"“the only sources for the early (pre-Achaemenid) history of the eastern Iranian peoples are the Avesta, the Old Persian inscriptions, and Herodotus. … In view of the dearth of historical sources it is of paramount importance that one should evalute the evidence of the Avesta, the holy book of the Zoroastrians, parts at least of which antedate the Old Persian inscriptions by several centuries.” ..."