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The *Yellow Emperor's Inner Canon* states: "When essence and qi are depleted, deficiency occurs." It also states: "Where pathogenic factors gather, the body's righteous qi must be deficient." Deficiency refers to emptiness and lack. This is analogous to a country that is internally empty and whose populace has scattered; in such a case, thieves and bandits will rise like bees, making suppression and pacification difficult. Unless wise and virtuous individuals are appointed to stabilize society, allow for rest and recuperation, and help the people recover, peace cannot be guaranteed. The deficiency of the body is exactly the same. If a physician is not wise and perceptive, who can stabilize the righteous qi, gather what has scattered, and restore the original state?

The *Inner Canon* says: "If one cannot treat deficiency, how can one ask about other conditions?" This implies that deficiency is the root of all diseases, so it should be listed first among various syndromes. However, what determines whether the body is sufficient or deficient is qi and blood; and what transforms into qi and blood is water (Kidney Yin) and fire (Heart Yang). Water and fire are the foundation of life and the basis for the activity of the spirit (*Shen*). The *Ling Shu* (Spiritual Pivot) states: "The essence of water manifests as Zhi (Will), and the essence of fire manifests as Shen (Spirit)." However, water and fire should be balanced rather than biased, and they should interact rather than separate. Fire has a nature of flaming upward, so it should be made to descend; water has a nature of flowing downward, so it should be made to ascend. When water is above and fire is below, this is called "interaction" (*Jiao*). Interaction leads to *Ji Ji* (Completion/Success), while non-interaction leads to *Wei Ji* (Incompletion/Failure). Interaction is the sign of life, and non-interaction is the sign of death. For example, in Xiao Ke syndrome (Thirst-Knowledge disease, characterized by excessive thirst and urination), the lack of interaction indicates an excess of fire; in Shui Qi syndrome (Water-Qi disorder, characterized by edema), the lack of interaction indicates an excess of water.

Therefore, fire belongs to Yang and Qi, standing in opposition to water. Water is Yin Essence, and fire is Yang Qi. When these two match properly, it is called "Harmony of Yin and Yang," also known as "Sha Huo" (Lesser Fire, referring to physiological warm fire). If vital energy is thus harmonious, various diseases will not arise, and longevity can be achieved. If one fails to nourish oneself properly, leading to Yin depletion and drying up of water, then fire will become excessive. This is what is meant by "when Yin is insufficient, Yang will encroach upon it," resulting in a state described here as "Yin Excess and Yang Deficiency" (contextually implying relative Yang hyperactivity due to absolute Yin deficiency, or pathological fire consuming Qi), also referred to as "Zhuang Huo Shi Qi" (Excessive Fire consumes Qi). From this, we know that fire is Qi, and Qi is fire. Thus, the *Immortal's Classic* states that medicine is fire, and fire is medicine; they are one yet two, and two yet one. Li Dongyuan also said: "Fire and Yuan Qi (Original Qi) cannot coexist." This refers to the same principle.

Consider water as an analogy: water is naturally cold. When flowing, it is cold; if it becomes extremely cold, it freezes into ice and stops flowing. Upon thawing, it returns to its normal state; it is not a different substance. Generally speaking, when in balance, water and fire achieve *Ji Ji*, and at this time, fire is the True Yang Qi. However, when it becomes biased, it turns into pathological fire, only then opposing Yuan Qi and creating a state of stagnation and obstruction. Therefore, Dai Ren (Zhang Congzheng) said: "Do not merely treat Wind, do not merely treat Dryness; when you treat Fire, Wind and Dryness will be resolved." This means that if one understands this, one has reached the origin of Yin-Yang and Water-Fire, allowing for flexible application in any situation. This specifically points out that Fire has many transformations and causes various diseases. Understanding this allows one to distinguish other conditions.

However, those who emphasize nourishing Yin argue that in the human body, there is only one source of Water, but two types of Fire (Ministerial Fire and Ministerial Fire/Yang), meaning Yang is often in excess and Yin is often insufficient. From youth to old age, diseases arise almost entirely from a deficiency of True Yin. Moreover, few people restrain their desires, while many indulge them, causing Yin Water to become more depleted and Yang Fire to become more vigorous. Unfortunately, Yin Essence is difficult to regenerate, and drastic tonification yields no immediate effect. Therefore, Yin-nourishing herbs should never be interrupted from youth to old age. Regarding Yang-tonifying herbs, the warnings are earnest. Although individual constitutions differ, those with Yang excess should indeed tonify Yin, and those with Yin excess particularly need to tonify Yang. Furthermore, Yin grows with Yang; solely nourishing Yin damages Stomach Qi and cuts off the source of postnatal transformation. Purely Yin-natured herbs possess the energy of desolation and closure, lacking the功 (merit) of Yang-like warmth and nurturing.

Moreover, Heaven takes Yang as the root of life, and humans take Fire as the gate of life. Heaven opens at Zi hour, where Yang is born; thus, Zi is the root of Yang and belongs to Pre-Heaven. Humans are born at Yin hour, where Fire appears; thus, Yin is the mother of Fire and belongs to Post-Heaven. Fire is the root of life; Yang is the function of Fire. Therefore, it is said: Without this [principle/person], Heaven cannot transform and generate all things; without this Fire, humans cannot have life. The Yang Qi of Heaven can interact downward; the Yin Qi of Earth can interact upward; the True Fire of humans can hide below, allowing True Water to spread above. This is the image of Yang imparting and Yin transforming, which can manifest the long-term prosperity of harmonious Qi and Blood.

Essentially, the essence of Yin and Yang hides within each other's dwellings; Yin contains Yang, and Yang contains Yin. Thus, although the Heart is Fire, it contains red fluid (Blood); although the Kidney is Water, it stores white Qi (Yuan Qi/True Yang). Red fluid is Yin, white Qi is Yang. They circulate continuously day and night; this is the normal measure. If one does not know how to nourish oneself and indulges in desires, depleting True Yin, Yang will have no attachment and will therefore flare up and ascend. This is the meaning of "Fire flares when empty." Consequently, Qi of the whole body converges on Yang. When Qi converges on Yang, Yang becomes even more excessive, and Yin becomes even more deficient. As a result, heat arises in the Upper Jiao, coughing produces phlegm, forced blood leads to hematemesis and epistaxis, headache and irritability occur, pain is felt in the chest bones, the mouth and tongue feel dry and bitter, the five palms feel hot, tidal fever and bone-steaming heat appear, and urine is short and red. These are the signs of this syndrome.

Over time, solitary Yang cannot flourish alone, and rootless fire cannot burn forever. As the *Canon* says, "Excessive Fire consumes Qi," Qi becomes weak, and Yang becomes deficient. Consequently, food is not digested, diarrhea is uncontrolled, the Dantian feels cold, muscles and bones feel weak, nocturnal emission and spermatorrhea occur, dizziness and spontaneous sweating happen, and sudden collapse may ensue. These are the signs of another syndrome.

However, in the Sha Yin organs (Heart and Kidney), the focus is on True Yang; if Yang does not return, pathogens cannot be expelled. In the Jue Yin organ (Liver), the function is storing Blood; if Blood is not nourished, the pulse cannot rise. Therefore, in treatment: if Yang is very deficient, tonify Yang to generate Yin, so that Yin grows with Yang; if Yin is very deficient, tonify Yin to accompany Yang, so that Yang transforms with Yin. When Yin and Yang are harmonized, hundred diseases dissolve. If one is biased, adhering strictly to either Yin or Yang, not only will the medication fail to correct the imbalance, but it will instead exacerbate the harm caused by the bias.