ISÆUS, the fifth in order of the ten Attic orators comprised in the Alexandrian canon, was most probably a native of Chalcis, though Athens, the scene of his oratorical triumphs, claimed him for her son. The dates of his birth and death are alike unknown; but this much is certain, that he rose into eminence in the interval between the close of the Peloponnesian War, B.C. 404, and the accession of Philip to the throne of Macedon, B.C. 348. His father's name was Diagoras. In his youth he seems to have been at one time rather dissipated and extravagant, but with advancing years he reformed his mode of life, and became frugal and self-denying. Little more is known of his life, except that having mastered the arts of eloquence under Lysias and Isocrates, he opened a school of oratory, taught with great success, numbered Demosthenes among his pupils, and enjoyed the friendship of all the leading literati of his day.
In the Lives of the Ten Orators, attributed to Plutarch, Isæus is mentioned as the author of sixty-four orations.
Fourteen of these were condemned by the ancient critics themselves as spurious. Of the remainder, only eleven have come down to us entire; but fragments are preserved of no fewer than fifty-six attributed to him. The eleven extant are all forensic, and bear on subjects connected with disputed inheritances. Their chief value to us lies in the knowledge they afford of Athenian law in matters relating to wills, the transmission of property, the rights of succession in cases of intestacy, and many of the forms of procedure. Of all these Isæus seems to have been thoroughly master. Little was written by the ancients themselves upon the place and character of Isæus as an orator. Indeed the only extant criticism of him is in the parallel drawn by Dionysius of Halicarnassus, between him and his master Lysias. This, along with the actual remains of Isæus, supplies materials for a pretty accurate estimate of him. With infinite grace Lysias combined an artless and natural simplicity that gained belief for his statements, even when they were known to be false, and conviction for his reasoning when it was plainly untenable. Isæus, on the other hand, disregarded nature too much, and was too manifestly an artist. More polished, and at the same time more vehement than his master, he always seemed to study effect, and to aim at conviction more by his manner of stating the case than by a regard to the ultimate truth. On this account, says Dionysius, "Lysias was believed, even when he stated what was false; people hesitated to believe Isæus even when he was known to speak the truth." Isæus, however, is entitled to special praise as the first who practised and recommended the scientific cultivation of political oratory. To him is due the credit of introducing that powerful and impressive style, which was afterwards perfected by his pupil Demosthenes. Ten of the eleven extant orations of Isæus have been known since the revival of letters. The eleventh, on the Inheritance of Menecles, was first published by Th. Tyrwhitt, Lond., 1785, from a manuscript out of the Laurentine library of the Grand Duke of Tuscany at Florence. In 1815 Angelo Mai discovered the larger half of the oration on the Inheritance of Cleonymus, which he published at Milan, and afterwards at Rome. There have been numerous editions of Isæus in the collective works of the Attic Orators, of which the best is Bekker's. There have been also several separate editions, of which the best is that of Schömann, Greifswald, 1831. In 1779 Sir William Jones published an English translation of Isæus, with a learned commentary, and critical and historical notes. Besides being somewhat loose and feeble, this version is said to be not even very correct.
This Isæus ought not to be confounded with Isæus, another celebrated orator, who lived at Rome in the time of Pliny the younger, about the year 97.