I. (i) Against Eunomius. The work under this title comprises five books, the first three generally accepted as genuine, the last two sometimes regarded as doubtful. Gregory of Nazianzus,  Jerome,  and Theodoret  all testify to Basil's having written against Eunomius, but do not specify the number of books. Books IV. and V. are accepted by Bellarmine, Du Pin, Tillemont, and Ceillier, mainly on the authority of the edict of Justinian against the Three Chapters (Mansi ix., 552), the Council of Seville (Mansi x., 566) and the Council of Florence (Hardouin ix., 200). Maran (Vit. Bas. xliii.) speaks rather doubtfully. Boehringer describes them as of suspicious character, alike on grounds of style, and of their absence from some mss. They may possibly be notes on the controversy in general, and not immediately directed against Eunomius. Fessler's conclusion is "Major tamen eruditorum pars eos etiam genuinos esse censet."
The year 364 is assigned for the date of the publication of the three books.  At that time Basil sent them with a few words of half ironical depreciation to Leontius the sophist.  He was now about thirty-four years of age, and describes himself as hitherto inexperienced in such a kind of composition.  Eunomius, like his illustrious opponent, was a Cappadocian. Emulous of the notoriety achieved by Aetius the Anomoean, and urged on by Secundus of Ptolemais, an intimate associate of Aetius, he went to Alexandria about 356 and resided there for two years as Aetius' admiring pupil and secretary. In 358 he accompanied Aetius to Antioch, and took a prominent part in the assertion of the extreme doctrines which revolted the more moderate Semiarians. He was selected as the champion of the advanced blasphemers, made himself consequently obnoxious to Constantius, and was apprehended and relegated to Migde in Phrygia.
 Or. xliii. S: 67.
 De Script. Eccl. 116.
 Dial. ii. p. 207 in the ed. of this series.
 Maran, Vit. Bas. viii.
 cf. Ep. xx.
 1 Eunom. i.
Reference address : https://www.elpenor.org/basil/life-works.asp?pg=43