The religious belief taught by Zarathustra is based on the dual conception of a good principle, Ahura Mazda, and an evil principle, Anra Mainyu, and the leading idea of his teaching is the constant conflict between the two, which must continue until the end of the period ordained by Ahura Mazda for the duration of the world, when evil will be finally overcome; until then the god's power is to some degree limited, as evil still withstands him. Zarathustra's doctrine was essentially practical and ethical; it was not in abstract contemplation, or in separation from the world, that man was to look for spiritual deliverance, but in active charity, in deeds of usefulness, in kindness to animals, in everything that could help to make the world a well-ordered place to live in, in courage and all uprightness. To build a bridge or dig a canal was to help to lessen the power of evil. As Reinach has concisely expressed it, "a life thoroughly occupied was a perpetual exorcism." The two figures of Ahura Mazda and Anra Mainyu, the one with his attendant archangels and angels, and the other with his arch-demons and demons, or Divs, compose the Zarathustrian celestial hierarchy, as represented in the earlier sacred writings; in the later ones other figures are introduced into the pantheon. The sacred writings that have been preserved are of different periods, and outside the range of Zarathustra's moral system of religion there are traces in them of revivals of an older primitive nature worship, and of the beliefs of an early nomadic shepherd life, as, for instance, the sacredness in which cow and dog are held, as well as reminiscences of general Indo-Germanic myths. Ahura Mazda was the creator of the universe for the duration of which he fixed a certain term. It seems uncertain whether the Persians pictured the world as round or flat, but according to their idea it was divided into seven zones, of which the central one was the actual habitable earth. Between these zones and enveloping the whole was the great abyss of waters. Between earth and heaven rose the celestial mountain whence all the rivers upon earth had their source, and on which was deposited the Haoma.