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The life and death of all things fundamentally depend on Yang Qi. However, nowadays nine out of ten people suffer from Yin deficiency. Why is this? It is because one does not realize that this "Yin" is precisely the root of Yang Qi.

Generally speaking, Yin cannot exist without Yang; without Qi, there can be no generation of Form. Yang cannot exist without Yin; without Form, there can be no bearing of Qi. Therefore, the growth of things arises from Yang, while their maturation is accomplished by Yin. This is referred to as Original Yang and Original Yin, also known as True Essence (Zhen Jing) and True Qi (Zhen Qi). To understand the causes of life and death, one must examine Yang; examining Yang involves observing whether it is declining or not. To understand the causes of survival and demise, one must examine Yin; examining Yin involves observing whether it is deteriorating or not. This is the essential method for preserving life.

Reviewing the works of predecessors, many failed to recognize the true nature of True Yin, leading to rigid and dogmatic theories. Since Liu Hejian's theory emphasizing Fire became prevalent, Zhu Danxi advocated using bitter-cold herbs to tonify Yin, a view universally adopted and unchallenged. The reason for this lies in the fact that Heat syndromes are obvious and easily observed, whereas Cold syndromes are subtle and often overlooked. Furthermore, distinguishing between Deficient Fire and Excess Heat is particularly difficult. Few realize that among cases caused by Excess Heat, only three or four out of ten occur, whereas those caused by Deficient Fire are common, occurring six or seven times out of ten. Excess Heat represents the flourishing of Fire, where Original Qi is initially uninjured; thus, it can be subdued with bitter-cold herbs. However, treatment should cease once the Heat is resolved; overuse will inevitably damage Original Qi. Deficient Fire, on the other hand, results from the depletion of True Yin. When True Yin is insufficient, how can harsh, bitter, and cold substances possibly replenish it? Moreover, the nature of deeply cold herbs lacks vitality; they not only fail to tonify Yin but also tend to destroy True Fire. Due to the gentle and lingering nature of Yin pathology, improper use secretly damages longevity without being perceived. Thus, some elderly individuals who prefer coolness do so because their Original Yang is sufficient to withstand cold; it is not that coolness extends life.

I request to elaborate in detail on the manifestations, residence, functions, pathologies, and treatments of True Yin, to fully elucidate its principles.

Regarding the manifestations of True Yin: Yang takes Yin as its root. The *Classic of Internal Medicine* states: "The Zang organs that store Essence must not be injured; injury leads to loss of control and Yin deficiency, Yin deficiency leads to lack of Qi, and lack of Qi leads to death." Does this not designate Essence as True Yin? It also states: "If flesh and form have wasted away, even if the nine pulse positions are harmonious, death will ensue." Does this not designate Form as True Yin? Observing whether physical structure is intact or deteriorated reveals whether True Yin is injured or not. These are the manifestations of True Yin, which must be examined.

Regarding the residence of True Yin: Each of the five Zang organs and five fluids has its specific governance. The *Classic of Internal Medicine* states: "The Kidney governs Water and receives the Essence of the five Zang and six Fu organs to store it." Thus, the five fluids return to Essence, and the five Essences are unified by the Kidney. Within the Kidney lies the chamber of Essence, called the Gate of Life (Ming Men), which is the dwelling place of Heavenly One, i.e., the Fu organ of True Yin. Essence is stored here; Essence is the Water of True Yin. Qi transformation occurs here; Qi is the Fire within Yin. The Gate of Life resides between the two Kidneys, possessing both Water and Fire, governing their waxing and waning. Hence, it is the beginning of conception and the root of life and destiny. Treating True Yin while neglecting the Gate of Life is incorrect. This is the residence of True Yin, which must be examined.

Regarding the function of Yin: The functions of Water and Fire are indispensable. The Fire of the Gate of Life is called Original Qi; the Water of the Gate of Life is called Original Essence. When the five fluids are abundant, the body relies on them for strength; when the five Qi are regulated, Ying and Wei (Defensive and Nutritive Qi) rely on them for harmony. The Water and Fire of the Gate of Life are the source of transformation for the twelve Zang-Fu organs. Therefore, the Yin and Yang of the twelve organs all depend on it for regulation. These are the functions of True Yin, which must be examined.

Regarding the pathology of True Yin: Yin Qi is never in excess; Yin diseases always arise from insufficiency. For instance, what appears as Yin dominance in the lower jiao is originally not Yin excess, but rather the decline of Fire in the Gate of Life. What appears as Yang dominance at the surface is originally not Yang excess, but rather the depletion of Water in the Gate of Life. Depletion of Water leads to successive Yin deficiency diseases; decline of Fire leads to recurring Yang deficiency syndromes. Examples include Dai Yang syndrome (face red like cinnabar) and Ge Yang syndrome (external heat like fire). Symptoms may include thirst and dry throat seeking water for relief; restlessness, agitation, and mania desiring to lie in mud; five-palm heat accompanied by emaciation and bone-steaming fever; constipation of both bowels and urine resembling juice; hematemesis and epistaxis; coughing and spermatorrhea; macules, jaundice, and anhidrosis due to exhaustion of body fluids; or wind-stroke with convulsions due to damage to essence and blood.

Such conditions belong either to rootless flames or to Fire failing to return to its origin. All result from Yin being insufficient to balance Yang, indicating disease in the Water of Yin. Conversely, if Fire is deficient in the lower jiao, it may manifest as mental dullness or physical fatigue. Dizziness and visual/auditory impairment indicate upper jiao Yang deficiency. Throat obstruction, nausea, and shortness of breath indicate upper jiao Yang deficiency. Indigestion, acid regurgitation, and vomiting indicate middle jiao Yang deficiency. Fullness, blockage, and phlegm from water flooding indicate middle jiao Yang deficiency. Mixing of clear and turbid, intestinal murmurs, and diarrhea indicate lower jiao Yang deficiency. Impotence, cold sperm, and abdominal pain indicate lower jiao Yang deficiency. Aversion to cold and shivering indicate Yang deficiency of the Fire Zang (Heart), unable to resist cold. Muscle distension indicates Yang deficiency of the Earth Zang (Spleen), unable to control water. Spasms and painful bi-syndromes indicate Yang deficiency of the Wood Zang (Liver), unable to nourish sinews. Cold cough, deficiency asthma, cool body, and spontaneous sweating indicate Yang deficiency of the Metal Zang (Lung), unable to protect the Lung. Spermatorrhea, bloody stool, incontinence, broken-back sensation, and extreme bone pain indicate Yang deficiency of the Water Zang (Kidney), with internal exhaustion of essence and marrow. Such conditions result from Yang being insufficient to overcome Yin, indicating disease in the Fire of Yin. Wang Taipu stated: "When cold fails to treat cold, blame the lack of Water; when heat fails to treat heat, blame the lack of Fire. Both lack of Water and Fire reside in the Gate of Life." Therefore, Yin deficiency diseases must be examined.

Regarding the treatment of True Yin: Since disorders have origins and diseases have causes, treating disease requires seeking the root. The root of the five Zang organs lies in the Gate of Life; the root of Spirit and Qi lies in Original Essence. This is True Yin. Wang Taipu stated: "Strengthen the master of Water to control the glare of Sunlight; benefit the source of Fire to dissipate the shadows of Yin." Only Xue Lizhai mastered this subtlety, frequently using Zhang Zhongjing's Ba Wei Wan (Eight-Ingredient Pill) and Qian Yi's Liu Wei Wan (Six-Ingredient Pill), which are formulas for benefiting Fire and strengthening Water. When cold pathogens invade, although presenting as exterior syndromes, the transformation of sweat depends on Yin. In wind-stroke, where hemiplegia is common, the deterioration of sinews and vessels depends on Yin. In deficiency consumption generating fire, how else can one extinguish the prairie fire but by strengthening Water? In dysentery causing Yin loss, how else can one secure the gates but by tonifying the Kidney? In distension caused by water pathogens, one must seek the Water Zang to govern water; in Guan Ge syndrome rooted in Yin deficiency, one cannot strengthen Yin by abandoning Yin. These few points are the major guidelines in disease management. Those with insight can reflect upon them repeatedly.