Several words were also introduced by him in philosophy. Of these are "element," which before his time was confounded with "principle." He distinguished them thus: "Principle is that which has nothing before it whereof it might be generated; elements are compounded." The word "poem" was first used by him. So were "superficies" and "antipodes." "Divine providence," words since appropriated by Christian theologians, was first an expression of Plato's. He, too, first considered the force and efficacy of grammar as the organ of pure thought. His school was the pride of Athens, and drew into it its most gifted youth, as well as scholars from abroad. His most distinguished disciples were Speusippus, his nephew, whom he reformed by his example and teachings, and who became eminent as a philosopher, succeeding him in the Academy; Xenocrates, whom he much loved; Aristotle, the Stagirite, whom Plato used to call a wild colt, foreseeing that he would oppose him in his philosophy, as a colt, having sucked, kicks its dam. Xenocrates was slow, Aristotle quick, in extremity; whence Plato said of them, "See what an unequal team this of mine. What an ass and horse to yoke together!" Isocrates the orator, and Demosthenes, were among his auditors; Dion of Syracuse was an intimate friend of his, and by his persuasions he made two journeys to Syracuse, at one of which he was sold into slavery by the tyranny of Dionysius, and being redeemed by his friend, returned to Athens, as is related by Plutarch. Xenophon was his contemporary. At home he lived quietly in the Academy, not taking part in public affairs, the laws and customs of the Athenians not being in harmony with his ideas of republican institutions. "Princes," he said, "had no better possessions than the familiarity of such men as could not flatter, wisdom being as necessary to a prince as the soul to the body; and that kingdoms would be most happy if either philosophers ruled, or the rulers were inspired with philosophy, since nothing is more pernicious than power and arrogance accompanied with ignorance. Subjects should be such as princes seem to be." And he held that a philosopher might retire from the commonwealth if its affairs were unjustly administered. "A just man was a perpetual magistrate." He affirmed that philosophy was the true helper of the soul, all else but ornamental; that nothing is more pleasing to a sound mind than to speak and hear the truth spoken, than which nothing is better or more lasting.