The Yin–Yang theory , the five phases theory (Wu Xing, ), the meridian system theory , and the viscera-state theory (manifestation of Zang-viscera, ) are the key theories of TCM, which properly align various viscera in an orderly fashion, according to the disemboguement sequence of the human body-field. These theories explain the dialectic relationships between the outside and the inside , between the engendering , and the restraining and between the overwhelming and the rebelling . The viscera organs are not isolated from each other. Instead, they are widely connected with the surroundings, and correspond and mutually function with each other. It is the connection that makes the human body a whole life system. These connections include the meridian system and gaps or spaces in the body.
The Yin–Yang theory and the five phases theory are not independent of each other but unified. The idea that “nature and man are united as one system” is well recognized in the Yin–Yang theory. Human beings can resist changes of the surroundings, which include changes in social relations and the natural environment, because everyone is naturally part of this system. These changes are closely connected with each other. For example, changes in the universe would be related to gravitational waves which influence atoms of the human body; or changes of the location of stars would influence the transportation and transmission of bodily fluids (substance) and its existing condition through electromagnetic waves. Each cell of the human body in this environment would change with bodily fluids. The physiological alteration of the human body can be considered an adaptation to the changes in the environment.
Human beings are not only living along with the surroundings, but are also unified with changes in the surroundings. Because of its small size relative to the whole of nature, the human body’s requisite for existence is to adapt to changes in the surroundings. If one opposed or could not stay in balance with changes in the outside world, then one would not maintain normal body function and would get sick.