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The *Nan Jing* (Classic of Difficult Issues) states: There are two kidneys, but they are not both kidneys. The left is the Kidney, belonging to Water; the right is the Mingmen (Gate of Life), belonging to Fire. Later scholars opposed this, asserting that both kidneys belong to Water, and the Mingmen is located between the two kidneys, specifically in the depression of the fourteenth vertebra counting from the top of the spine; if counted from the bottom up, it is the seventh vertebra, directly opposite the umbilicus. They cite the *Nei Jing* (Inner Canon): "Above the diaphragm and huang, there are Father and Mother" (referring to the Heart), and "Beside the seven segments (i.e., the seventh vertebra), there is the Little Mind" as evidence. They argue that since the two kidneys are beside the seventh segment, the space between them is exactly the depression of the seventh segment, and the "Little Mind" is the Mingmen.

The *Yi Guan* (Canon of Medicine) interprets "middle" as Taiji (Supreme Ultimate), using Taiji as a metaphor. It posits that dividing the one yields Taiji; the seal script for "middle" resembles φ and ○, representing Taiji; the vertical line │ divides Taiji into parts. Here, │ symbolizes the human spinal column, and ○ symbolizes the two kidneys. When the two kidneys ○ join, they remain one Taiji ○. The blank space within symbolizes the Mingmen. Furthermore, since the two kidneys are divided into the Two Modes (Liangyi), the left kidney is Yin Water, and the right kidney is Yang Water. Yang Water is the fluid aspect of Qi, corresponding to the Kan trigram (Water). Kan consists of one Yang line trapped between two Yin lines; water qi moves secretly underground, serving as the fundamental source upon which all things receive their life. In the *Yue Ling* (Monthly Ordinances), during mid-autumn, it says: "The killing energy is at its peak, Yang Qi declines, and water begins to dry up." This represents the drying of water and the death of the earth. During mid-winter, it says: "Spring waters move." This is because one Yang arises in this month, representing the movement of water and the vitality of the earth. Yin Water refers to form. The Dui trigram represents Marsh. One Yin line extends above two Yang lines; Dui uses tangible water to universally benefit all things, descending to provide nourishing benefits. This explanation is quite reasonable. However, it is largely consistent with what the *Nan Jing* states. Essentially, both assert that although the Kidney belongs to Water, there is Fire within the Water. The herbs used to tonify Fire are fundamentally the same; how could there be a distinction where one herb enters the right Kidney and another enters the seventh vertebra? The concern about divergence and error necessitates this clarification.

As for Zhao Xianke (Zhao Shi), he argued that the Fire of the Mingmen is the Primordial Yang of the Pre-Heaven, and Kidney Water is the Primordial Yin of the Pre-Heaven, constituting the root of life. Therefore, treating disease must seek the root. Thus, when cold conditions do not respond to cold treatment (using Cold-natured herbs to treat Heat, yet Heat does not recede), this indicates a lack of Water (Kidney Yin deficiency). For those lacking Water, one should strengthen the master of Water (the Kidney) to control the Sunlight (deficient Fire), primarily using Liuwei Wan (Six-Ingredient Pill with Rehmannia). When heat conditions do not respond to heat treatment, this indicates a lack of Fire (Kidney Yang deficiency). For those lacking Fire, one should augment the source of Fire to dissipate Yin shadows (Yin-cold pathogens), primarily using Bawei Wan (Eight-Ingredient Pill, i.e., Jin Gui Shen Qi Wan, also seen in Xu Sun/Deficiency Damage contexts). Following his theory and using these formulas yields miraculous results for severe deficiency syndromes. However, administering these tonics immediately at the onset of illness would be erroneous. Why? Because initial illness only damages Post-Heaven Qi and Blood, without immediately affecting Pre-Heaven Water and Fire. Therefore, applying Heat resolves Cold, and applying Cold resolves Heat, with immediate effect. Why seek other remedies? If one fails to eliminate the pathogen and instead hastily tonifies the Righteous Qi, will it not lead to prolonged illness and missed opportunities for treatment?

Currently, many physicians adhere rigidly to the theories of the *Yi Guan*. Regardless of whether the disease is new or chronic, they use either Liuwei Wan or Bawei Wan, or Bu Zhong Yi Qi Tang (see Qi deficiency), or Guipi Tang (see Blood deficiency), as if all ancient formularies could be discarded. This is extremely confusing. Someone asked: "Qi is Fire, and Blood is Water. In the fetus, Qi and Blood are already present, implying that Qi and Blood are also generated by the Pre-Heaven. Liuwei Wan tonifies Water, and Bawei Wan tonifies Fire, implying that Water and Fire are also nurtured by the Post-Heaven. Now you say Qi and Blood belong to the Post-Heaven, while Water and Fire belong to the Pre-Heaven. Is this not slighting Qi and Blood while valuing Water and Fire?" I replied: "Qi and Blood are the general principles of Water and Fire. Respiration, movement, perception, and spiritual consciousness in the human body are all functions of this Fire. Qi can describe respiration and movement, but cannot encompass perception and consciousness; thus, although Qi is Fire, it does not fully exhaust the concept of Fire. Blood, marrow, and essence in the human body are all functions of this Water. Blood is merely the red component within Water and cannot represent the whole; thus, although Blood is Water, it does not fully exhaust the concept of Water. At the beginning of life, when conception first forms, resembling a dewdrop, it is the essence and Qi of the parents. Therefore, Water and Fire are the 'Pre-Heaven of the Pre-Heaven.' After several months of forming the physical structure, Qi and Blood become complete; thus, Qi and Blood are the 'Post-Heaven of the Pre-Heaven.' As for the Qi and Blood nurtured by diet after birth, their status as Post-Heaven needs no further explanation. This is how Water/Fire and Qi/Blood are distinguished as Pre-Heaven and Post-Heaven. The Pre-Heaven is truly the root of the Post-Heaven; therefore, Water and Fire are the origin of Qi and Blood. The Lower Jiao is the root of the Middle and Upper Jiao; thus, the Kidney and Mingmen are the foundation of Water and Fire. Their distinction in importance naturally cannot be ignored. (Pre-Heaven Water and Fire root each other mutually: exhaling air condenses into water, showing Yin rooting in Yang; evaporating water generates Qi, showing Yang rooting in Yin.)"