The systematising power of Aristotle, his faculty for bringing the isolated parts of a surface into co-ordination and continuity, is apparent even in those sciences with whose material truths he was utterly unacquainted. Apart from the falseness of their fundamental assumptions, his scientific treatises are, for their time, masterpieces of method. In this respect they far surpass his moral and metaphysical works, and they are also written in a much more vigorous style, occasionally even rising into eloquence. He evidently moves with much more assurance on the solid ground of external nature than in the cloudland of Platonic dialectics, or among the possibilities of an ideal morality. If, for example, we open his _Physics_, we shall find such notions as Causation, Infinity, Matter, Space, Time, Motion, and Force, for the first time in history separately discussed, defined, and made the foundation of natural philosophy. The treatise _On the Heavens_ very properly regards the celestial movements as a purely mechanical problem, and strives throughout to bring theory and practice into complete agreement. While directly contradicting the truths of modern astronomy, it stands on the same ground with them; and anyone who had mastered it would be far better prepared to receive those truths than if he were only acquainted with such a work as Plato’s _Timaeus_. The remaining portions of Aristotle’s scientific encyclopaedia follow in perfect logical order, and correspond very nearly to Auguste Comte’s classification, if, indeed, they did not directly or indirectly suggest it. We cannot, however, view the labours of Aristotle with unmixed satisfaction until he comes on to deal with the provinces of natural history, comparative anatomy, and comparative psychology. Here, as we have shown, the subject exactly suited the comprehensive observation and systematising formalism in which he excelled. Here, accordingly, not only the method but the matter of his teaching is good. In theorising about the causes of phenomena he was behind the best science of his age; in dissecting the phenomena themselves he was far before it. Of course very much of what he tells was learned at second-hand, and some of it is not authentic. But to collect such masses of information from the reports of uneducated hunters, fishermen, grooms, shepherds, beemasters, and the like, required an extraordinary power of putting pertinent questions, such as could only be acquired in the school of Socratic dialectic. Nor should we omit to notice the vivid intelligence which enabled even ordinary Greeks to supply him with the facts required for his generalisations. But some of his most important researches must be entirely original. For instance, he must have traced the development of the embryo chicken with his own eyes; and, here, we have it on good authority that his observations are remarkable for their accuracy, in a field where accuracy, according to Caspar Friedrich Wolff, is almost