• Exterior and Interior
• Cold and Heat
• Deficiency (xu) and Excess (shi)
• Yin and Yang

There are eight principles, namely yin, yang, exterior, interior, cold, heat, deficiency (xu) and excess (shi), that used as four pairs of basic guidance for clinical differentiation in traditional Chinese medicine (TCM).

The concept of eight principles was discussed early in the Yellow Emperor’s Classic of Internal Medicine, the bible of traditional Chinese Medicine (TCM). Later on a famous TCM scholar, Zhang Zhongjing, wrote a book known as Treatise on Cold-induced and Miscellaneous Diseases, that began to use the eight principles for disease diagnosis or identifying syndromes. It was not until the Ming dynasty, that the eight principles began to use regularly for clinical diagnosis. For example, Zhang San Xi wrote in the book Six Essence of Medicine that: "ancient physicians treated on the basis of eight methods. They were yin, yang, exterior, interior, cold, heat, deficiency (xu) and excess (shi)." Wang Zhizhong, also from the Ming dynasty, said in his book Dong Yuan Xian Sheng Shang Han Zheng Mai that: “they are the eight keys for disease treatment.” In the Qing dynasty, Cheng Zhongling further clarified the implications, and claimed that the eight principles are the basic guidelines for examining patients and treating diseases. Today, TCM physicians still use the eight principles to understand the location and nature of pathological changes, the course of disease, and the relationship between pathogenic factors and body’s resistance.

Summary on the Eight Principles

The underlying idea of clinical differentiation is that signs and symptoms of a disease can be generalized according to the eight principles. For instance, the location or severity of the disease may be termed as interior or exterior type; the nature of the disease may be divided into cold or heat type; and the condition of the disease may belong to either excess (shi) or deficiency (xu) type. As a whole, yin and yang is the root of other six principles, that the interior, cold and deficiency are categorized as yin type, while the exterior, heat and excess are categorized as yang type. Although these four pairs of categories have different characteristics, the patterns sometimes interact.

The eight principles serve as the guiding rules for all syndrome differentiation, each of them generalizes a unique aspect of clinical condition, a whole picture of patients can only be made through comprehensive applied of all these principles. For example, when a patient develops aversion of wind blowing, coughing, body aching, and fever, physicians will claim that the problem is located in the exterior body, belongs to a heat nature and the body's healthy energy is still sufficient, and thus the patient’s condition can be summarized as an exterior, heat and excess (shi) syndrome. When the condition left untreated, it will be aggravated and then develop into a mixed nature of cold and heat, or have both deficiency (xu) and excess (shi) symptoms.

Exterior and Interior

Exterior and interior determine the severity of a disease and the affected body part. If a disease occurs in the body surface, muscles, or meridians, it is usually regarded as an exterior type (affected superficial portion of the body) and unlikely to be serious. If a disease occurs in the organs, blood, qi (vital energy) and bone marrow, it is regarded as an interior type and likely to be relatively serious. The exterior and interior principles are especially important for diagnosing exogenous diseases. Since their causal factors are originated from outside the body, they tend to have typical course of development, such as from outer to inner part of body, from superficial to deep regions, and even aggravated progressively, therefore physicians are able to evaluate and predict the clinical stage of patients according to the two principles.

• Syndromes of exterior type : This refers to conditions resulting from an invasion of exogenous pathogens that enter through the mouth, nose or body surface, and affect the superficial portion of the body. The conditions are marked by sudden onset, relative gentle and short duration, that typically seen in early stage of feverish conditions. The clinical features of exterior type are fever, aversion to wind blowing, chills, headache, general aches, a thin tongue coating and a floating pulse. Other accompanying symptoms are nasal congestion, runny nose, sneezing, itchy and sore throat, and coughing.

• Syndromes of interior type : This refers to pathological changes deep inside the body that involves the organs, bone marrow, qi or blood. Besides exogenous pathogens, factors like mental distress, improper diet and physical strain also lead to this type. Unlike the exterior type, clinical features of interior syndromes vary from person to person, and are mainly associated with organ dysfunctions, which have discussed in the section of Syndromes Differentiation of Organs. Interior syndromes tend to be relative serious and longer course of disease. The clinical features of interior type are marked by NO aversion to wind and chills, never show a floating pulse, and there are usually tongue and the coating changes.

Disease symptoms usually change over time, and therefore, so can the type of syndrome a person experiences. Exterior and interior syndromes can mutually interchange, and even coexist such as simultaneous exterior and interior type or half exterior-half interior type. Based on the clinical features, an exterior or interior syndrome is further linked with other principles.

Exterior & Interior In Relation to Other Principles
	Exterior syndrome	Interior syndrome
Cold type	Fever, chills, headache, body aching, nasal congestion, sweating or lack of sweating, floating and tense pulse, thin white tongue coating	Chills, cold limbs, pale complexion, lack of thirst or preference for hot drinks, nausea, vomiting, abdominal pain, diarrhea, clear profuse urine, deep and slow pulse, white and glossy tongue coating
Heat type	Fever, slight aversion to wind and chills, sweating, slight thirst, sore throat, floating and rapid pulse, a red tongue	High fever, sweating, thirst that preference for cold drinks, irritability, restlessness, red face, chapped lips, abdominal distention, constipation, yellow scanty urine, red tongue with yellow and dry coating, surging and rapid pulse, or deep and rapid pulse
Deficiency (xu) type	Aversion to wind, sweating, floating and slow and forceless pulse. The tongue body is slightly bulky	Feeble voice, breath shortness, apathy, general weakness, poor appetite, palpitation, dizziness, abdominal pain that feel alleviated when pressed, incontinence, deep and weak pulse, the tongue is pale and bulky and tender with white coating
Excess (shi) type	Chills, fever, lack of sweating, headache, body aching, floating and tense pulse, or floating and forceful pulse, white tongue coating	Coarse breathing, irritability, fullness in chest, abdominal distention and pain that become worse when pressed, constipation, yellow scanty urine, sweating in the palms and soles, deep and excess pulse, a rough and dry tongue with thick coating

Cold and Heat

Cold and heat are two commonly seen symptoms from a TCM viewpoint. They reflect the yin yang balance state of the body and relate to two different natures of disease.

• Syndromes of cold type : Cold syndromes are recessive conditions resulting from exposure to exogenous cold or a disharmony of yang deficiency and yin excess inside the body. Cold syndromes can present various according to the causes or affected body regions, common clinical features are sensitive to low temperatures, pale complexion, preference for warmth, a bland taste, cold limbs, fatigue, clear profuse urine, loose bowels, thin and clear body discharges. There is a pale tongue with white glossy coating, and a slow or tense pulse.

• Syndromes of heat type : Heat syndromes are excitatory conditions caused by invasion of exogenous heat or a disharmony of yin deficiency and yang hyperactive. Heat syndromes can present various according to the causes or affected body regions, common clinical features are red face, aversion to heat, thirst and preference for cold drinks, irritability, yellow and thick body discharges, constipation, and yellow scanty urine. The tongue is red with yellow and dry coating, and the pulse is rapid.

Although cold and heat are opposite in nature, they are closely related to each other and may even exist simultaneously, manifesting as a complex syndrome of both cold and heat. They can also be transformed into each other under certain circumstances, for example diseases caused by pathogenic cold are cold in nature, however when lingering for a period of time, patients are likely to turn into heat in nature. Furthermore, when diseases have developed to a late stage, a heat condition may be covered by fake cold signs, while a cold condition may be covered by fake heat signs.

Deficiency (xu) and Excess (shi)

Deficiency (xu) and excess (shi) are two principles that help analyze and generalize the opposing forces between the body’s resistance and pathogenic factors during the course of a disease. Basically, deficient (xu) indicate conditions caused by weakness and insufficiency of the body’s resistance. Excess (shi) refer to conditions induced by an overabundance of pathogens.

• Syndromes of deficient (xu) type : This refers to conditions that are dominated by weakness of the body's healthy energy rather than by abundance of pathogenic factors. Owing to various disease preventing factors inside the body, patterns of deficiency (xu) can be further classified into such as qi deficiency, blood deficiency, yang deficiency, yin deficiency, body fluid deficiency, bone marrow depletion, nutrient stage deficiency and protective qi deficiency. Each of them has particular clinical features, some common signs of deficient syndromes are fatigue, low-spirited, pale or sallow complexion, palpitation, breath shortness, cold limbs, sweating, diarrhea, loose bowels, urinary frequency or incontinence. The tongue is pale, and the pulse is thready pulse and weak. Deficient syndromes usually occur in those with constitutional defects, have chronic and ongoing diseases, or are in a depleted physical state.

• Syndromes of excess (shi) type : This refers to conditions that are dominated by abundance of pathogenic factors, the body's resistance has not yet weakened, resulting in hyperactive, precipitating struggles between the two forces. Besides exogenous pathogen invasions, excess syndromes can also be caused by internal damage such as qi stagnation, blood stasis, phlegm obstruction or stagnancy of fluid flow. Patients with an excess syndrome can develop complex symptoms, some common signs are fever, distension pain that becomes worse when pressed, chest stuffiness, irritability, coarse breathing, excessive throat discharges, constipation, urinary difficulty, and even unconsciousness. The tongue coating is thick and greasy, and the pulse is excess and forceful. Excess syndromes usually occur in those with a strong physique, newly developed cases, sudden attacks, and fierce clinical manifestations.

In reality, conditions that mix deficient and excess types are not uncommon. Excess and deficient syndromes are not steady, they can be transformed into each other, and may even appear fake signs to cover the underlying condition. For example, those who suffer from stagnation of blood and qi belong to an excess syndrome; however, when most of the body's disease preventing forces depleted, the body can be shifted to a mixed condition. TCM physicians do not use the deficiency and excess principles alone, they are closely linked with another category – the cold and heat types. Excess type and heat type are often combined, while deficient type is combined with cold type.

Cold & Heat In Relation to Deficiency (xu) & Excess (shi)
	Clinical features	Causes
Cold-excess type	Acute onset, chills, cold limbs, excess sputum, asthmatic breathing or breath shortness, abdominal pain that becomes worse when pressed, and constipation. The pulse is deep or taut and tense; the tongue may appear green-blue or purple color, and its coating is white, greasy and thick	Invasion by exogenous cold; over-eating food with cold energies.
Cold-deficient type	Gradual onset, chills, cold limbs, breath shortness, general weakness, abdominal pain that can be alleviated by pressed, loose bowels, and clear profuse urine. The pulse is weak, deep and slow; the tongue is pale, bulky and tender	Internal damages or long term illness leads to yang-qi depletion, and a relative excess of yin has developed inside the body. These are also termed as yang deficient syndrome
Heat-excess type	Acute onset, relative short duration, persistent or high fever, thirst, red face, fullness and distending pain in the abdomen that becomes worse when pressed, constipation, urinary difficulty or yellow urine. There may also be unconsciousness in serious cases. The backside is hotter than the palm side when touching the hand, a rapid and excess pulse, the tongue is red or deep red with yellow coating	Heat or fire evil invasion; over-eating spicy food or food with hot energies; excessive yang qi inside the body
Heat-deficient type	Gradual onset, relative long duration, remittent low-grade fever, night sweating, bone pain, dry mouth, skinny appearance, breath shortness and general weakness. When touching the hand, the palm side is warmer than the backside. The pulse is thready and rapid; the tongue is red with little coating, or the tongue body may be bulky and tender.	Internal damages or long term illness leads to depletion of yin fluid and a relative excess of yang qi has formed inside the body. This is also called yin deficiency or "yin deficiency with internal heat."

Yin and Yang

Yin and yang represent two opposite yet complementary aspects of objects, the nature of a disease and its clinical features can usually be categorized as a yin or yang character. Therefore the yin and yang pair becomes the root of other six principles. Syndromes of exterior, heat and excess (shi) belong to the category of yang, while interior, cold, and deficient (xu) syndromes belong to the category of yin. In clinical diagnosis, physicians will first differentiate whether the syndrome is ascribed to yin or ascribed to yang, because although the syndrome may be complex with many changes, it still does not depart from the two major categories. Yang types are characterized by excitation, restlessness, hyperactivity and optimism, while those characterized by inhibition, quietude, deterioration and gloominess are categorized as yin type.

• Samples of yang symptoms include irritability, red face, a hot sensation of body, coarse breathing, loud voice, thirst that preference for cold drinks, abdominal pain that becomes worse when pressed, constipation, scanty yellow urine, a deep red tongue with yellow coating, floating and surging pulse or rolling and rapid and forceful pulse.

• Samples of yin symptoms include low-spirit, pale complexion, cold limbs, feeble voice, loose bowels, clear profuse urine, a pale tongue with white coating, and the pulse is deep, slow and weak.

The yin yang principles are also applied to explain the changes in the organs and tissues. They can be detailed as the yin deficiency, yang deficiency, collapse of yin, collapse of yang.

• Yin deficiency: The condition indicates consumption or loss of yin fluid which fails to depress the yang part, and leads to depletion in nourishment and moisture of the body. Symptoms or signs include afternoon fever, flushed cheeks, feverish sensation in the palms and soles, irritability, insomnia, night sweating, dry mouth and throat, scanty and yellow urine and dry stool. On examination the tongue is red with little coating, and the pulse is thready and rapid.

• Yang deficiency: The condition indicates insufficiency of yang qi inside the body that fails to provide the functions of warmth, motivation and promotion. Symptoms or signs include aversion to cold, cold limbs, bland taste in the mouth, preference for hot drinks, pale complexion, spontaneous sweating, general swelling, profuse and clear urine or loose stool. The tongue is pale, bulky with a white slimy coating, and the pulse is deep and slow or thready on examination.

• Collapse of yin: This indicates a dangerous condition resulting from an excessive loss of yin fluid. Patients present with sticky hot sweat, high fever, irritability, shrunken skin, mildly warm hands and feet, increased thirst, flushed cheeks and very little urine production. The pulse is thready, rapid and weak on examination.

• Collapse of yang: Yang collapse refers to a serious condition due to the profuse consumption of the body's yang qi. Patients present with profuse cold sweating, apathy, temperature below normal, cold limbs, feeble breathing and a pale white complexion. On examination there is a pale and moist tongue and thready and fading pulse.

It should be noted that the eight principles only provide a preliminary understanding about the conditions of patients. Clinically, they should also be combined with other diagnostic parameters, such as the organ conditions, qi (vital energy) and blood evaluations, and the kinds of causal factors, so as to make a precise diagnosis. To take feverish condition as a sample, after evaluated the patient by the eight principles, physicians have to further clarify whether the fever occurs in the qi or blood aspect? What are the causal factors? Which organs are affected? When all the details are made clear, then physicians can easily select a suitable remedy for the patient.