Zoroastrian cosmology is dualist: alongside the good creator stands Angra Mainyu, the hostile or destructive spirit, called Ahriman in the later Pahlavi books. The Gathas describe two primal spirits who, at the beginning, made a choice between good and evil. Spenta Mainyu, the bounteous spirit, sided with Ahura Mazda and asha; Angra Mainyu chose the Lie, the druj, and from that choice come death, disease, deceit and every marring of the good creation. This dualism is genuine and cosmic, but it is not a stalemate of two eternal equals.
Zoroastrian teaching holds that the conflict is bounded within time and that good is destined to win it outright at the frashokereti, the final renovation, when the hostile spirit and all evil are abolished. The struggle gives human life its moral seriousness: every good thought, word and deed strengthens the side of Ahura Mazda, and every evil one serves his adversary.