The river surrounding the earth, and source of all waters. This early Greek idea probably originated in the great river civilizations of Egypt and Mesopotamia, whose cosmologies pictured the earth floating like a raft on the primal waters. Even Thales, the first Greek philosopher, held in the sixth century bc that water was the first principle from which everything is fashioned and takes its life-giving force. Homer twice calls the earth-encircling river ‘the begetter of all’, though no cosmological theory is derived from the statement. In the fourth century bc Aristotle took Homer to mean that creation was the offspring of flux and motion. Mythological Oceanos was the husband and brother of Tethys, who gave birth to the rivers, including the poisonous Styx in the underworld. Another old sea god was Nereus, a wise prophet dwelling in the depths with his followers the Nereids.