Theory of Yin-Yang

The theory of Yin and Yang, derived from prolonged observations of nature, describes the way phenomena naturally group in pairs of opposites - heaven and earth, sun and moon, night and day, winter and summer, male and female, up and down, inside and outside, movement and stasis. These pairs of opposites are also mutual complements.

The Basic Principles of Yin-Yang Theory

All phenomena in the universe may be ascribed to Yin and Yang. Each individual phenomenon possesses both a Yin and a Yang aspect. Yin and Yang are natural complements in the sense that they depend upon and counterbalance each other. Further, they are mutually convertible, since either may change into its complement.

1) Yin and Yang as the fundamental categories of all phenomena

In medicine, the concepts of Yin and Yang are generally used to categorize both anatomic parts and physiologic functions. For example, the back is Yang and the abdomen is Yin; the six bowels are Yang and the five viscera are Yin; Qi is yang and blood is Yin; agitation is Yang and depression is Yin. Similarly, diseases may be categorized according to Yin and Yang. For example, exterior, repletion, and heat disease patterns are Yang, while interior, vacuity, and cold disease patterns are Yin. Pulses may be similarly categorized: floating, rapid, and slippery pulses are Yang, while deep, slow, and rough pulses are Yin.

2) Yin and Yang are divisible

Every phenomenon may be classified as Yin or Yang in contrast to another. Each Yin or Yang phenomenon itself possesses both Yin and Yang aspects that may be further divided in the same way. This process of division may be carried on ad infinitum.

Medicine makes extensive use of this infinite divisibility of Yin and Yang not only in anatomy, physiology, and pathology, but also in pattern identification and treatment. 

3) Yin and Yang are rooted in each other

The notion that Yin and Yang are rooted in each other means that they are mutually indispensable and engendering. Yin and Yang are interdependent. Yin exists by virtue of Yang and Yang exists by virtue of Yin.

In medicine, the concept of the interdependence of Yin and Yang is widely used in physiology, pathology, and treatment. Qi and blood, two fundamental elements of the human body, provide an example: Qi is Yang and blood is yin. It is said that Qi engenders blood, i.e. blood formation relies on the power of Qi to move and transform food; “Qi moves the blood,” meaning that blood circulation relies on the warming and driving power of Qi. Furthermore, “Qi contains the blood,” i.e., it keeps the blood within the vessels. The functions of engendering, moving, and containing the blood are summed up in the phrase, “Qi is the commander of the blood.” Conversely, Qi is dependent on the provision of adequate nutrition by the blood; thus it is said that Qi has its abode in the blood and blood is the mother of Qi. Because Qi has the power to engender blood, treatment of blood vacuity involves dual supplementation of Qi and blood. Massive bleeding, where Qi deserts with the blood, is first treated by boosting Qi, since blood-nourishing formulas should not be administered until Qi is secured. Similarly, formulas used to treat Qi vacuity often include blood-nourishing agents to enhance Qi supplementation.

Another example of the interdependence of Yin and Yang, seen in the development of diseases, is the principle that “detriment to Yin affects Yang” and “detriment to Yang affects Yin.” Since “without Yang, Yin cannot be born,” when Yang vacuity reaches a certain point, the production of Yin humor is affected and Yin also becomes vacuous. Most cases of chronic nephritis indicate Yang vacuity and are characterized by water swelling due to the inability of the kidney to transform fluids. However, when the Yang vacuity reaches a certain point, fluid formation is affected and a Yin vacuity pattern evolves. This demonstrates the principle of “detriment to Yang affects Yin.” Similarly, Yin vacuity, when reaching a certain peak, leads to simultaneous Yang vacuity, since “without Yin, Yang cannot arise.” What is termed high blood pressure in Western medicine usually corresponds to hyperactivity of yang caused by vacuity of Yin. In severe cases, this condition may develop into a dual Yin-Yang vacuity, illustrating the principle that “detriment to Yin affects Yang.”

4) Yin and Yang counterbalance each other

The Yin and Yang aspects of the body counterbalance each other. A deficit of one naturally leads to a surfeit of the other, while a surfeit of one will weaken the other. In both cases, Yin and Yang no longer counterbalance each other, and disease arises as a result. In medicine, the notion of counterbalancing is widely applied in physiology, pathology, and therapy.

In physiology, for example, liver Yin counterbalances liver Yang, preventing it from becoming too strong. If liver Yin becomes insufficient and fails to counterbalance its complement, ascendant hyperactivity of liver Yang develops. In the relationship of evils to the human body, Yang evils invading the body will cause a surfeit of Yang, which may lead to damage to Yin humor and the emergence of a heat pattern. Conversely, a Yin evil entering the body will lead to a surfeit of Yin, causing damage to the body’s Yang Qi and the emergence of a cold pattern.

In therapy, if a disease is caused by heat evil, it is treated with cool or cold agents according to the principle that “cold can counteract heat,” meaning Yin agents combat Yang evils. Similarly, disorders caused by cold evil are treated with warm or hot agents. since “heat can overcome cold,” i.e., Yang agents can combat Yin evils. This is summed up in a guiding principle of therapy, “heat is treated with cold; cold is treated with heat.” It is most often applied in patterns of repletion characterized by a surfeit of either Yin or Yang.

In conditions caused by deficit of Yin or Yang, the opposing complement is no longer kept in check and becomes disproportionately strong. If Yin is vacuous, Yang is no longer kept in check and its strength will grow out of proportion to that of Yin. Such a condition is at root a Yin vacuity, manifesting itself as vacuity heat. For this reason, treatment by draining fire and clearing heat alone is not only ineffective but also detrimental to the patient’s health. It is replaced by a method such as enriching Yin and downbearing fire, or fostering Yin and subduing Yang, whereby clearing heat and draining fire are secondary to enriching Yin. By supplementing Yin, the Yang surfeit will naturally diminish. This explains the principles, “where cooling is to no avail, water is lacking,” and “invigorate the governor of water to counteract the brilliance of Yang.” In the reverse situation, where Yang is vacuous and fails to keep Yin in check, there is exuberant internal Yin cold manifesting in such forms as clear-food diarrhea, fifth-watch diarrhea, and water swelling. Here, treatment should aim not simply at dissipating cold evil, but also at supplying the Yang vacuity through such methods as assisting Yang, boosting fire, and supplementing Qi. This demonstrates the principle, “where warming is to no avail, fire is lacking,” and “boosting the source of fire to eliminate the entrenched surfeit of Yin.”

It is important to note the difference between the natural ebb and flow of Yin and Yang and a surfeit of one or the other complement. The natural flux of Yin and Yang refers to their normal relationship in the human body, which is one of constant fluctuation, rather than a rigid, immutable balance. “When Yin rises, Yang ebbs,” and “when Yang swells, Yin subsides.” This constant fluctuation is apparent in all the body’s physiologic functions, such as fluid production and metabolism, the role of the five viscera in storing essential Qi, and the role of the six bowels in conveyance, and transformation of food. By contrast, “deficit” and “surfeit” denote the disturbance of the normal relative balance and failure to rectify the imbalance immediately. This is known as imbalance of Yin and Yang, which is the underlying cause of all disease.

5) Mutual convertibility of Yin and Yang

In medicine, examples of Yin-Yang conversion are found mainly in pathology where yang patterns can develop into Yin patterns and vice versa. In practice, this means that heat patterns can either turn into or develop from cold patterns and vacuity can give way to, or supercede, repletion. For example, what Western medicine calls infectious hepatitis in its acute icteric phase manifests, in Chinese medical terms, in damp-heat signs such as yellowing of the face and eyes, fever, nausea, vomiting, rib-side pain, oppression in the chest, torpid intake, and thick, slimy tongue fur. However, when the condition becomes chronic and develops into cirrhosis of the liver, the patient will show signs of vacuity such as lassitude of spirit, lack of strength, dizziness, a dull pain in the chest and rib-side, no enjoyment of food, and a dark red tongue. This indicates that the condition of repletion has turned into one of vacuity. If the condition develops further, stagnation of water-damp gives rise to ascites, manifesting as distention and fullness in the chest and abdomen, showing that the condition has reverted from vacuity to repletion. However, the resultant condition of repletion is different from the original one. In the initial condition, although evil Qi is strong, right Qi is still relatively unaffected. However, in the resultant condition, evil Qi is exuberant in a body left frail by serious damage to right Qi.

Cold-heat and vacuity-repletion conversion are subject to specific variables such as the strength of the patient’s defenses, the nature of the evil, and choice of treatment. For instance, wheezing and panting may change from the original cold pattern to a heat condition, owing to repeated contraction of external evils. In cases of pyelonephritis, the original disorder is expressed in Chinese medicine as damp-heat in the lower burner. Pyelonephritis may, owing to unthorough treatment, resistance of bacteria to drugs, or repeated relapses, develop into insufficiency of kidney Yin, manifesting as effulgent Yin vacuity fire, a form of vacuity heat.