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 This was the Stagirite’s achievement,—the clear analytic separation of the different sciences, and the statement, in outline at all events, of the questions which each science had to answer. Aristotle generally attempted to furnish his own answer to these questions, and often gave wrong answers; yet to have posited the questions at all was a great matter, and cleared the way for the thoughts of subsequent generations. There is no one to whose work the saying is more appropriate than to that of the Stagirite—prudens quæstio dimidium scientiæ est—“It is half-way to knowledge when you know what you have to inquire.”

The leading questions started in the Natural Philosophy of Aristotle are as to the nature of causation, time, space, and motion. On the subject of motion he went astray by taking up the idea that celestial and terrestrial motions were different in kind—that the heavenly bodies “naturally” revolved, while bodies on earth had each a natural motion in them, either downward or upward. This belief in the absolute levity of certain bodies—as, for instance, fire—was, of course, a mistake. “Truth is the daughter of Time;” and a few of the great discoveries of modem ages, which appear so simple, though they were so hardly and so late achieved,—such as the Copernican system, and the law of gravitation,—have shattered the Cosmos of Aristotle. Still it required at least fifteen centuries before anything like a demonstration was brought against the reality of that Cosmos and its arrangements. Thus, if Aristotle be censured for the incorrectness of his theories, succeeding generations of thinkers