陰陽者，天地之道也，萬物之綱紀，變化之父母，生殺 之本始，神明之府也，治病必求於本。

Yin and Yang are the Way of heaven and earth. They are the governing principles of everything in the universe, source of all changes and transformations, the basis of beginnings and ends, and the house of one's soul and spirit. In order to treat illnesses, one must search for the root causes. 

[Chapter 5, Huang Di Nei Jing Su Wen, compiled approx. during 475-221 BC)