"At the center of Zoroastrian theology was a unique monotheistic system based on the sole god, Ahura Mazda ("the Wise Lord"). Like most ancients, Zarathustra could not easily conceive of his god as being the source of both good and evil. He therefore developed an ethical dualism in which two opposing spirits, Spenta Mainyu ("the beneficent spirit") and Angra Mainyu ("the hostile spirit"), were responsible for good and evil, respectively."