In regard to the antiquity of the Avesta, then, we can only build upon the simple facts that it cannot have been written down for the first time when the Buddhists found adherents in Bactria (IV. 542), or when the kingdom of the Greek princes arose in Bactria, or when the Seleucidæ and Alexander, before them, reigned over Iran. It has been proved that the Avesta was in existence before Alexander overthrew the kingdom of the Achæmenids. The series of the successors of Zarathrustra, which western writers could trace backwards from this point--Osthanes II., Pazates, Gobryas, Osthanes I., Astrampsychus, Apusodorus--plainly shows that even under the Achæmenids the West was seriously occupied with religious questions. As Osthanes I. had written on the doctrine of Zoroaster about the time of Xerxes (p. 92), it is at least more probable than not that the Avesta was already in existence at that time. If in the West there was a series, and as the Greeks point out, a continuous series, of priestly teachers, round whom naturally pupils and schools grew up, and after the beginning of the fifth century a theological literature, similar teachers and schools must have existed long before in the East, and this greatly strengthens the conclusion drawn from the contents of the Vendidad, that it must have been written down before the rise of the Medes. But for any more precise determination of the date of the Avesta between the two limits obtained--the year 750 B.C., _i.e._ the beginning of the formation of a priesthood in the West and the contemporary use of writing in the East, and the year 350 B.C.--we are confined wholly to internal evidence.
Scriptures of such extent as the Avesta is shown to have been, by the accounts of the Greeks and Arabs, and the list of contents (p. 51), and the existing fragments, could not have been written down at once or within a brief space of time. We saw (p. 33) that it set up a religious canon, which not only regulated the doctrine and the worship, the duties of priests and laity, but also comprised the law, and in a word all the relations of life. A codification of this kind is only possible when belief and doctrine, culture and ritual, have arrived at fixed and complete formulæ, have been arranged in a system and developed, and the consequences bearing on life, morality, and law have been drawn from them by an active and influential priesthood. Hence before the Avesta was written down and collected there must have been a priestly order in the East, in the circles of which the doctrine and practice went through this developing, revising, and fixing process. Various sketches, lists of prayers for certain offerings, collections of rules belonging to this or that priesthood or school, must have been in existence, and combinations of the traditional material must have been made, before a canon comprising the whole wisdom of the priests, and far exceeding in extent the law of Manu, could have been compiled.