After the first two chapters of Book II, in which, as we have said, Aristotle determines the subject - "formal object," as it were - of physical philosophy, he goes at once to the causes of mobile being. This is a logical step, since science (any science) in the Aristotelian sense consists, basically, in knowledge through causes. Hence, one of the first things to be done in the science of nature is to ascertain the causes of mobile being. Causes, moreover, are principles of demonstration in a science. Consequently, to treat of the causes of mobile being is also to clarify the method one should follow in the study of nature.
Aristotle addresses himself to the topic with great thoroughness. Introduced are a variety of chapters, dealing not only with cause in the familiar sense but also with more cryptic expressions of nature, such as chance, fortune, necessity, and especially, teleology. On first acquaintance with these chapters it may not be instantly clear how one thing relates to another or to the discussion as a whole. One train of thought, however, is soon discernible and becomes increasingly apparent. In physics (in the Aristotelian sense) an explanation by final cause excels all others; especially does it surpass the deterministic kind of explanation for which Democritus was known. Aristotle, as we know, had found the out-and-out materialism of Democritus a very unfinished view of nature, revealing at most one aspect. Nature, he was sure, was far more resourceful than this Atomist had surmised. Indeed, for the study of nature Plato's theory of celestial forms or archetypes, though not without difficulties of its own, was by comparison far more illuminating.1