This ethical dualism gives Zoroastrian belief its strong moral seriousness: because the cosmos is contested, human choices genuinely matter, and each person's thoughts, words, and deeds add to the side of order or of chaos. The teaching, rooted in the Gathas attributed to Zarathushtra and elaborated in the later Avesta, is often noted for its influence on ideas of a final judgement, resurrection, and the victory of good found in later traditions. It distinguishes Zoroastrianism from both strict monism and any dualism in which evil is co-eternal and undefeated, since here the good God prevails.
One historical school within Zoroastrianism itself, Zurvanism, attempted to resolve the tension in the dualist scheme by positing Zurvan, Infinite Time, as a prior unifying principle from which both Ahura Mazda and Angra Mainyu were born as twin offspring, a myth attested in Sasanian-era sources and in reports by later Christian, Manichaean, and Islamic writers describing Zoroastrian belief they encountered. Zurvanism appears to have had real currency, particularly among the Sasanian priesthood, before the mainstream Zoroastrian tradition that survived into the modern period rejected it as heretical, so that the version of the good-evil struggle now taught by Zoroastrian communities represents the position that outlasted, rather than the only position ever held within, the tradition's own history.