one of the most celebrated rhetoricians of Athens, was born, b.c. 436, five years before the beginning of the Peloponnesian war, twenty-two years after the birth of Lysias, and fifty-four after that of Demosthenes. He died b.c. 338. He was son of Theodorus, who was a manufacturer of musical instruments, but, though of humble parentage, he seems to have enjoyed the best education which Athens could furnish. His early years were spent under Gorgias of Leontium, one of the most celebrated sophists of the age; Prodicus of Ceos, whose beautiful apologue of Hercules between Virtue and Vice, has immortalized his name; and Theramenes, who was afterwards condemned to death by the thirty tyrants, because, though their colleague, he refused to participate in their tyranny. With their assistance, Isocrates soon began to display the talents of which he was possessed, and became anxious to illustrate his name as a statesman and legislator. Nature, however, it would appear, had placed an impassable barrier against his entrance on public life, by furnishing him with a weak voice and timidity of nature which he found himself unable to overcome. Thus prevented from pursuing the path to which his inclination pointed, he determined to make his talents subservient to his fortune. He opened a school for oratory, and soon found himself surrounded by a numerous body of young men anxious to profit by his instructions. Amongst the more celebrated we may mention Theopompus, Ephorus, Isaeus, Timotheus, Philiscus, and Xenophon. So numerous were the disciples of Isocrates, that Hermippus composed a work in several books respecting them. The sophists of his time were accustomed to discuss subtle points of logical casuistry; Isocrates first distinguished himself by discussing the great political interests of the time, and examining important questions of morality. As his speeches were not intended to be delivered, but to be perused in the retirement of the closet, Isocrates was obliged to pay particular attention to beauty of style, to the rounding of his periods, and avoiding whatever might prove offensive to the ear. He is said to have been employed ten years in polishing one of the most celebrated of his productions, entitled the Panegyric. It might be expected that this mode of proceeding would be attended with some defects; it occasioned a want of animation, a constant monotony, and often an entebling of the ideas, which were enveloped in a multitude of words, useful only to round the period and to produce the necessary rhythm and cadence. He was not always able to secure himself against the envy of his countrymen, and they even accused him of holding a suspicious intercourse with Philip of Macedon, with whom he kept up a constant correspondence; but he proved, by the closing scene of his life, that his intentions had always been pure, and that he had sincerely loved his country. After the battle of Charonea, b.c. 338, so fatal to the liberties of Greece, he determined no longer to drag on a life which would be embittered by seeing his country subject to the Macedonians. He preferred to starve himself to death.

Of his works we still possess twenty-one orations and nine letters. The first is addressed to Demoricus, of whom we know nothing, and is a collection of remarks and maxims for the direction of a young man's conduct. It is attributed by some critics to Isocrates of Apollonia, who is mentioned by Suidas and Harpocration. One of the finest of his productions is the Panegyric (Πανεγυρικός), which was written n. c. 380, with the view of putting an end to the internal dissensions of the Greeks, and inducing them to unite against the Persians. We may also mention the oration entitled Ἐπιστολῆς, as one of the best specimens of his peculiar style, though it was composed, n. c. 342, at the advanced age of ninety-four. It is a high panegyric on the Athenians and their ancestors. Numerous editions of his works have been published. The editio princeps was printed at Milan in 1493, by Demetrius Chalcondylas, but the best is that by Coray, Paris, 1807. The Panegyric has been published separately by Morus, 1803, by Pinzger, Lips. 1825, and by Dindorf, Lips. 1826. Many of his other works have also been published separately.

ISOPERIMETRICAL figures, in Geometry, are such as have equal perimeters or circumferences.

ISOSCELES triangle, in Geometry, one that has two equal sides.