Zoroastrian ethics turns on a single opposition: asha against druj. Asha is truth, order and the right pattern of the world as Ahura Mazda made it; druj is its contrary, the Lie, falsehood, deceit and the disorder that corrodes creation. The two divide the world and everyone in it. A person who stands with truth is an ashavan, a possessor of asha; one who serves falsehood is a dregvant, a follower of the Lie. Druj is tied to the hostile spirit, Angra Mainyu, whose primal choice of the Lie made him the adversary of the good creation, but druj is not only cosmic. It is present in every actual lie, every broken promise or contract, every act of deceit. Zoroastrian religion frames the moral life as a continual taking of sides, asha over druj, in what one thinks, says and does, so that the term names at once a metaphysical principle and the plain wrongdoing of daily life.
Historians of Zoroastrianism trace the asha and druj polarity to the Gathas, seventeen hymns tradition ascribes directly to Zarathushtra himself, where Yasna 30 describes the two primal spirits choosing, at the beginning, between the Lie and the Truth, an act taken as the model of every subsequent human choice. Scholars including Mary Boyce have argued that this developed dualism, together with its doctrine of a final renovation of the world (frashokereti) in which druj is defeated for good, influenced later Second Temple Jewish and Christian ideas of a cosmic adversary and an end-time reckoning, though the degree and direction of that influence remains debated among historians of religion rather than settled fact.