Asha (Avestan; Old Persian arta) is the central concept of Zoroastrian ethics and cosmology: truth, right order, righteousness, the way things properly are and ought to be. It is cognate with the Vedic rta, the cosmic order of the Rigveda, and the correspondence between the two terms is among the clearest pieces of evidence for a shared Indo-Iranian religious inheritance.
Asha's opposite is druj, the lie or falsehood, and the framing of the fundamental moral choice as one between truth and the lie, rather than between obedience and disobedience, or between good and evil conceived as substances, is distinctive. The Gathas, the hymns attributed to Zarathushtra himself and the oldest layer of the Avesta, present existence as structured by this choice. Two primordial spirits choose between truth and the lie at the beginning, so that the condition of the world follows from a decision rather than from a necessity, and every subsequent person faces the same choice in their own case. Asha Vahishta, "best truth", is one of the Amesha Spentas, the beneficent immortals through whom Ahura Mazda acts, and is associated with fire, which is why fire occupies the place it does in Zoroastrian ritual.
The concept's reach is deliberately wide. It covers truth-telling and the keeping of contracts, an emphasis reflected in Herodotus' report that Persians taught their sons three things, to ride, to shoot, and to tell the truth, and it extends to ritual correctness, to the right ordering of society, and to the physical order of the natural world. Asha is what is being maintained and druj what is being resisted in all these domains simultaneously, which is why the tradition summarises the whole of practice as good thoughts, good words and good deeds. Zoroastrian ethics places unusual weight on individual free and responsible choice, and the Gathas are explicit about it.
Asha also frames the eschatology. The renovation of the world, frashokereti, in which the lie is finally defeated and existence restored to its proper condition, is the horizon toward which the moral struggle runs, and the individual judgement at the Chinvat bridge measures a life against the standard asha sets. Because Zoroastrian ideas were current in the Near East during the periods when Jewish, and later Christian and Islamic, eschatologies took shape, the question of influence has been discussed since the nineteenth century; the dating of the Avestan and Pahlavi sources makes it extremely difficult to settle, and specialists continue to differ.