The Theory of Yin and Yang is one such philosophy. It is said to date back nearly 6,000 years to the third or fourth millennium B.C. and is attributed to an enlightened philosopher named Fu Shi The basic premise of yin and yang is the notion that the only constant factor in natural phenomena is universal change. In other words, nothing remains the same; no disease, no condition, no emotion, no treatment or diagnosis, absolutely everything is in a constant state of flux and, therefore, subject to the laws of change.
The entire universe may be viewed as the interplay and alternation of yin and yang. Originally the Chinese characters for yin represented the moon and yang represented the sun. Gradually these terms were broadened to include yin as night and yang as day, yin as winter and yang as summer, and yin as female and yang as male. In fact, there is nothing which cannot be viewed from the standpoint of yin and yang.
It is important to remember that yin and yang are not static concepts and that they are constantly influencing and determining one another. There is always some measure of yin within yang and vice versa. To use the analogy of a hillside; during the day the sunlit side of the hill is yang within yang, while the shaded side is yin within yang.
These types of relationships become significant when they impact the body's anatomy and physiology and it is precisely these designations that are used in the diagnosis of imbalances in TCM. For a TCM practitioner, the name of the disease is of secondary importance. The primary key to the proper diagnosis of syndromes is the identification of the condition in terms of yin or yang. There is no Yin without some Yang and vica versa. To maintain this harmony guarantees health and happiness.