The Iranians came to believe in the existence of two co-eternal and co-equal Persons, one good and the other evil, between whom there had been from all eternity a perpetual and never-ceasing conflict, and between whom the same conflict would continue to rage through all coming time.
It is impossible to say how this development took place.

We have evidence, however, that at a period considerably anterior to the commencement of the Median Empire, Dualism, not perhaps in its ultimate extravagant form, but certainly in a very decided and positive shape, had already been thought out and become the recognized creed of the Iranians.

In the first Fargard, or chapter, of the Vendidad--the historical chapter, in which are traced the only movements of the Iranic peoples, and which from the geographical point whereat it stops must belong to a time when the Arians had not yet reached Media Magna---the Dualistic belief clearly shows itself.

The term Angro-mainyus has now become a proper name, and designates the great spirit of evil as definitely and determinately as Ahura-mazda designates the good spirit.
The antagonism between Ahura-mazda and Angro-mainyus is depicted in the strongest colors; it is direct, constant and successful.

Whatever good work Ahura-mazda in his benevolence creates, Angro-mainyus steps forward to mar and blast it.