(Ἰσαῖος).
1. One of the ten Attic orators, whose orations were contained in the Alexandrian canon. The time of his birth and death is unknown, but all accounts agree in the statement that he flourished (ἤκμασε) during the period between the Peloponnesian war and the accession of Philip of Macedonia, so that he lived between B. C. 420 and 348. (Dionys. Isaeus, 1; Plut. Vit. X. Orat. p. 839; Anonym. γένος Ἰσαίου.) He was a son of Diagoras, and was born at Chalcis or, as some say, at Athens, probably only because he came to Athens at an early age, and spent the greater part of his life there. He was instructed in oratory by Lysias and Isocrates (Phot. Bibl. Cod. 263; Dionys. Plut. Il. cc.) He was afterwards engaged in writing judicial orations for others, and established a rhetorical school at Athens, in which Demosthenes is said to have been his pupil. Suidas states that Isaeus instructed him gratis, whereas Plutarch relates that he received 10,000 drachmas (comp. Plut. de Glor. Ath. p. 350c.; Phot. l.c.); and it is further said that Isaeus composed for Demosthenes the speeches against his guardians, or at least assisted him in the composition. All particulars about his life are unknown, and were so even in the time of Dionysius, since Hermippus, who had written an account of the disciples of Isocrates, did not mention Isaeus at all.