Yin and Yang
The Chinese cosmological pair denoting complementary, opposing forces whose interplay constitutes all natural and social phenomena.
Yin and yang are the two complementary principles of classical Chinese cosmology. The original Chinese characters refer to the shaded and sunlit sides of a hill, and the philosophical use generalizes from this contrast: yin names the receptive, dark, soft, cold, female, and earthly; yang names the active, bright, hard, hot, male, and heavenly. The two are not moral opposites of good and evil but ontological complements whose continuous interplay constitutes the natural and social world.
The yin-yang framework is foundational to the Yijing (Book of Changes) and to the cosmological speculation of the Han period, where it is integrated with the doctrine of the five phases (wuxing). It informs Daoist alchemy, classical Chinese medicine, and the cosmological background assumptions of Neo-Confucianism. Zhu Xi and the Neo-Confucians drew on yin-yang theory to articulate the relation between the underlying principle (li) and the material force (qi) through which it is realized.