is one of the most fundamental concepts in Traditional Chinese Medicine (TCM), as it is the foundation of diagnosis and treatment. The earliest reference to Yin and Yang is in the I Ching (Book of Changes) in approximately 700 BC. Yin & Yang are general terms for the two opposite aspects of matters and phenomena in nature, which are interrelated and contradicted to each other. They coexist in a dynamic state, an excess of one will lead to the decrease of the other and vice versa. Yin & Yang are the unceasing movement, variation and development of everything. In Chapter five of Familiar conversation: Yin & Yang is the principal to heaven and earth, the guiding discipline of all things under the sun, the origin of change, the root and starting point of life and death and the mansion of divinities.
Yin and yang can be thought of as complementary (rather than opposing) forces that interact to form a dynamic system in which the whole is greater than the assembled parts. Everything has both yin and yang aspects, (for instance shadow cannot exist without light and light cannot exist without dark, high and low, hot and cold, fire and water, life and death, male and female, sun and moon,). Either of the two major aspects may manifest more strongly in a particular object, depending on the criterion of the observation. In Chinese philosophy, Yin & Yang, are concepts used to describe how apparently opposite or contrary forces are actually complementary, interconnected and interdependent in the natural world, and how they give rise to each other as they interrelate to one another. Many tangible dualities are thought of as physical manifestations of the duality of yin and yang. This duality lies at the origins as well as being a primary guideline of traditional Chinese medicine, and a central principle of different forms of Chinese martial arts and exercise.