1. Yin and Yang

1. Yin and Yang

Yin and Yang are the foundation of the Eight Principles. The nature of any disease can broadly be categorized into two main types: Yin syndrome and Yang syndrome. No matter how complex or variable the clinical presentation may be, it ultimately falls into one of these two categories: either excessive Yin leading to deficiency, or excessive Yang leading to excess. When conducting syndrome differentiation, we first distinguish between Yin and Yang, and then further differentiate between Exterior and Interior, Cold and Heat, and Deficiency and Excess, ensuring clarity and accuracy in our diagnosis. The "Plain Questions: On the Great Principle of Yin and Yang" states: "Yin and Yang are the way of heaven and earth, the guiding principles of all things, the parents of change, the origin of life and death, the abode of the divine, and the foundation of healing—therefore, in treating illness, one must seek the root." The "Plain Questions" also says: "A skilled diagnostician observes color and takes the pulse, first distinguishing Yin from Yang." Zhang Jingyue further notes: "In diagnosing and treating, one must first examine Yin and Yang—this is the fundamental principle of medical practice. If Yin and Yang are neglected, treatment will inevitably fail. Although medical practice may seem complex, it can be summarized in one word: Yin and Yang." Clearly, Yin and Yang hold a crucial position in syndrome differentiation and treatment.

(1) Basic Manifestations of Yin and Yang Syndromes

Basic manifestations of Yin syndrome: listlessness, dull complexion, vacant gaze, weak voice, feeling cold with cold limbs, slow movements, poor appetite and little drinking, clear and long urination, loose stools, thin white tongue coating, and a fine, slow pulse. In short, any condition involving decline in bodily functions, nutritional deficiency, or degeneration of organs generally falls under the category of Yin syndrome.

Basic manifestations of Yang syndrome: high spirits, flushed face, bloodshot eyes, loud voice, aversion to heat and preference for coolness, restlessness and hyperactivity, thirst and coarse breathing, short and reddish urine, dry and hard stools, thick yellow and dry tongue coating, and a rapid, strong pulse. In short, any condition involving heightened bodily functions, adequate nutrition, or new organic changes in organs generally falls under the category of Yang syndrome.

In clinical practice, however, the manifestations of Yang and Yin syndromes are rarely as straightforward as described above. More often, they present as combinations of Yin and Yang, with overlapping exterior and interior features, alternating Cold and Heat, and simultaneous Deficiency and Excess. Some patients exhibit Yin deficiency with Yang excess, others show excessive Yin with deficient Yang, some have both Yin and Yang deficiencies, while others have both Yin and Yang excess. Given the complexity of Yin and Yang manifestations, it is essential in clinical diagnosis to clearly identify whether a symptom is truly Yin or Yang, understand their mutual transformation, and determine which is dominant, in order to provide an accurate basis for treatment.

(2) Yin Deficiency and Yang Deficiency

The kidneys are the foundation of innate vitality and the source of true Yin and true Yang. The right kidney is the "Mingmen," housing true Yang, while the left kidney is simply the "kidney," housing true Yin. When true Yin is insufficient, internal fire rises, causing facial flushing and red cheeks, chapped lips, dry mouth and throat, bone-steaming heat, five-heart vexation, night sweats and nocturnal emissions, a red tongue with little coating, and a fine, rapid, weak pulse. When true Yang is insufficient, the complexion becomes pale, with spontaneous sweating and fatigue, poor appetite, shortness of breath and reluctance to speak, feeling cold and afraid of the cold, abdominal bloating and loose stools, diarrhea at dawn, impotence and nocturnal emissions, heavy and sluggish legs, a swollen tongue with tooth marks, a thin white coating, a large but weak pulse, and a weak pulse at the cun position.

(3) Loss of Yin and Loss of Yang

The "Plain Questions: On the Great Principle of Yin and Yang" states: "When Yin and Yang are severed, vital energy is also severed." Loss of Yin and loss of Yang are signs that Yin and Yang are on the verge of being completely lost, representing dangerous syndromes that appear during the progression of disease and marking the inevitable path toward death. In such situations, it is crucial to make an accurate diagnosis and promptly implement the most rapid and effective rescue measures to turn danger into safety.

Clinical manifestations of loss of Yin include chest tightness and shortness of breath, warm hands and feet, hot skin, thirst for cold drinks, salty and non-sticky sweat, a red tongue with no moisture, and a fine, rapid, weak pulse.

Clinical manifestations of loss of Yang include a pale face, cold limbs, feeling cold and afraid of the cold, spontaneous sweating, a pale and swollen tongue, and a floating, hollow or extremely weak pulse.

Loss of Yin represents the extreme stage of Yin deficiency syndrome. According to the principle that Yin deficiency generates internal heat, patients in the loss of Yin stage still retain symptoms such as warm hands and feet and hot skin—symptoms akin to internal heat. However, as the condition progresses, these internal heat symptoms may eventually disappear due to the fact that solitary Yin cannot generate heat and solitary Yang cannot sustain growth. Loss of Yin can quickly lead to loss of Yang, followed by death.

Loss of Yang represents the extreme stage of Yang deficiency syndrome. According to the principle that Yang deficiency generates external cold, patients in the loss of Yang stage still retain symptoms such as a pale face, cold limbs, and feeling cold—symptoms akin to external cold. However, as the condition progresses, these external cold symptoms intensify until finally life comes to an end.

All fatal cases ultimately end in loss of Yang, meaning that loss of Yin precedes loss of Yang, and the transition from one to the other happens very quickly. The "Plain Questions: On the Law of Organ Qi According to Season" states: "Yang qi is like heaven and the sun—if it is lost, lifespan is shortened and vitality is diminished." Thus, among loss of Yin and loss of Yang, loss of Yin is the critical warning sign, and the progression from loss of Yin inevitably leads to loss of Yang. Of course, there are also cases where loss of Yang occurs first, followed immediately by loss of Yin, resulting in both loss of Yin and loss of Yang occurring simultaneously.