Book XXIII, 6:32-36
In these parts are the fertile fields of the Magi, about whose sects and pursuits — since we have chanced on this point — it will be in place to give a few words of explanation. According to Plato, the most eminent author of lofty ideas, magic, under the name of hagistia [holy rites], is the purest worship of the gods. To the science of this, derived from the secret lore of the Chaldeans, in ages long past the Bactrian Zoroaster made many contributions, and after him the wise king Hystaspes, the father of Darius.
When Zoroaster had boldly made his way into the unknown regions of Upper India, he reached a wooded wilderness, whose calm silence the lofty intellects of the Brahmins control. From their teaching he learned as much as he could grasp of the laws regulating the movements of the earth and the stars, and of the pure sacrificial rites. Of what he learned he communicated something to the understanding of the Magi, which they, along with the art of divining the future, hand on from generation to generation to later times. From that time on for many ages down to the present a large class of men of one and the same descent have devoted themselves to the service of the gods. The Magi also say (if it is right to believe them) that they guard on ever-burning braziers a fire sent down from heaven in their country, and that a small portion of it, as a good omen, used to be carried before the Asiatic kings. The number of Magi of this origin in old times was very small, and the Persian potentates made regular use of their services in the worship of their gods. And it was sin to approach an altar, or touch a sacrificial victim, before one of the Magi, with a set form of prayer, poured the preliminary libations. But they gradually increased in number and became a strong clan, with a name of their own; they possessed country residences, which were protected by no great walls, and they were allowed to live in accordance with their own laws, and through respect for religion were held in high esteem. From this seed of the Magi, as the ancient records relate, seven men after the death of Cambyses mounted the Persian throne, but (we are told), they were overthrown by the party of Darius, who made himself king by the neighing of a horse.