The Vendidad (properly Videvdat, "the law for keeping away the demons/daevas") is the Zoroastrian book of purity and ritual law, and it is unusual within the Avesta for surviving essentially complete. Cast largely as a dialogue in which Ahura Mazda answers the questions of Zarathushtra, it lays down the rules of ritual purity and the means of countering the pollution and the demonic forces that threaten the good creation, above all the pollution of death. It contains the tradition's elaborate law concerning corpses and their handling, the special ritual role of the dog, the penalties and purifications for contact with impurity, and, among its opening sections, the mythic geography of the lands Ahura Mazda created and the story of Yima (Jamshid) and the vara, the enclosure built to preserve life through a catastrophic winter. Later and more concerned with ritual detail than the Gathas, the Vendidad is recited whole in a night-long liturgy, and it is the principal source for the purity discipline that has always been central to Zoroastrian practice.