in Chinese philosophy, the ultimate source and motive force behind all reality. In the Book of Changes ( Yijing), the ancient philosophical text in which the concept is first mentioned, taiji is the source and union of the two primary aspects of the cosmos, yang (active) and yin (passive). The neo-Confucian philosophers of the Song dynasty (960–1279 CE) associated taiji with li (“principle”), the coherent structure of the changing world. Li engenders qi (“vital energy”), which is transformed through the yang and yin modes of development into the Five Phases (wuxing)—wood, earth, fire, metal, and water—which explain change and persistence in the cosmos.