The problem of the origin of evil, the most difficult to be solved, seems to have been constantly before his mind. It seemed to him impossible that the Truth, whom he conceived to be eternally pure, good, just, and perfect, had created evil. The ancient Aryans attributed the struggles in the physical world around them to the strife between good and evil; Zoroaster seized this idea, applied it with the deepest emphasis to the moral and spiritual world, and it became the basis of his system of dualism. Together with Ahura-Mazda, the good principle, he admitted the existence of an evil principle or spirit equal in power and of a similar nature[21]--Angra Mainyus; in Persian Ahriman. This spirit is the author of all moral and physical evil, sin, disease, suffering, and death. All things, created by Ahura-Mazda pronouncing the creating, pre-existing word "Honover," were pure, perfect, and beautiful as himself until spoiled by the evil influence of Ahriman. And though Ahriman, like Ahura-Mazda, has been eternal and self-existing in the past, Zoroaster declares that a day will come when three great prophets will arise, Ukhsyad-eremah, "the increasing Light," Ukhsyad-eretah, "the increasing Truth," Açtvad-ereta, "self-existent Truth," who will convert all mankind; everything created will become as pure as on the first day when it issued from the breath of the "Wisest of all Intelligence," and Ahriman will be destroyed and disappear for ever. Such is the real doctrine of Zoroaster, while the hymns of the Zend-Avesta glow and burn with the assurance of the mystic and essential life of the soul with the spiritual essence of all pure thought. The pure heavens are like light; thought is likened to a drop of pure light, and the departing spirit has a sunbeam for its guide to conduct it to immortal light. In the Gâthas, or Songs, he says: "God appears in the best thought, the truest speech, and the sincerest action. He gives through his pure spirit health, prosperity, devotion" (which, more properly translated, ought to be "love"), "and eternity to this universe. He is the Father of all truth and the Mother of all tenderness."