The Sumerians were polytheistic and believed various gods controlled natural phenomena they could not explain like weather, night and day, and seasons. The gods established rules called "Mes" that governed both good aspects like honesty and justice as well as bad aspects like pain, lies, and wars. While the gods planned for both good and evil, their will was never fully understood by humans who could only plead with the gods. Major gods included An the god of heaven, Enlil the god of air who became chief god, Enki the god of wisdom and lifegiving waters, and Ninhursag the mother of all living things.