Corpse pollution, nasu, is the strongest form of impurity in the system, and the logic is consistent rather than arbitrary. Death is the enemy's work; a dead body is where his power is concentrated; and the four elements are good creations under divine guardianship. Burial defiles earth; cremation defiles fire, which is the most sacred; disposal in water defiles water. Exposure on stone in an open tower — where sun and birds reduce the body and the dry bones fall into a central ossuary pit lined so that nothing leaches into the ground — pollutes nothing. The corpse-bearers, nasasalars, are a distinct hereditary group who work always in pairs and are subject to strict separation, and the elaborate purification apparatus of the Vendidad, including the nine-night barashnum, exists largely to manage contact with death. The system is coherent, and it depends entirely on the availability of vultures.