Taoists affirm yin and yang, the two complementary and interdependent principles whose interplay constitutes and moves all things. Yin is the receptive, dark, yielding, and cool; yang the active, bright, firm, and warm; neither is good or evil, and each contains the seed of the other, as the familiar diagram shows. All phenomena arise from their alternation and balance, and health, harmony, and the right ordering of life consist in their proper proportion rather than in the triumph of one over the other.
Yin and yang predate organized Taoism as a systematic concept: the terms appear in early form in the I Ching, the Book of Changes, and were developed into a more elaborate cosmological system during the Warring States period by thinkers associated with the Yin-Yang school, traditionally linked to the philosopher Zou Yan in the fourth and third centuries BCE, who combined the paired forces with the theory of the Five Phases (wood, fire, earth, metal, water) to explain cosmic and historical change. Taoism absorbed and integrated this already-developed cosmological vocabulary rather than originating it, one reason yin-yang thought pervades traditional Chinese medicine and Confucian cosmology as much as it does specifically Taoist philosophy.