As even now indicated, Aristotle's first concern is with the principles of natural being, or rather with their number. The principles of nature, he says, "must be either one or more than one. And if one, it must be either immovable . . . or in motion. . . . But if more than one, they are either limited or unlimited in number; and if limited, but more than one, there must be either two, or three, or four, or some other number." 2
This passage deserves the closest attention; it governs and throws light on all that follows in Book I, which may be outlined under these headings:
As the outline indicates, in the first part of Book I Aristotle makes a critical analysis of earlier doctrines. This analysis, thorough and trenchant, is an achievement by itself. Especially noteworthy are his refutation of the Eleatics and his criticism of Anaxagoras. The Eleatics had denied the very possibility of change, so, in effect, doing away with the problem of principles. Anaxagoras had gone to another extreme, saying that the principles were infinite. Aristotle confronts and dissects these and kindred views, and refuting them, consolidates his own thought. Interesting though it should be to record his every move in this performance, we shall, for brevity's sake, keep our account to the essentials.