Also yīn-yáng. Two mutually complementary and opposing principles in Chinese thought: one dark, female, receptive (yīn), and the other bright, male, active (yáng). The term also refers to the scheme that the two together form.

Yīn-yáng is applied in every aspect of Chinese medicine, including physiology, pathology, diagnosis, treatment, and pharmacy. Its most important application lies in physiology and pathology, where it explains health and sickness in terms of balance and imbalance.

Associations of Yīn and YángYīnYángOriginal meaningsFacing away from the sunFacing the sunKernel IdeasDark, cold, stillness, moistnessLight, heat, activity, drynessPositions / DirectionsDown, in, northUp, out, southTimeAutumn-winter, nightSpring-summer, dayLife cycleDecline, deathBirth, growth, developmentChange in size and densityContraction, concentrationExpansion, diffusionStateSolid, liquidGasWeightHeavyLightMovementDownward, inwardUpward, outwardGenderFemaleMale

Positions and directions: Because upward-facing surfaces receive more light than downward-facing surfaces, up is classed as yáng and down as yīn. Because the outside of an object receives more light than the inside, exterior is classed as yáng and interior as yīn. Because northern positions are generally colder and darker than southern positions, north is classed as yīn and south as yáng.

Time: Because nighttime and winter are darker and colder than daytime and summer, night and winter are yīn, while daytime and summer are yáng. Spring and autumn are intermediate seasons, neither being colder or warmer than the other. However, spring is classed as yáng because it marks a tendency toward increasing heat, while autumn marks a tendency toward increasing cold.

Life cycle: The yearly life cycle of nature follows the seasons. Life develops in the spring and summer, while it declines in the autumn and winter. Birth, growth, and development occur mainly in spring and summer, and hence are yáng, while decline, wilting, withering, and death occur mostly in autumn and winter, and hence are yīn.

Change in size and density: Heat generally causes things to expand and cold causes things to contract. Expansion and decrease in density are yáng, while contraction and condensation are yīn.

States: Solids and liquids are dense and tangible and therefore yīn, while gases are diffuse and intangible and therefore are yáng. Readers will remember that this distinction was important to early Chinese philosophers, who posited that the whole universe was made of a single substance qì that existed in different densities: yīn qì (tangible substance), yáng qì (intangible substance).

Weight: Heavy things are dense and therefore yīn, while light things are yáng.

Movement: As an activity, movement is classed as yáng, while stillness is classed as yīn.

Gender: Female is yīn, while male is yáng. This classification is based on physical characteristics, traditional activities, and related qualities.

• Males work for sustenance and defense of territory, so they have the capacity to develop greater physical strength; they spend more time outside the home. Hence, physical strength, assertiveness, aggression, risk-taking, and object-orientation are yáng male qualities.
• Females nurture children and care for the aged; these tasks traditionally kept them at home. Hence relative physical weakness, sensitivity, people-orientation, desire for security, and a tendency to avoid aggression are yīn female qualities.

These characterizations of gender are only relative. People in the modern world may find them objectionable, especially if they are held up as standards of expected behavior. However, they are reflected in such biomedically recognized facts as that men are more prone than women to risky behavior, bouts of heavy drinking, acts of violence, and strokes than women, and less prone than women to rumination. In Chinese medicine, this manifests in a difference in incidence of disease patterns.

Apart from these general yīn-yáng characteristics, Chinese medicine recognizes differences between the sexes related to reproductive function. Discharge of blood through menstruation means that the blood is accorded more importance in females than males. Yáng qì is considered more important in male physiology.

Water and Ice, Men and Women

When water freezes, it increases in volume and decreases in density. Because of this, ice floats on water. This phenomenon is a notable exception to the notion of cold being associated with contraction. Male is classed as yáng and female as yīn because men are stronger and more active, while women are weaker but more nurturing. It is true that men are generally heavier than women, which means that the sexes might be classified in the opposite fashion. Nevertheless, greater physical strength for strenuous activity trumps weight.

Interactivity

The yīn-yáng system explained not only correspondences between different things. It was also used to formulate interactions between paired objects and phenomena. For example, natural philosophers observed how day gradually gives way to night as summer gradually gives way to autumn and winter in the diurnal and annual cycles. Early cosmologists saw the interaction of yīn and yáng as powerful force shaping our world.

Very importantly, they also noticed the interactions between fire and water. Fire causes water to evaporate, while water can extinguish fire. This latter observation was of especial significance in Chinese medicine because it provided a seminal idea for understanding basic physiological and pathological processes. Most illnesses are marked by a tendency toward heat/activity/dryness or cold/inactivity/moistness, which indicates an imbalance between yīn and yáng.

Chinese Cosmogeny

A passage from Huái Nán Zǐ translated in Sources of Chinese Tradition. Volume 1, Wm. Theodore de Bary, Wing-Tsit Chan, and Burton Watson, eds. (New York, Columbia, 1960), pp.192–3, describes one of the early Chinese cosmogenic theories highly influenced by the doctrine of yīn and yáng. Before heaven and earth had taken form, all was vague and amorphous. Therefore, it was called the Great Beginning. The Great Beginning produced emptiness and emptiness produced the universe. The universe produced material-force (i.e., qì), which had limits. That which was clear and light drifted up to become heaven, while that which was heavy and turbid solidified to become earth... The combined essences of heaven and earth became the yin and yang; the concentrated essences of the yin and yáng became the four seasons, and the scattered essences of the four seasons became the myriad living things of the world. After a long time, the hot force of the accumulated yang produced fire and the essence of the fire force became the sun; the cold force of the accumulated yin became water and the essence of the water force became the moon. The essence of the excess force of the sun and moon became the stars and planets. Heaven received the sun, moon, and stars while earth received water and soil.

Relationships and Interactions Between Yīn and Yáng Phenomena

Things that are paired as yīn and yáng relate to each other and interact with each other in similar ways. These are interdependence, mutual restraining, divisibility, waxing and waning, and yīn-yáng conversion.

Interdependence: Yīn and yáng are rooted in each other (阴阳互根 yīn yáng hù gēn). Objects and phenomena that form yīn-yáng pairs depend upon each other: there is no up without down, no summer without winter, no cold without heat.

Mutual restraining: Yīn and yáng restrain each other (阴阳互约 yīn yáng hù yuē). They counterbalance each other. Heat can restrain cold; cold can restrain heat. Fire evaporates water and water douses fire.

Divisibility: Yīn and yáng are divisible (阴阳可分 yīn yáng kě fēn), which means that any item forming a yīn-yáng pair with a complementary opposite item can be divided into yīn and yáng. For example, movement in relationship to stillness is yáng, but upward movement is more yáng than downward movement. Thus, upward movement, being relatively yáng in all regards, is yáng within yáng (阳中之阳 yáng zhōng zhī yáng), while downward movement, being a yáng phenomenon that is relatively yīn, is yīn within yáng (阳中之阴 yáng zhōng zhī yīn). North is yīn, while south is yáng. Yet both northern and southern positions can be divided into yīn and yáng depending on whether they face toward or away from the sun.

Although yīn and yáng are divisible, they are not separable. They cannot be isolated from each other. Downward movement cannot exist without upward movement.

Waxing and waning: Yīn and yáng wax and wane (阴阳消长 yīn yáng xiāo zhǎng), which means that when one grows, the other diminishes. Many yīn-yáng phenomena evince a waxing and waning relationship. For example, from winter to summer, cold qì decreases and heat qì increases, embodying the principle of yīn waning and yáng waxing, while from summer to winter, heat qì declines and cold qì increases, evincing the principle of yáng waning and yīn waxing.

Mutual conversion: Yīn and yáng convert into each other (阴阳转化 yīn yáng zhuǎn huà) means that when a yīn or yáng phenomenon reaches its extreme, it can change into its opposite.

In Chinese, the tendency of things in general to give rise to their opposite is called things reaching their extreme must turn into their opposite (物极必反 wù jí bì fǎn). This phenomenon is most apparent in things that are classified as a yīn and yáng.

The waxing and waning of yīn and yáng are quantitative changes in yīn or yáng. However, when waxing and waning reach their extremes, a qualitative change occurs: yīn gives way to yáng or yáng gives way to yīn. Hence, it is said that yīn and yáng convert into each other (阴阳转化 yīn yáng zhuǎn huà). In terms of the seasons, after the greatest cold of the winter, warmth returns in the spring. This is yīn converting into yáng.

Yīn and Yáng in Medicine

When yīn and yáng were adopted in medicine, they were found to have great explicatory power in every aspect from physiology and pathology to diagnosis and therapy. All the features of yīn-yáng relationships (independence, mutual restraining, divisibility, waxing and waning, and mutual conversion) were found to be useful.

Yīn and yáng are fundamental to the Chinese understanding of the body. The distinction made by natural philosophers between yīn qì and yáng qì is central to physiology and pathology. Qì as the basic stuff of the universe can assume different forms: Yīn qì (阴气 yīn q ì) is tangible, concentrated, relatively inert qì, while yáng qì (阳气 yáng qì) is intangible, diffuse, active qì. Early physicians adopting this idea believed that the whole body was composed of qì. Yīn qì is the solid matter (flesh, sinews, bones, fluids, blood), while yáng qì is a diffuse form of qì that powers all activity within the body.

Fallacies Concerning Yīn and Yáng

Some Western students have difficulty understanding yīn and yáng because they confuse them with evil and good. In our culture, light and darkness have connotations of good and evil and therefore relate to each other in a clear hierarchy of value. Such connotations do not apply in the yīn-yáng system. Yīn and yáng are interdependent, so one cannot be preferable to the other. Another common Western fallacy lies in the belief that yīn and yáng (and every other aspect of Chinese medicine) originate from Daoism. The concepts of yīn and yáng were devised by cosmologists and protoscientific thinkers long before the appearance of Daoism. The supremacy of yīn and the female over yáng and the male is one of the better-known notions of early Daoist philosophy. However, this does not reflect a Daoist preoccupation with yīn-yáng cosmology, but rather its general emphasis on balance and on the importance of things that are undervalued in human society.

Yīn and Yáng in Anatomy and Physiology

According to the principles outlined above, anatomical parts and physiological substances and functions are classified as yīn and yáng.

Yīn and Yáng in the Body
Yáng	Yīn
Upper body	Lower body
Exterior	Interior
Bowels	Viscera
Yáng qì	Yīn humor

Yīn-Yáng in Anatomy

The upper body is classed as yáng and the lower body as yīn, the back as yáng and the abdomen as yīn.

The exterior (表 biǎo) of the body, also called fleshy exterior (肌表 jī biǎo) because it is composed of skin and flesh that clad the body, is yáng, while the interior (里 lǐ), which includes the sinew, bones, and internal organs, are yīn.

The distinction between interior and exterior is important in diagnosis and treatment. If common cold or flu manifests in heat effusion (i.e., fever) with aversion to cold, this means that the exterior is affected and that the treatment involves resolving the exterior. If the disease is affecting the interior, the treatment will be different.

Since yīn and yáng are further divisible, certain internal organs are relatively yīn (yīn within yīn), while others are relatively yáng (yáng within yīn).

The organs principally concerned with the internal workings of the body, the liver, heart, spleen, lung, and kidney, are classified as yīn. These are understood to be yīn within yīn. They are called the zàng (脏) and are referred to in this text in English as the viscera. Their main function is the production and storage of essence.

The stomach, small intestine, large intestine, gallbladder, and bladder deal with substances entering the body and/or leaving it. They are called fǔ (腑) and are referred to in this text in English as the bowels. These are yáng within yīn.

Yīn and Yáng as Body Parts
Yīn or yáng are used in certain names of body parts. In Chinese, the external genitalia in the male or female are called yīn parts or simply yīn, because they are normally kept hidden (private parts). However, the penis is often referred to as the yáng thing, obviously because it is a male organ. Impotence is expressed as yáng wilt. The vagina is called the yīn pathway (阴道 yīn dào).

The channel system provides connections between all parts of the body, including the bowels and viscera. Each of the twelve main channels belongs to (homes to) a bowel and viscus and connects with (nets) the corresponding its exterior-interior partner. Its full name includes the extremity it runs to or from, but also the name of the bowel or viscus to which it homes as well as its yīn-yáng designation, e.g., foot greater yīn (tài yīn) spleen channel, foot yáng míng (yáng míng) stomach channel.

Yīn-Yáng in Physiology

The distinction between yīn qì and yáng qì provides the basis of the Chinese medical understanding of physiological processes. Yīn qì is the physical basis of the body, while yáng qì explains all activity. The two are complementary opposites that are constantly interacting. The Sù Wèn (Chapter 3) states, Yīn stores essence and is the ultimate beginning; yáng defends the outer body and ensures security (阴者, 藏精而起亟；阳者，卫外而为固也 yīn zhě, cáng jīng ér qǐ jí yě; yáng zhě, wèi wài ér wéi gù yě). The ultimate beginning is taken to mean the material foundation of the body.

In simple terms, yīn has the qualities of water, while yáng has the qualities of fire. Fire heats water and water cools fire. In a healthy body, fire and water restrain each other, preventing excesses of one or the other. Thus, the yīn and yáng aspects of the body are in harmony—or, to use a more familiar modern metaphor, in balance. In this sense, yīn and yáng connote cold/heat, stillness/movement, moistness/dryness, that is, all the kernel ideas of yīn and yáng except light and dark.

The term yáng qì is commonly seen in discussions of physiology and pathology. The term yīn qì is not so commonly used. While the solid aspects of the body remain relatively stable, it is the bodily fluids and blood that most regularly interact with yáng qì. Hence, the yīn side is more commonly referred to as yīn humor (阴液 yīn yè). Depletion of yīn is affects the fluids of the body first; in severe cases, however, it can cause wasting of the flesh.

Although the yīn-yáng concept of physiological balance may sound exotic to us, we are all familiar with the idea, since we spend a good deal of our lives coping with it. We consciously make yīn-yáng adjustments by choosing food, drink, and clothing, by regulating our amount of exercise, and by controlling our indoor ambient temperature.

In hot weather and warm parts of the day, our yáng qì is stronger than in cold weather and at colder times of the day. This is the natural waxing and waning of yīn and yáng in the daily and yearly cycles of nature. Heat in the environment makes us feel hot and tend to sweat. Since we sweat to disperse heat, our fluids suffer depletion that is often reflected in scant, darker-than-normal urine. Fluid depletion causes us to experience thirst, which makes us drink more fluids to maintain our yīn-yáng balance. Because activity produces heat, hot weather makes us restrict our activity (e.g., by taking the bus to work rather than cycling), again to preserve our yīn-yáng balance. In hot weather, we tend to eat cold foods, which also helps keep our temperature down. We tend to eat less than in winter, because we do not need as much energy to keep our bodies warm. In our indoor environment, we open the windows to allow cool air in or turn on the air-conditioning.

When the ambient temperature is cold, we often feel cold. To deal with this, we try to stay warm by additional clothing and staying inside and by choosing warm foods and drinks. We consume more food to boost our body’s yáng qì. We tend to consume fewer fluids, because we sweat less than in summer. We find that physical activity tends to keep us warm. Nevertheless, we tend to be less active in winter. This is probably because winter for our more primitive ancestors was a time when food was scarce and farming activity was minimal, so people relied on the limited food supplies preserved from the summer.

Because of changing environmental conditions, we must constantly adjust the yīn-yáng balance within our bodies. The measures we take are prompted by physical signs: sensations of heat and cold; thirst; desire for cold or hot foods; desire for more food or less food.

The significance of yīn-yáng in physiology can be seen in terms of kernel features of yīn and yáng. Yīn is cold, stillness, and moistness; yáng is heat, movement, and dryness. Cold weather tends to produce cold sensations, stillness, and moistness in the body, while hot weather tends to produce heat sensations, movement, and dryness.

Yīn and Yáng Feature Prevalent in Pathology
Yīn	Yáng
Cold	Heat
Stillness	Movement
Moistness	Dryness

However, individuals are not all quite the same. There are constitutional biases, so that some people have more yīn than yáng, while other have more yáng than yīn. People with less yáng than yīn tend to cold, stillness, and moistness. They tend to feel the cold and have cold extremities. They are relatively inactive, lack in energy, and easily get tired. They are fat, flabby, which Chinese medicine attributes to moistness in the body (dampness or phlegm). Sometimes they are overweight, even though owing to the clogging effect of dampness, they may not eat too much. People with more yáng than yīn tend to heat, movement, and dryness. They tend to feel hot rather than cold. They are the active type that can eat a lot without getting fat. They tend to be lean and sinewy with a dark, sometimes oily complexion. These two basic constitutional types are important in diagnosis because each type is vulnerable to different illnesses. People more yáng than yīn are prone to illness taking the form of heat patterns, while those with more yīn than yáng are prone to illness marked by cold patterns. See constitution.

The features of yīn and yáng can also be seen in the inner workings of the body. The bowels and viscera each have yīn and yáng aspects. The yīn aspect is the substance of the viscus and the blood and fluids it contains. The yáng aspect includes its qì, which powers its functional activity. Function is classified as a yáng entity, because it involves activity. However, according to the principle that yīn and yáng are divisible, function has both a yīn and a yáng aspect: a tendency to quiescence and a tendency toward activity.

Names for Yīn and Yáng

Nowadays, everyone refers to yīn and yáng in Chinese medicine by their Chinese names. However, in the eighteenth and nineteenth century, yīn was often translated as radical moisture, and yáng as innate/natural, vital heat. These translations reflect only the fire-water duality of physiology. Such renderings are only valid within the confines of human physiology and pathology and fail to reflect the broader cosmological significance of yīn and yáng.

Typical of this is the example of the heart. Heart yīn includes the physical organ of the heart and the blood and yīn humor that ensure its health and enable it to perform its functions. Heart yáng consists of the qì that powers the function of the heart as manifest in the heartbeat. The heart’s function is divisible into a yáng tendency toward activity, related to the power of its qì, and a yīn tendency toward quiescence related to its blood and yīn humor. A normal heartbeat is neither too fast nor too slow. Heart yáng makes the heartbeat strongly, but this is balanced by heart yīn. When heart yáng becomes stronger than heart yīn, the heart beats faster; when heart yīn becomes stronger than heart yáng, the heart beats slower.

Yīn and Yáng in Pathology

Classification of Disease Evils

The six excesses (wind, cold, summerheat, dampness, dryness, and fire) are classified according to yīn and yáng in the following way: Wind, summerheat, dryness, and fire are yáng, while cold and dampness are yīn.

A certain degree of congruence with yīn-yáng classification of body parts is observable. The yáng evils all tend to affect the upper and outer body, while dampness tends to affect the lower body. Cold is an exception, because it can easily affect the outer body and lung.

Yīn-Yáng Surfeits and Deficits

Transient yīn-yáng imbalances in healthy individuals arise as a result of changes in ambient conditions, such as temperature and humidity. The yīn-yáng balance of the body is upset to a greater degree whenever we fall ill.

Illness is a disturbance of normal functions. Given the multiplicity of functions, there are many illnesses. However, illness of any kind is reflected in yīn-yáng imbalances that manifest in signs similar to those of transient imbalances resulting from changes in ambient temperature: sensations of heat and cold, thirst, changes in urine, desire for hot or cold foods in certain amounts. However, these often differ in their intensity or their combinationsand are often accompanied by additional signs that do not normally occur as a result of transient ambient changes. Sensations of heat and cold, for example, may be stronger and less easily mitigated by food, clothing, and ambient temperature adjustments. A fever that is palpable to another person may be hidden to the patient, who only feels cold sensations (aversion to cold). Signs such as pain that are only ever experienced in illness may occur. Amongst the welter of signs, a yīn-yáng imbalance can always be discerned.

The yīn-yáng balance can be upset in one of two ways. Either one side grows disproportionately strong, or one side grows disproportionately weak. These two imbalances manifest in different ways.

Surfeits of yīn and yáng are the result of disproportionate strengthening of yīn or yáng. They occur when, for example, the body is affected by untoward environmental influences (external evils) or by inappropriate diet or lifestyle (e.g., excessive consumption of hot spicy food, alcohol, and smoking, which can give rise to heat or damp-heat). When they are caused by external evils, they are often of rapid onset.

A surfeit of yīn, or disproportionate exuberance of yīn, arises when the yīn evil dampness and particularly cold invades the body or as a result of eating raw and cold (yīn) foods. The yīn from outside the body merges with the yīn of the body creating an overall imbalance. At the same time, the surfeit of yīn damages yáng qì and obstructs its movement, thus exacerbating the imbalance further.

Yīn surfeits vary in form but can always be analyzed in terms of cold, stillness, whiteness, and moistness.

• Cold: If the body’s natural heat is insufficient to drive out the cold, the patient feels cold and shivery, which is called aversion to cold. This is the principle that when yīn prevails, there is cold (阴胜则寒 yīn shèng zé hán).
• Stillness: The patient may huddle up in a curled-up lying posture to preserve bodily warmth. Cold slows the movement of yáng qì and blood, so the pulse often becomes slow. Obstruction of yáng qì further gives rise to pain in various parts of the body (when there is stoppage, there is pain), such as the head, nape, limbs, or abdomen.
• Whiteness: Obstruction of the movement of yáng qì causes contraction of the blood vessels that manifests in a somber-white facial complexion and a pale tongue.
• Moistness: Because the fire of the body is failing to warm and evaporate fluids, signs such as clear copious nasal mucus may be observed.
• Contraction and tension: Cold causes contraction and tension. Contraction of the interstices (sweat glands and sweat pores) prevents sweating. Contraction of the sinews causes stiff nape or hypertonicity of the limbs. Contraction of the blood vessels causes the pulse to be tight or sunken.

The damage to yáng qì is expressed in the classical phrase when yīn prevails, yáng ails, while the prominence of cold signs is reflected in the phrase when yīn is exuberant, there is cold. Because the yīn-yáng imbalance stems from a surfeit of yīn, the resulting disease pattern is called a repletion cold pattern.

Surfeits of yáng: Conversely, when yáng evils affect the body, they disturb yīn-yáng harmony by making the yáng stronger than normal and damaging yīn humor. Yáng surfeits can be caused by contraction of heat evil or other evils that transform into heat in the inner body. They can also develop from imbalances arising within the body from the general wear and tear of life or from eating hot spicy or rich sweet and fatty foods, or from overuse or misuse of hot-natured medicinals.

Yáng surfeits can manifest in multiple ways. However, they can be analyzed in terms of heat, movement, redness, and dryness.

• Heat: The body becomes hotter than normal so that the patient experiences a vigorous heat effusion (high fever). Heat causes increased sweating, which consumes the fluids of the body. This is the principle that when yáng prevails, there is heat.
• Movement: Heat can cause movement. Accelerating the movement of qì and blood, it makes the pulse rapid. Affecting the spirit, it can cause vexation and agitation.
• Redness: The acceleration of the movement qì and blood gives rise to a red facial complexion and red tongue.
• Dryness: At the same time, the body’s fluids will be subject to more heat than normal and will tend to evaporate more quickly, giving rise to thirst, dry nose, and dry tongue fur. Sometimes there are short voidings of reddish urine and constipation.

The damage to yīn qì is expressed in the phrase when yáng prevails, yīn ails, while the prominence of heat signs is expressed in the phrase when yáng is exuberant, there is heat. Because the imbalance stems from a surfeit of yáng, the resulting disease pattern is a repletion heat pattern.

Heat Effusion vs Fever

The Chinese term 发热 fā rè, or simply 热 rè, can be translated as fever. However, since it also includes localized or subjective heat sensations that may not involve a rise in body temperature, we usually render the term as heat effusion. Hence, a low grade fever is traditionally referred to as slight heat effusion, while the classic sign of an exterior pattern is referred to as heat effusion with aversion to cold, and high fever is referred to as vigorous heat effusion. The milder manifestations of heat, which are typical of yīn vacuity, are usually referred to simply as heat, e.g., heat in the palms and soles, postmeridian tidal heat and tidal heat.

Deficits of yīn and yáng: When yīn or yáng grows weaker than its opposite, a deficit arises. This is a problem of internal imbalance, which arises when unhealthy lifestyle, overwork, lack of sleep, dietary irregularities, emotional imbalances, or enduring or repeated illness gradually takes its toll. The signs resemble those of yīn-yáng surfeits, but tend to be of gradual onset, chronic, intermittent, and localized.

Deficits of yīn: If yīn weakens, then the yáng side becomes relatively stronger. Deficits of yīn can arise when people overwork and do not get adequate rest, when their yīn humor is damaged by severe or enduring illness or by fire arising from, for example, emotional imbalances.

This yīn-yáng imbalance is expressed in the phrase when yīn is vacuous, yáng becomes exuberant. Because heat is the main manifestation, it is said that when yīn is vacuous, there is heat. This kind of heat, stemming from a surfeit of yīn rather than an insufficiency of yáng, is called vacuity heat.

As with repletion heat, the signs of vacuity heat can be analyzed in terms of heat, movement, redness, and dryness but are less overt.

• Heat: Heat manifests in slight heat effusion (low-grade fever), usually only experienced in the latter part of the day when yīn qì is normally strongest. This is called postmeridian tidal heat effusion. The heat gives rise to increased sweating during sleep, which is called night sweating.
• Movement: Vacuity heat makes the pulse beat faster. However, since this stems from the insufficiency of yīn, the pulse is not only rapid but also fine. In vacuity heat, the surfeit of yáng qì deep within the body causes unrest of the heart spirit, which is called heart vexation.
• Redness: The exuberance of yáng causes qì and blood to move more forcefully and rapidly than normal, giving rise to a facial complexion and tongue that are redder than normal. Unlike the complexion of repletion heat, the redness of the complexion in vacuity heat is usually confined to the area of the cheek bones and becomes most visible in the latter part of the day. It is called reddening of the cheeks or sometimes postmeridian reddening of the cheeks.
• Dryness: The heat causes a dry throat and mouth with a mildly increased desire for fluids.

Symptoms of Yīn-Yáng Surfeits and Deficits

Yīn surfeits: When yīn is exuberant there is cold (repletion cold) Cold: Aversion to cold Stillness: Pain; curled-up posture; slow pulse; pain (when there is stoppage, there is pain) Whiteness: Somber-white facial complexion; pale tongue Moistness: Clear copious snivel Yáng surfeits: When yáng is exuberant, there is heat (repletion heat) Heat: Heat effusion; sweating Movement: Rapid pulse Redness: Red face Dryness: Thirst; dry nose; dry tongue fur; short voidings of reddish urine; constipation Yīn deficits: When yīn is vacuous, there is heat (vacuity heat) Heat: Postmeridian tidal heat; heat in the palms and soles; night sweating Movement: Heart vexation; rapid fine pulse Redness: Reddening of the cheeks Dryness: Dry throat and mouth; emaciation Yáng deficits: When yáng is vacuous, there is cold heat (vacuity cold) Cold: Fear of cold; cold limbs Stillness: Scantness of breath (mild breathing difficulty); laziness to speak; curled-up posture Whiteness: Bright-white facial complexion; pale enlarged tongue Moistness: Long voidings of clear urine; sloppy stool or diarrhea; glossy white tongue fur

Deficits of yáng: When the yáng side grows weaker, the yīn side becomes relatively stronger. This happens when poor lifestyle, overwork, or enduring illness causes damage to yáng qì.

Because yáng is weaker than normal, it fails to restrain yīn. This is expressed as when yáng is vacuous, yīn becomes exuberant. Because cold is the main manifestation of this yīn-yáng disharmony, the idea is also expressed as when yáng is vacuous, there is cold. This kind of cold, stemming from an insufficiency of yáng rather than exuberance of yīn from the presence of yīn evils, is called vacuity cold.

Like repletion cold, vacuity cold manifests in signs that can be analyzed in terms of cold, stillness, whiteness, and moistness, although they tend to be less overt. Signs of contraction and pain associated with repletion cold are generally not present.

• Cold: The patient will experience feelings of cold that are referred to as fear of cold. The hands and feet are often palpably cold. Fear of cold is a chronic condition and differs from aversion to cold, which develops rapidly when the body is invaded by an external evil.
• Stillness: Liking for quiet and lying in curled-up posture; a forceless pulse.
• Whiteness: Bright-white complexion and a pale tongue.
• Moistness: Because yáng qì’s power to transform fluids is weakened, there is increased urination, which is called long voidings of clear urine. When its power to cope with ingested food and fluids is weakened, there may be sloppy stool or even diarrhea.

YinYang-YYSurfeitsDeficits

Yīn-yáng surfeits and deficits affecting the viscera: Because the viscera each have yīn and yáng aspects, yīn-yáng imbalances can affect specific viscera. Let us take the heart as an example.

When the heart is healthy, yīn and yáng remain in a state of general balance, and the heart beats at a normal pace. When the heart is unhealthy, signs of dysfunction appear. These vary depending on whether yīn or yáng is affected.

• When heart yīn is insufficient, heart yáng becomes proportionately stronger. This causes an accelerated heartbeat, fine and rapid pulse, and heat signs such as heart vexation and heat in the center of the palms and soles.
• When heart yáng is insufficient, heart yīn becomes proportionately stronger. This can give rise to oppression and pain in the chest, a decelerated heartbeat, a slow and fine pulse that may be intermittent (i.e., it skips beats), and pronounced cold signs, such as cold limbs and a dull gray facial complexion.

These conditions are called heart yīn vacuity and heart yáng vacuity, respectively. Both are vacuity patterns characterized by a fine pulse, but the two differ in the speed of the pulse and the presence of heat or cold signs, as well as in attendant signs (heart vexation as opposed to oppression in the chest). This is because yīn tends toward stillness and cold, while yáng tends towards movement and heat.

Another example is the kidney. The kidney governs water, that is, by the power of kidney yáng, it distills waste fluids, sending the clear part back into circulation and draining off the turbid part in the form of urine. The kidney also governs reproduction, as well as being the root of the yīn and yáng of the entire body. When kidney yīn is insufficient, urine is scant, menstruation is scant, and heat signs are observed. When kidney yáng is insufficient, it fails to distill the fluids, so that there are long voidings of clear urine. There are cold signs such as cold hands and feet. In males, there may be impotence, and in females, infertility due to uterine cold.

Differences between deficit and surfeits imbalances: In both surfeits and deficits of yīn and yáng, the result is either cold or heat signs. However, the presenting signs differ slightly. Vacuity patterns are usually marked by mild signs of gradual onset, while repletion patterns are marked by pronounced signs typically of rapid onset.

Chief Principles in Yīn and Yáng in Pathology
Surfeits: When yáng prevails, yīn ails; when yīn prevails, yáng ails. (When yáng is exuberant, there is heat; when yīn is exuberant, there is cold). Deficits: When yáng is vacuous, yīn becomes exuberant (there is cold); when yīn is vacuous, yáng becomes exuberant (there is heat).

For example, the heat associated with yáng prevailing and yīn ailing tends to be a high fever of swift onset. This is because the yīn-yáng balance is disturbed quickly when heat enters. By contrast, the heat associated with the gradual weakening of the body’s yīn side is only a low-grade fever (slight heat effusion) and develops gradually. In terms of repletion and vacuity, the high fever condition arising when yáng prevails is a pattern of repletion heat, while the slight fever (low fever) caused by an insufficiency of yīn is one of vacuity heat.

Similarly, a repletion cold pattern that arises when yīn prevails differs from a vacuity cold pattern that arises when the yáng qì of the body grows weak. Repletion cold is usually of sudden onset, while vacuity cold develops gradually. Repletion cold and vacuity cold may be characterized by cold limbs (palpable cold of the extremities). However, repletion cold usually appears suddenly, and since it stems from external cold that often affects the exterior, there is often acute shiveriness called aversion to cold that the patient may feel despite extra clothing or bedclothing and that may be accompanied by mild heat effusion, i.e., palpable heat in the forehead. Repletion cold is often characterized by pain. By contrast, vacuity cold appears gradually and is often marked by a chronic sensation of lack of warmth in the extremities that is more easily relieved by adequate clothing or bedclothing. It is associated with chronic intolerance of cold environments called fear of cold, but not with the shivery chills that we call aversion to cold. Vacuity cold is less likely to be characterized by pain.

In terms of the basic relationships between yīn and yáng, yīn-yáng surfeits and deficits are both breakdowns of healthy mutual restraining of yīn and yáng. Surfeits are conditions in which one side restrains the other excessively. Deficits are conditions in which one side fails to restrain the other sufficiently.

Detriment of yīn or yáng affecting its opposite: Because yīn and yang are mutually dependent, severe yīn vacuity or yáng vacuity can give rise to vacuity of the opposite. The resulting simultaneous presence of yīn and yáng vacuity is characterized by both yīn vacuity and yáng vacuity signs, i.e., vacuity cold and vacuity heat signs, e.g., cold limbs with sensations of internal heat.

Unlike surfeits and deficits, where the mutual restraining of yīn and yáng breaks down, detriment of yīn or yáng affecting its opposite is a breakdown of yīn-yáng interdependence. The Yi1 Guan1 Bian1 (医贯砭 The Stone of Medical Mastery) states, Without yáng, yīn has no means of arising (无阳则无以生阴 wu2 yīn zé yáng wu2 yi3 hua4); Without yīn, yáng has no means of forming (无阴则阳无以化 wu2 yīn zé yáng wu2 yi3 hua4). When the loss of yīn or yáng is total, its opposite is left alone (solitary yīn, solitary yáng) and incapable of surviving. Hence, this notion is also expressed as solitary yīn cannot arise and solitary yáng cannot grow 孤陰不生, 獨陽不長 gu1 yīn bu4 sheng1, du2 yáng bu4 zhang3. So, the total loss of either yīn or yáng results in the loss of both. As the Neì Jing1 states, When yīn and yáng are severed, essential qì expires (阴阳离决, 精气乃绝 yīn yáng li2 jué, jing1 qì nai3 jué). In other words, the patient dies.

Yīn-yáng conversion: The mutual conversion of yīn and yáng is described in the Neì Jing1 as follows:

The Ling2 Shu1 (Chapter 74): cold engendering heat and heat engendering cold, these [processes] are mutations of yīn and yáng (寒生热, 热生寒, 此阴阳之变也 hán sheng1 rè, rè sheng1 yáng, ci3 yīn yáng zhi1 bian4 y e3). The Su4 Wen4 (Chapter 7): double yīn becomes yáng (重阴必阳 chong2 yīn bì yáng) and double yáng become yīn (重阳必阴 chong2 yáng bì yīn); extreme cold engendering heat (寒极生热 hán ji2 sheng1 rè), extreme heat engendering cold (热极生寒 hán ji2 sheng1 hán).

In practice, not only can cold patterns change into heat patterns and vice versa, but interior and exterior patterns can give way to each other, as can vacuity and repletion patterns.

Heat patterns giving way to cold patterns can be seen in warm-heat disease. Virulent heat toxin giving rise to a heat pattern can cause severe sweating and damage to liquid that are so severe that original qì discharges outward with the fluids. When this happens, a vigorous heat effusion (persistent high fever) suddenly gives way to a drop in temperature, and the appearance of a somber-white facial complexion, cold extremities up to the elbows and knees (called reversal cold of the limbs), a faint pulse on the verge of expiration, which are signs of a sudden and severe loss of yáng qì. This is often described as fulminant desertion of yáng qì. It is a repletion heat pattern converting into a vacuity cold condition.

Cold patterns converting into heat patterns are seen in the two following examples. Cold evil invading the exterior and manifesting in an exterior cold pattern, can, if unresolved, pass into the interior and transform into heat, giving rise to an exterior heat pattern. Similarly, when severe cold rheum (a thick mucus accumulation) damages the center (i.e., the spleen and stomach), the yīn pattern can, under certain circumstances, transform into heat. Note that these examples of cold converting into heat represent more gradual change than the previous example of heat suddenly converting into cold.

The pathological imbalances of yīn-yáng are complex. See also pathomechanism.

Yīn and Yáng in Diagnosis

Yīn and yáng provide not only a way to understand the overall nature of disease; they also help to make sense of individual signs. In the four examinations, many of the signs that the practitioner observes can be broadly classified as yīn or yáng.

Classification of signs.

• Heat effusion and liking for cold are yáng, while aversion to cold and a liking for warmth are yīn.
• A bright complexion is yáng, while a dull complexion is yīn.
• Floating, rapid, surging, and slippery pulses are yáng, while sunken, slow, fine, and rough pulses are yīn.
• A powerful resonant voice is yáng, while a faint low voice is yīn.

Yīn-yáng significance of the time of symptoms: In the body, yīn and yáng wax and wane between day and night. Yáng qì strengthens in the morning and weakens in the afternoon and evening. Yīn qì strengthens in the evening and grows weaker in the early morning. The changes in the yīn-yáng balance throughout the day mean that certain symptoms become worse or only appear at certain times of the day.

Yīn and Yáng in Pulses
Rapid	Slow
Floating	Sunken
Slippery	Rough
Surging, large	Fine
Replete	Vacuous

• Postmeridian tidal heat effusion: Tidal heat effusion that occurs after midday is called postmeridian tidal heat effusion. It most commonly occurs in yīn vacuity, when the heat effusion starts in the evening to abate in the morning. Normally, toward nighttime, defense yáng gradually enters the interior, where it moves during the night. During the night, its activity is normally kept in check by the body’s yīn qì. However, when yīn humor is depleted, defense qì is no longer kept in check and fails to fully enter the interior, manifesting in abnormal bodily heat. This is often described as yáng failing to enter yīn (阳不入阴 yáng bù rù yīn), that is, failing to be enveloped and subdued by yīn.
• Night sweating: Sweating in sleep is called night sweating (or as in Chinese thief sweating). This usually occurs with postmeridian tidal heat effusion and is a sign of yīn vacuity. During sleep, defense yáng enters the interior giving rise to heat and can no longer keep the sweat pores tight and secure; hence, sweating.
• Fifth-watch diarrhea: Diarrhea sometimes heralded by pain or rumbling intestines before dawn (4–6 a.m.) each day is called fifth-watch diarrhea. This attributed to spleen-kidney yáng vacuity. It is explained by the fact that yáng qì is weakest at this time of day.
• Profuse urination at night: The need to urinate 2 or 3 times or more during the night, usually during the early hours of the morning, is called profuse urination at night. It is a sign of kidney yáng vacuity and is explained by the fact that yáng qì, responsible for the retention of urine, is weakest at this time.

Classification of patterns: Yīn and yáng forms the basis of eight-principle pattern identification. The eight principles (八綱 bā gāng) are exterior-interior, cold-heat, vacuity-repletion, and yīn-yáng. Yīn and yáng are the most general parameters. The others are facets of them; exterior, heat, and repletion are yáng pattern; interior, cold, and vacuity are yīn patterns.

Exterior and interior represents an important distinction. Exterior patterns stem solely from invasion of external evils and are invariably characterized by heat effusion and aversion to cold occurring at the same time. The patient feels cold but is felt to have a fever. All other conditions are considered interior.

Cold/heat and vacuity/repletion have already been discussed in the context of surfeits and deficits of yīn and yáng, so it is not necessary to discuss them in any greater detail at this time.

Different combinations of the eight principles are possible. Exterior patterns can take the form of heat or cold and vacuity or repletion. Thus, when wind-heat invades the exterior, an exterior heat (表热 biǎo rè) pattern arises. When wind-cold invades the exterior, exterior cold (表寒 biǎo hán) patterns develop. When yáng qì weakens internally, interior cold (里寒 lǐ hán) develops. When yīn humor weakens internally, interior heat (里热 lǐ rè) patterns develop.

Eight-Principle Pattern Identification
Yīn	Yáng
Exterior	Interior
Cold	Heat
Vacuity	Repletion

Classification of diseases: Some diseases fall into yīn-yáng pairs. Here are two examples:

• Mania and withdrawal: These are both mental and emotional disturbances. Mania, characterized by agitation, pronounced bodily movement, and even violent behavior, is yáng, while withdrawal, marked by quietness, sullenness, and lack of interest in the outside world is yīn.
• Flat-abscesses and welling-abscesses: Flat-abscesses, which are suppurations under the skin that are not swollen or clearly circumscribed, are yīn in contrast to welling-abscesses, which swell up above the body's surface and are clearly circumscribed, are yáng.

Yīn and Yáng in Principles of Treatment

Just as yīn-yáng disharmony is the most general way of understanding disease, the most fundamental treatment principles are supplementing insufficiency and draining superabundance.

Treating yīn-yáng surfeits: Yīn-yáng surfeits give rise to repletion patterns.

• In repletion cold, the principle of cold is treated with heat (寒者热之 hán zhě rè zhī) applies. This means that medicinals with a hot or warm nature are used to treat cold patterns (see yīn-yáng in pharmacy below).
• In repletion heat, the principle of heat is treated with cold (热者寒之 rè zhě hán zhī) applies. This means that medicinals with a cold or cool nature are used to treat heat patterns.

Treating yīn-yáng deficits: Yīn-yáng deficits give rise to vacuity patterns, which are treated by supplementing.

• Yáng deficits present as vacuity cold, which is treated by supplementing yáng. This is often described as supporting yáng to restrain yīn. This principle is based on the Sù Wèn (Chapter 74) principle of boosting the source of fire to disperse the shroud of yīn (益火之源以消阴翳 yì huǒ zhī yán yǐ xiāo yīn yì).
• Yīn deficits present as vacuity heat, which is treated by supplementing yīn. This is often described as enriching yīn to repress yáng. This principle is based on the Sù Wèn (Chapter 74) principle of invigorating the governor of water to restrain the light of yáng. (壮水之主以制阳光 zhuàng shuǐ zhī zhǔ yǐ zhì yáng guāng).

Also applied in the treatment of vacuity patterns is a twofold principle of seeking yīn in yáng and seeking yáng in yīn, advocated by Zhāng Jǐng-Yuè (1563–1640). According to this principle, which is based on the interdependence of yīn and yáng, yáng-supplementing medicinals are added to formulas designed to supplement yīn (seeking yīn within yáng), while yīn-supplementing medicinals are added to formulas designed to supplement yáng (seeking yáng within yīn). The practice of adding qì-boosting medicinals to formulas designed to nourish the blood is also an expression of this principle. Qì and blood constitute a yīn-yáng pair. Blood relies on qì for its production so that adding qì supplementing medicinals, the production of blood is enhanced.

Similarly, the treatment of detriment to yīn or yáng affecting its opposite also involves dual supplementation..

• Detriment to yáng affecting yīn is treated by supplementing both yīn and yáng, but with emphasis on supplementing yáng.
• Detriment to yáng affecting yīn is treated by supplementing both yīn and yáng, but with emphasis on supplementing yīn.

Yīn and Yáng in Medicinal Therapy

Yīn and yáng are applied in pharmacy in the following aspects:

Nature: Each medicinal is ascribed a hot, warm, cold, or cool nature, or qì, depending on whether it increases or decreases heat in the body. Warm means mildly increasing heat, while hot means greatly increasing it. Similarly, cool means mildly decreasing heat, while cold means greatly decreasing it. Heat and warmth are yáng, while cold and coolness are yīn. In addition, there are balanced medicinals, which are neither warm-hot nor cool-cold. Warm-hot medicinals treat cold patterns, while cool-cold medicinals treat heat patterns.

Flavor: Each medicinal possesses one or more of five flavors: acrid, sweet, sour, bitter, and salty. Of these, acrid and sweet are yáng, while sour, bitter, and salty are yīn.

Bearing: Bearing is the tendency of medicinals to move toward certain parts of the body to produce their effect. Upbearing medicinals move to the upper body; downbearing medicinals to the lower body; floating medicinals to the exterior; and sinking medicinals to the interior. Upbearing and floating (upward and outward movement) are yáng, while downbearing and sinking (downward and inward movement) are yīn.

Upbearing and floating medicinals, labeled together as upfloating medicinals, are ones that promote sweating and resolve the exterior, that open the orifices, and that promote ejection (cause vomiting to rid the stomach of unwanted contents).

Downbearing and sinking medicinals, known as downsinking medicinals, for example, are used to clear heat and drain fire, to calm liver and extinguish wind, and to disinhibit water.

Yīn-yáng correspondences exist between nature, flavor, and bearing. Upfloating medicinals tend to be from the upper and outer parts of plants that are light in weight: leaves, flowers, skins and twigs. These tend to be acrid or sweet in flavor. Downsinking medicinals tend to be heavier parts such as seeds, fruits, shells, and minerals. These tend to be bitter, salty, or sour.

Yīn and Yáng in Diet

Yīn and yáng are applied in traditional Chinese dietary therapy. Traditional Chinese dietetics has the notion of a balanced diet based on a yīn-yáng balance. A healthy diet is based on grains combined with meat, vegetables, and fruit. Foods, just like medicinals, are warm-hot, cold-cool, or balanced in nature. Green vegetables and many fruits are cold in nature. Certain meats such as lamb and venison are warm, and strong alcoholic beverages (liquor) are hot. To ensure a balanced diet in the Chinese medical sense, care is taken to avoid excesses of hot and cold foods. Since cooking tends to make cold foods warmer in nature, avoidance of too many raw and unheated foodstuffs helps to maintain the balance. Likewise, cold drinks and ice-cream are considered potentially damaging to the health.

Following is a list of foodstuffs classified according to yīn and yáng in. A more comprehensive list is given in dietary therapy.

Balanced: Rice, some legumes, many meats, certain vegetables (mainly root crops).

• Rice.
• Legumes: soybeans, rice beans, broad beans
• Meats are largely balanced, e.g., pork and beef.
• Vegetables: white cabbage, potato, carrot, and turnip.

Warm-hot: Alliums (onions, garlic, etc.), spices, certain meats.

• The most commonly used fresh condiments in the Chinese cooking―scallions, ginger, garlic, chili, and cilantro―are all warm in nature. In general, all species of the genus allium, including scallions, garlic, onions, garlic chives, and shallots, are warm in nature.
• Fermented soybeans and hot bean paste are also warm in nature, as too are dried spices such as pepper, Sìchuan1 peppercorns, dried chili, star anise, cinnamon bark. Chinese wine used in cooking is also warm, as is grape wine.
• Chicken, and especially lamb, and venison are warm. In China, dog meat was also traditionally prized for its warm nature.

Cold-cool: Grains, tofu, green vegetables, and salt.

• Grains: wheat, millet, buckwheat.
• Tofu (bean curd), the Chinese vegetarian’s main source of protein, is cool in nature, because the heat-clearing medicinal gypsum (shi2 gāo) is added as a thickening agent to soybean milk to make it.
• Green leafy vegetables, beans in their pods (e.g., French beans), and all gourds (except pumpkin, which is warm), tomato, and eggplant are cold in nature.
• Mung beans.
• Salt is cold in nature. Soy sauce, which has a high salt content, is also cool in nature.

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