What great elegance there is in the Peripatetic system I have explained a little time ago, as briefly as I could. Butthe form of the system, as is the case with most of the other schools, is threefold: one division being that of nature; the second, that of arguing; the third, that of living. Nature has been investigated by them so thoroughly that there is no part of heaven, or earth, or sea (to speak like a poet), which they have passed over. Moreover, after having treated of the origin of things, and of the universal world, so as to prove many points not only by probable arguments, but even by the inscrutable demonstrations of mathematicians, they brought from the subjects which they had investigated abundant materials to assist in attaining to the knowledge of secret things. Aristotle investigated the birth, and way of living, and figure of every animal; Theophrastus examined the causes, and principles, and natures of plants, and of almost everything which is produced out of the earth; by which knowledge the investigation of the most secret things is rendered easier. Also, they have given rules for arguing, not only logically, but oratorically; and a system of speaking in both these manners, on every subject, has been laid down by Aristotle, their chief; so that he did not always argue against everything, as Arcesilas did; and yet he furnished one on every subject with arguments to be used on both sides of it.
But, as the third division was occupied about the rules of living well, it was also brought back by those same people, not only to the system of private life, but also to the direction of affairs of state. For from Aristotle we have acquired a knowledge of the manners, and customs, and institutions of almost every state, not of Greece only, but also of the Barbarians; and from Theophrastus we have learnt even their laws: and each of them taught what sort of man a leader in a state ought to be, and also wrote at great length to explain what was the best constitution for a state. But Theophrastus also detailed very copiously what were the natural inclinations of affairs, and what the influences of opportunities which required regulating as occasion might demand. And as for living, a quiet method of life appeared to them to be the best, passed in the contemplation and knowledge of things; which, inasmuch as it had the greatest resemblance to the life of the gods, appeared to them to be most worthy of a wise man; and on these subjects they held very lofty and dignified language.