Before Zhou Dunyi, the Confucian revival of the Song dynasty had ethics and classical scholarship but no settled account of the cosmos to ground them in. His Taijitu Shuo, a commentary keyed to a diagram he drew himself, supplied one: the Supreme Ultimate (Taiji), itself rooted in the Ultimate of Non-Being (Wuji), moves and generates yang, rests and generates yin, and the interaction of the two produces the Five Phases, water, fire, wood, metal and earth, from which the ten thousand things, and eventually human beings, take shape.
What made the diagram matter for later Confucians was the last step. Humanity alone receives this cosmic process in its most refined form, so that the same pattern which orders the stars also constitutes human moral nature. Self cultivation, on this account, is not obedience to an external rule but the work of clearing away the turbid material force that obscures a goodness already built into the cosmos. Zhou Dunyi never built a school around the idea in his own lifetime; he held minor provincial posts and briefly tutored two boys, Cheng Hao and Cheng Yi, whose father asked him to instruct them in the classics. A century and a half later Zhu Xi, assembling the genealogy that became Neo-Confucianism, placed Zhou Dunyi at its head for exactly this reason: he was the first to give Confucian ethics a metaphysics able to answer Buddhist and Daoist cosmology on its own ground.