with the focus on matter and its quantitative structure.