Eduard Meyer

“ A semi-legendary king (kava) , praised by Zoroaster as his protector and a true believer, son of Aurvataspa (Lohrasp) . The later tradition and the Shahname of Firdousi makes him (in the modern form Kai Gushtāsp) king of Iran. As Zoroaster probably preached his religion in eastern Iran, Vishtāspa must have been a dynast in Bactria or Sogdiana. The Zoroastrian religion was already dominant in Media in the time of the Assyrian king Sargon (c. 715 B.C.) , and had been propagated here probably in much earlier times (cf. Persia) ; the time of Zoroaster and Vishtāspa may therefore be put at c. ”