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What is Yin Yang theory
Ancient Chinese people were greatly interested in the relationships and patterns that occurred in nature. Instead of studying isolated things, they viewed the world as a harmonious and holistic entity. In their eyes, no single being or form could exist unless it was seen in relation to its surrounding environment. By simplifying these relationships, they tried to explain complicated phenomena in the universe.

What is the Yin Yang Theory?

Yin yang theory is a kind of logic, which views things in relation to its whole. The theory is based on two basic components: yin and yang, which are neither materials nor energy. They combine in a complementary manner and form a method for explaining relationships between objects. Gradually, this logic was developed into a system of thought that was applied to other areas. Traditional Chinese Medicine (TCM) is an example of one area where the yin yang theory is used to understand complicated relationships in the body.|

The Origin of the Yin Yang Theory

The original concept of yin and yang came from the observation of nature and the environment. "Yin" originally referred to the shady side of a slope while "yang" referred to the sunny side. Later, this thinking was used in understanding other occurrences, which occurred in pairs and had complementary and opposing characteristics in nature. Some examples include: sky and earth, day and night, water and fire, active and passive, male and female and so on. Working with these ideas, ancient people recognized nearly all things could have yin and yang properties. Yin and yang can describe two relative aspects of the same phenomena such as the example of the slope, or they can describe two different objects like sky and earth.

Usually, yang is associated with energetic qualities. For example, movement, outward and upward direction, heat, brightness, stimulation, activity and excitement are all yang qualities. Yin, on the other hand, is associated with the physical form of an object and has less energetic qualities such as rest, inward and downward direction, cold, darkness, condensation, inhibition, and nourishment. See Table 1 for a description of yin and yang characteristics.

Table 1 Examples of Yin Yang Pairs

	Yang	Yin
Light	Bright	Dark
Temperature	Hot	Cold
Position	Upper	Lower
Action	Movement	Rest
Direction	Outward	Inward
Physiological functions	Excitatory	Inhibitory

Properties of Yin and Yang

By describing how things work in relation to the universe and to each other, the yin yang theory establishes a dynamic thought process that can be applied to everyday life.

1. Yin and Yang oppose each other.
Yin yang theory believes everything has an opposing yin and yang aspect. These aspects are mutually controlled and inhibited by each other, which results in a continuous state of dynamic balance. For example, heat can dispel cold while cold can reduce heat. If there is not enough heat, it will become cold and vice versa. Another example is the physiological functions in our body. Both the excitatory (yang) and the inhibitory (yin) functions are in mutually controlled balance. If the dynamic balance is disturbed, one aspect may become excessive causing serious health problems.

2. Yin and Yang mutually create and depend on each other.
Both yin and yang cannot exist without each other or stand alone. They depend on each other for definition and can only be measured by comparing themselves to each other. For example, heat ceases to exist (yang aspect) if there is no such thing as cold (yin aspect). Without an understanding of hot and cold, there would only be one temperature. Height (yang aspect) cannot be measured if there is not a low reference point (yin aspect); otherwise, everything would be at one level. In addition, the comparisons between yin and yang are relative to the objects being compared. For example, when soup is first cooked it is hot but after it a while it becomes cold, but the hot cold distinction is relative to a cold an ice cube and boiling water.

According to the yin yang theory, our physical body is closely related to its physiological functions. The activity (yang) of our body is nourished by its physical form (yin), and the physical form is created and maintained by the body's activity. They rely on each other to achieve a balanced state of health.

3. Yin and Yang change and grow in a cyclic and balanced manner.
Yin and yang achieve a state of balance by mutual control and inhibition. The balance is neither static nor absolute, but is maintained within certain limits. At certain times, yin expands while yang diminishes. At other times, the opposite is true. The change of seasons illustrates this concept. From winter through spring and summer, the weather changes from cold to hot. This is a process where yang (heat) grows and yin (cold) diminishes. On the other hand, the weather will change from hot to cold from summer through autumn and winter. A process where yin expands and yang diminishes. Over time, the proportion of hot (yang) and cold (yin) weather will be balanced and in harmony.
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4. Yin and Yang transform into each other.
When one aspect goes to an extreme, it will undergo a reverse transformation into the opposite character. This sudden transformation usually takes place in a particular situation. For example, when summer reaches the hottest day (extreme yang), the weather begins changing in a reverse manner. Instead of becoming hotter, it starts to become cooler. When winter reaches its coldest day (extreme yin), the weather reverses its direction and becomes warmer. This transformation is the source of all changes, which allow both yin and yang to create each other. In the body, the pattern of yin yang transformation happens when excitatory and inhibitory functions transform into one another.

Application in human body structures
Traditional Chinese Medicine (TCM) believes the human body has organic unity. The sense of unity is based on the opposing and complementary relationships of yin and yang. The body's organs and tissues can be classified according to yin yang theory based on their functions and locations. The upper body belongs to yang while the lower body belongs to yin. Other yin yang pairs in the body include the interior (yin) versus the exterior (yang), the front (Yin) versus the back (yang), the inside (yin) versus the outside (yang) of the limbs and the five yin organs versus the six yang organs. Each organ can also be further divided into yin and yang aspects such as heart yin and heart yang and kidney yin and kidney yang.

Physiological application
TCM believes health is achieved when yin and yang are in harmony. As already mentioned, the body's physical form belongs to yin while the body's activities or functions belong to yang. Because both the body's physical form and functions are dynamically balanced, they mutually restrict and depend on one another. The body cannot function if it doesn't have a physical form in which to perform them. Furthermore, physiological functions can consume certain physical forms (material) and metabolize these materials to obtain energy.

Pathological application
TCM believes yin yang disharmony is the cause of disease and physiological disorders. Disharmony means the proportions of yin and yang are unequal and unbalanced. When one aspect is deficient, the other is in excess. There are many factors that cause yin and yang disharmony, but they are all related to the "evils" (outside influences that cause disease) and the flow of qi throughout the body. When a person has normal qi flow, their body functions well and has good immunity allowing them to recover easier from illness. Normal qi is composed of yang qi (physiological functions and energy) and yin fluid (physical form and the physiological fluid of body) while the "pernicious evils" are composed of six evils. Cold and dampness belong to yin evils while wind, dryness, summer heat, and fire belong to yang evils. Hence in TCM, disease results from either a deficiency of normal qi (deficiency of yin fluid or a deficiency of yang qi) or an excess of the "pernicious evils" (excess of yin evils or yang evils). The conflict between resuming normal qi and getting rid of the "pernicious evils" is what allows the disease to either progress or transform back to a healthy state. By applying the yin yang theory to treat and diagnose diseases, yin yang harmony can be restored and health maintained.

Diagnostic application
TCM diagnoses patients according to their disharmony pattern. Preliminarily, all patterns are classified into a system known as the "eight principal patterns." The "eight principal patterns" contain four pattern pairs: interior and exterior, cold and heat, deficiency and excess, and yin and yang. Among the eight patterns, yin and yang are the most fundamental and essential pattern pair. (See Table 2.). Generally speaking, yang signs are associated with excitatory, active, hot, progressing externally and developing upward and improving manifestations. Yin signs, on the other hand, are associated with inhibitory, resting, passive, cold, progressing internally and developing downward and worsening manifestations. Table 3 illustrates the clinical signs of yin and yang patterns.

Table 2 Eight Principal Disharmony Patterns

	Disharmony Pattern	Yin/Yang	Manifestations
1	Exterior	Yin	An exterior pattern is generated by "external influences" such as wind and cold. (i.e. common cold)
2	Interior	Yang	An interior pattern is generated by internal disharmony such as a disorder of organ function.
3	Cold	Yin	A cold pattern is manifested by the signs such as a pale face, cold limbs, aversion to cold, clear urine, or watery stools. The signs are usually related to non-excitatory physiological functions.
4	Heat	Yang	A heat pattern is manifested by signs of a red face, high fever, dislike of heat, dark urine, or constipation. The signs are usually related to excitatory physiological functions.
5	Deficiency	Yin	Signs of frail and weak movements, tiredness, shortness of breath, low voice, or dizziness indicate a deficiency pattern. The signs are usually related to lack of energy of normal functions.
6	Excess	Yang	An excess pattern exhibits signs of heavy movements, heavy and coarse respiration, or discomfort when touched with pressure. The signs are usually related to an excess/accumulation of evils/metabolic waste.
7	Yin	Yin	General pattern groups for Yin manifestations include interior, cold and deficiency patterns.
8	Yang	Yang	General pattern groups for Yang manifestations include exterior, heat and excess patterns.

Table 3 Yin and Yang signs in the body

Body signs	Yang signs	Yin signs
Face	Red face, fever, hot feeling, agitated and active manner	Pale face, low spirit, feels cold, cold limbs, tired and weak,
Voice	Coarse and strong voice, coarse breath, dry moth, thirst	Low voice, reduced appetite, no taste in mouth, little thirst
Urine	Constipation with awkward smell, dark urine	Copious and clear urine
Tongue	Red and scarlet tongue or yellow and black moss present on tongue	Pale and swollen tongue material
Pulse	Rapid and floating, flooded and strong, slippery and full pulse	Slow and sinking, weak, frail pulse

Therapeutic application of yin yang theory
Resuming yin yang harmony is the universal treatment goal of TCM. The therapeutic strategy is to replenish the deficiency and remove the excess. When one aspect is excessive and the other aspect remains normal, the treatment aims at clearing away the excess. On the other hand, when one aspect is deficient and the other remains normal, the treatment is to replenish the deficiency. If excess and deficiency take place simultaneously, both removing the excess and replenishing the deficiency are necessary.

1. Excess of Yang
When the yang aspect is in excess and the yin aspect is normal, disharmony occurs. For example, persons with pneumonia (an infection of the lungs) may have a high fever, red face, coarse respiration and a rapid and big pulse. The physiological signs like fever and a fullness of the pulse are considered to be in relative excess of "heat." Since the yin aspect is still normal, the heat symptoms are an excessive type.

The therapeutic strategy for treating the pneumonia relies on cooling down "excessive heat" with "cold" herbs. (The word "cold" is used to describe the nature of certain herbs that have yin properties.) Once the heat is removed, yin yang balance and health are restored.

2. Deficiency of Yin
Normally, yin and yang mutually control and balance each other. If the yin aspect becomes deficient suddenly, a relative excess of yang develops resulting in a relative excess of heat. This type of heat is also called "virtual heat" because it is caused by a yin deficiency and not yang excess. For example, in hyperthyroidism, a state of yin deficiency, people can experience symptoms of insomnia, palpitations, irritability, and have a thin and rapid pulse. Unlike the condition of "excessive heat" described under yang excess, "deficiency or virtual heat" cannot be treated with "cold" herbs. Rather, the disharmony must be treated using yin nourishing herbs.

In TCM, yin deficiency also refers to the deficiency of yin fluids such as blood and body fluids. Without sufficient nourishment, excitatory functions become dominant leading to symptoms of afternoon fevers or night sweats. Table 4 illustrates more detailed symptoms of yin deficiency. In general, virtual heat symptoms will automatically disappear when the deficient yin is replenished, and the body resumes its balance.

Table 4 Signs of Yin Yang disharmony patterns

	Yin/Yang harmony	Signs	Tongue	Pulse
Excess Heat	Excess Yang	High and sustaining fever; thirst; abdominal distention and pain that intensifies with pressure, dark urine	Think yellow moss; red tongue material	Rapid and flooded;slippery and full
Virtual Heat	Deficient Yin	Low grade fever; dry mouth; hot feeling in palms; night sweat; thin appearance; malaise	Little moss; red tongue material	Rapid and thin
Excess Cold	Excess Yin	Cold limbs; fear of cold; abdominal pain that intensifies with pressure; constipation	Pale tongue material; white thick and moist moss	Sinking; tight or wiry
Virtual Cold	Deficient Yang	Cold limbs; fear of cold; abdominal pain and pressure relieves discomfort; frail and weak manner; watery stool; copious and clear urine	Pale or swollen tongue material	Frail; sinking, slow and weak

Herb: DA HUANG
Taste: bitter
Nature: cold
YIN HERB

3. Importance of distinguishing disharmony pattern

Distinguishing the disharmony pattern is very important in TCM therapy. As in the previous example, both "virtual" and "excessive" heat have heat symptoms but there are differences in their presentations. "Excessive heat" can lead to a high fever while "virtual heat due to yin deficiency" may cause just a night fever or a low-grade fever. A person with "excessive heat" has a pulse that is rapid and full, while a person with "virtual heat due to yin deficiency" has a rapid and thin pulse.

Herb: BA JI TIAN
Taste: sweet, pungent
Nature: slightly warm
YANG HERB

Herbs and their Yin and Yang Properties

TCM also classifies herbs of different "natures" and "tastes" according to their yin and yang properties. Cold and cool nature herbs belong to yin while hot and warm herbs belong to yang. Yin herbs taste sour, bitter and salty, and yang herbs taste sweet and pungent. The yin yang theory also describes the effects of herbs. Herbs with floating and ascending properties are part of yang while herbs with sinking and descending properties are part of yin. In choosing the correct herbs for treatment using TCM, it is essential to first identify the disharmony pattern and then select herbs with the appropriate nature, taste and effect. By following these simple steps, Traditional Chinese Medicine can help maintain the body's balance and health.

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