“Yin-yang theory” is a methodology through which the ancients understood and explained the laws governing changes in the natural world, based on the perspective of the unity of opposites. It is a cornerstone of ancient Chinese philosophical thought and pervades every aspect of the traditional Chinese medicine (TCM) theoretical system, including physiology, pathogenesis, pattern identification and treatment, and the clinical reasoning behind formula selection and prescribing.

Suwen (Plain Questions), in chapter Yinyang Yingxiang Dalun (Great Treatise on the Correspondences of Yin and Yang), states: “Yin and yang are the Dao of heaven and earth, the binding principle of all things, the parents of change, the root and beginning of life and death, and the abode of the bright spirit. In treating disease, one must seek the root cause.” This demonstrates the paramount importance of yin and yang. Yin governs stillness; yang governs movement. Yin and yang mutually oppose and restrain each other, yet reciprocally root in and depend upon each other, maintaining equilibrium through their continual waxing and waning. The four seasons are one of the greatest manifestations of this yin-yang equilibrium. The seasons are not static divisions, but are formed through the dynamic waxing and waning and mutual transformation of yin and yang qi.

In spring, yang qi begins to rise and yin qi gradually recedes. All things are revived; tender shoots break through the earth, brimming with vitality.

In summer, yang qi reaches its zenith and yin qi lies dormant within. All things grow to their most luxuriant, radiating brilliance.

In autumn, yang qi begins to descend and yin qi gradually increases. The autumnal air is stern and severe, as all things wither and prepare for the coming winter.

In winter, yin qi is at its peak and yang qi lies concealed. Heaven and earth appear vast and desolate, as all things hibernate, conserving nutrients to await the arrival of spring.