We saw, in the case of Plato, that, as his literary activity lasted over a period of half a century, during which his philosophy was in constant development, it became important to trace this development in the {254} order of his Dialogues. The same thing is not true in the case of Aristotle. The whole of his writings, or rather those that have come down to us, seem to have been written during his last thirteen years, while he was at Athens, that is to say, after he had passed his fiftieth year. His system was then complete, mature, and fully developed. The question of the order in which they were written has no great importance. The result of critical investigations, however, is to show that he probably began with the various works upon logic, composed next the treatises upon physical science, next the ethical and political books, and lastly the "Metaphysics," which he left unfinished. It must not be forgotten that Aristotle was not only a philosopher in the modern restricted sense of that term. He was a man of universal learning. There is no branch of knowledge which did not receive his attention, and upon which he was not the greatest expert of his time, except perhaps mathematics. So far was he from being only an abstract philosopher, that his natural tastes seem to have lain rather in the field of physical science than of abstract thought. But his design seems to have been to work over the entire field of knowledge, thoroughly to overhaul the sciences already in existence, rejecting what seemed false in the work of his predecessors, and invariably adding to the residue valuable developments and suggestions of his own. Where there was no science already in existence, his plan involved the foundation of new sciences wherever necessary, and he thus became the founder of at least two sciences, Logic and Zoology. He thus attained to a pre-eminence in all branches {255} of knowledge which would be impossible for a single man in modern times. His works include treatises upon Logic and Metaphysics, upon Ethics, Politics, and Art. He wrote a treatise upon the principles of Rhetoric, another upon Astronomy, under the title "On the Heavens," another upon Meteorology. Several of his treatises deal with the biology of animal life, in which he was intensely interested. They include books entitled "On the Parts of Animals," "On the Movements of Animals," "On the Origin of Animals," as well as his great treatise, "Researches on Animals," which contains an enormous mass of facts collected from every possible source. It is true that a large proportion of these facts have turned out to be fictions, but this was inevitable in the infancy of science. It has been calculated that Aristotle shows himself acquainted with about five hundred different species of living beings, though they are not, of course, classified by him in the modern way. With these books upon animals he founded the science of