His philosophy comprised the elements of the school of Heraclitus, Pythagoras, and Socrates, combined in a system which he distributed into three parts,—moral, consisting of action; natural, in contemplation; rational, in distinction of true and false, which, though useful in all, yet belongs to pure thought. As of old, in a tragedy the chorus acted alone; then Thespis, making some intermissions of the chorus, introduced one actor, Æschylus a second, Sophocles a third; in like manner, philosophy was at first but of one kind,—physic; then Socrates added ethic; thirdly, Plato, inventing dialectic, made it perfect.
This third part, dialectic, consisting in reason and dissertation, he treated thus: Though judgments arise from the sense, yet the judgment of truth is not in the senses. The mind alone is the judge of things, and only fit to be credited, because the mind alone sees that which is simple, uniform, and certain, which is named Idea. All sense he conceived to be obtuse and slow, and nowise able to perceive those things which seem subject to sense; those things being so minute that they cannot fall under sense; so movable and various, that nothing is one, constant and the same; all are in continual alteration and movement, and subjects of opinion only. Science he affirmed to be nowhere but in the reasons and thoughts of the mind, whose objects are ideas, whence he approved definitions of things, and applied these to whatsoever subject he discussed, discriminating things and naming them etymologically. In this consisted the discipline of dialectic; that is, of speech concluded by reason. Though Socrates practised conversation by way of question and answer, or dialogue, yet Plato so much refined the form, both in speech and composition, that he deserves to be preferred before others, as well for invention as reformation. The analytical method, which reduces the thing sought into its principle, is his invention.[17]
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.
The study of philosophy, if it made him select in the choice of his associates, did not sour his temper, nor render him exclusive in his intercourse and fellowship with mankind. At the Olympian games, he once fell into company with some strangers who did not know him, upon whose affections he gained greatly by his affable conversation, dining and spending the day with them, never mentioning either the Academy or Socrates, only saying his name was Plato. When they came to Athens, he entertained them courteously. "Come, Plato," said the strangers, "now show us your namesake, Socrates' disciple. Take us to the Academy: recommend us to him, that we may know him." He, smiling a little, as he used, said, "I am the man." Whereat they were greatly amazed, having conversed so familiarly with a person of that eminence, who used no boasting or ostentation, and showed that, besides his philosophical discourse, his ordinary conversation was extremely winning.
He lived single, yet soberly and chastely. So constant was he in his composure and gravity, that a youth brought up under him, returning to his parents, and hearing his father speak vehemently and loudly, said, "I never found this in Plato." He ate but once a day, or, if the second time, very sparingly, abstaining mostly from animal food. He slept alone, and much discommended the contrary practice.
Of his prudence, patience, moderation, and magnanimity, and other virtues, there are many instances recorded. When he left his school, he was wont to say, "See, youths, that you employ your idle hours usefully. Prefer labor before idleness, unless you esteem rust above brightness."