From this point Aristotle proceeds to organic nature, and here his works not only contain a natural history, but also a physiology and anatomy. To the anatomy pertain his works on the Locomotion of Animals, and on the Parts of Animals. He deals with physiology in the works on the Generation of Animals, on the common Movement of Animals; and then he comes to the distinction between Youth and Age, Sleeping and Waking, and treats of Breathing, Dreaming, the Shortness and Length of Life, &c., all of which he deals with partly in an empiric, and partly in a more speculative manner. Finally, there comes the History of Animals, not merely as a history of Nature, but also as the history of the animal in its entirety—what we may call a kind of physiologico-anatomical anatomy. There is likewise a botanical work On Plants (περὶ φυτῶν) which is ascribed to him. Thus we here find natural philosophy in the whole extent of its outward content.
As regards this plan, there is no question that this is not the necessary order in which natural philosophy or physics must be treated. It is long since physics adopted in its conception the form and tendency derived from Aristotle, of deducing the parts of the science from the whole; and thus even what is not speculative still retains this connection as far as outward order goes. This is plainly to be preferred to the arrangement in our modern text-books, which is a wholly irrational succession of doctrines accidentally put together, and is undoubtedly more suitable to that method of contemplating nature, which grasps the sensuous manifestation of nature quite irrespective of sense or reason. Physics before this contained some metaphysics, but the experience which was met with in endeavouring unsuccessfully to work it out, determined the physicists, so far as possible, to keep it at a distance, and to devote their attention to what they call experience, for they think that here they come across genuine truth, unspoiled by thought, fresh from the hand of nature; it is in their hands and before their faces. They can certainly not dispense with the Notion, but through a kind of tacit agreement they allow certain conceptions, such as forces, subsistence in parts, &c., to be valid, and make use of these without in the least knowing whether they have truth and how they have truth. But in regard to the content they express no better the truth of things, but only the sensuous manifestation. Aristotle and the ancients understand by physics, on the other hand, the comprehension of nature—the universal; and for this reason Aristotle also calls it the doctrine of principles. For in the manifestation of nature this distinction between the principle and what follows it, manifestation, really commences, and it is abrogated only in genuine speculation. Yet if, on the one hand, what is physical in Aristotle is mainly philosophic and not experimental, he yet proceeded in his Physics in what may be called an empiric way. Thus, as it has been already remarked of the Aristotelian philosophy in general that the different parts fall into a series of independently determined conceptions, so we find that this is the case here also; hence an account can only be given of a part of them. One part is not universal enough to embrace the other part, for each is independent. But that which follows, and which has in great measure reference to what is individual, no longer comes under the dominion of the Notion, but becomes a superficial suggestion of reasons, and an explanation from the proximate causes, such as we find in our physics.