Ether ("upper air"), in Greek mythology, was the personification of the "upper sky", space and heaven. He is the pure, upper air that the gods breathe, as opposed to "aer", which mortals breathed. In Hesiod's Theogony he was the son of Erebus and Nyx, and brother of Hemera, both noted in passing in Cicero's De Natura deorum. He is the soul of the world and all life emanates from him. The aether was also known as Zeus' defensive wall; the bound that locked Tartaros from the cosmos.