Yin – Yang theory is an ancient theory that explains the nature of all phenomena. It is the Tao, the nature of things, the cosmos, and the universe. The characters of yin and yang demonstrate this concept and allow for interpretation. Imagine a mountain or river, there is a dark side and a light side. The dark or cloudy side is the yin aspect, the side that is bright or sunny is the yang. Examples of yin – yang pairs or principles are night – day, moon – sun, inside – outside, winter – summer, down- up, stasis – movement (see table).
It is easy to memorize this table but each word or component relates to the way things change, the interplay of fluid characteristics, the cyclical movement of peaks and bases. Yin – yang may seem like opposites but they are mutual complements. One cannot exist without the other and either may change into its counterpart. As discussed in the I Ching (Book of Changes), “Change is not meaningless- if it were, there could be no knowledge of it- but subject to the universal law, Tao.” 
Yin – Yang fundamentals are the basis of Chinese Medicine theory and everything, including anatomy, physiology, pathology, diagnosis, treatment principles, and treatment types can be described using this holistic concept.
“Be still like a mountain,
move like a great river.” 
1. The I Ching
2. The Essence of Tai Chi Chuan
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