《难经》谓∶肾有两枚，非皆肾也，左为肾，属水，右为命门，属火。后人非之，
谓两肾皆属水，命门在两肾之中，当脊骨自上数下第十四椎陷中；若自下数上，则
为第七椎，正与脐对。引《内经》∶鬲肓之上，中有父母，（谓心也。）七节（即第七椎）之
旁，中有小心为证。谓两肾在七节之旁，两肾中间，即七节之陷中也，小心即命门。
《医贯》又以中字为解，以太极形容。谓一中分太极，中篆作φ，○，
太极也；│ 分太极为○。者。│，象人之脊骨，○，象人之两肾，两肾○
相合，仍为一太极○。其中空白处，象命门。又两肾既分为两仪，则左肾为阴
水，右肾为阳水。阳水者，气之液也，坎水也。坎以一阳陷于二阴，水气潜行地中，
为万物受命根本。《月令》于仲秋云∶杀气如盛，阳气衰，水始涸。是水之涸，
地之死也。于仲冬云∶水泉动。是月一阳生，是水之动，地之生也。阴水者，形也。
兑，泽也。一阴上彻于二阳之上，兑以有形之水，普施万物，下降为资生之利泽，
其说甚有理。然与《难经》所言，大致亦自无殊。盖皆言肾虽属水，而水中有火耳。
所用补火之药，总属一样，岂有分别此味则入右肾，彼味则入七节？忧其岐误，
致烦辨正哉。至赵氏谓命火乃先天之元阳，肾水乃先天之元阴，为生命之根本，
治病必须求本。故凡寒之不寒，（用寒药以治热，而热不退。）是无水也。无水者，壮水之主
（主水者肾。）以制阳光，当主六味丸。热之不热，是无火也。无火者，益火之原以消阴
翳，当主八味丸。（并见虚损。）遵其说而用之败证。效诚如神。若初起遽以此投之，则
谬矣。何则？初病止伤其后天之血气，未遽累及先天之水火，故但热之则寒消，寒
之则热退，随手立应，何必他求。乃不去其邪，而遽补其正，有不迁延时日，坐失
事机者哉？何今之为医者，泥于《医贯》之说，不论新病久病，非六味则八味，非
补中（见气）。则归脾，（见血。）竟若历古方书，皆可删却，亦惑之甚矣。或问∶气即火也，
血即水也，儿在胎中，气血已具，是气血亦先天所生；六味补水，八味补火，是水
火亦后天所养。今谓气血为后天，水火为先天，毋乃轻气血而重水火乎？曰∶气血
者，水火之大纲也。人身呼吸运动，知觉神明，皆此火之为之也。气可以言呼吸运
动，不可以言知觉神明，是气虽即火，而不足以尽火。人身之血液精髓，皆此水之
为之也。血特水中之赤者耳，不可以概其余，故血虽即水，而不足以尽水。有生之
初，胎孕始结，形如露珠，父母之精气也。是水火乃先天之先天，数月形成，而后
血气具，是血气为先天之后天。若夫既生之后，饮食所长养之气血，其为后天，又
不待言矣。此水火、气血，先后天之分如此。先天实为后天之根，故水火为气血之
原，而下焦又为中上之根，故肾命为水火之本。其轻重之分，固不能以无别也。
（先天水火，又互相为根，如嘘气成水，阴根于阳也；蒸水成气，阳根于阴也。）
《难经》中说：肾脏有两个，并非两个都是肾。左边的称为肾，属水；右边的称为命门，属火。后世医家对此提出反对，认为两肾都属水，命门位于两肾之间，具体位置在脊柱从上往下数第十四椎的凹陷处；如果从下往上数，则是第七椎，正好与肚脐相对。他们引用《内经》中的话：“膈肓之上，中有父母”，（这里指心脏。）以及“七节（即第七椎）之旁，中有小心”作为证据。他们认为两肾位于第七椎的旁边，两肾中间正是第七椎的凹陷处，而“小心”就是命门。
《医贯》又将“中”字解释为太极，用太极来比喻。认为一中分出了太极，“中”字的篆书写法像φ和○，代表太极；竖线│将太极分为两部分。其中，│象征人的脊骨，○象征人的两肾，两肾○相合，仍然是一个完整的太极○。其间的空白处，象征命门。又说，既然两肾分为两仪，那么左肾为阴水，右肾为阳水。所谓阳水，是气的液体形态，即坎卦之水。坎卦是一阳陷于二阴之中，水气潜行于地中，是万物受命的根本。《月令》在仲秋时节说：“杀气极盛，阳气衰败，水开始枯竭。”这是水枯竭、大地死亡的表现。在仲冬时节说：“水泉涌动。”这是因为本月一阳初生，是水活跃、大地生机勃勃的表现。所谓阴水，是形体。兑卦代表泽。一阴上升覆盖于二阳之上，兑卦以有形之水，普遍施利于万物，下降成为滋养万物的恩泽，这种说法很有道理。但这与《难经》所说的，大体上也没有不同。大概都是说肾虽然属水，但水中含有火罢了。所用补火的药物，总归是一样的，难道会有分别说某种药入右肾，另一种药入第七椎吗？担心出现歧义错误，才烦劳进行辨正罢了。
至于赵献可（赵氏）认为命门之火是先天元阳，肾水是先天元阴，是生命的根本，治病必须求本。所以凡是寒症用寒药治疗却无效（用寒药治热证，但热不退），这是无水（肾阴亏虚）。无水者，应当壮水之主（主水的是肾），以制约阳光（虚火），主要使用六味地黄丸。凡是热症用热药治疗却无效，这是无火（肾阳不足）。无火者，应当益火之源（补命门之火）以消除阴翳（阴寒之邪），主要使用八味丸（即金匮肾气丸，并见于虚损病证）。遵循他的学说并使用这些方剂，对于败证（久病虚损重症）疗效确实神速。但如果疾病初起就立即投用这些补益药，那就错了。为什么呢？初病只是损伤了后天的血气，并未立刻累及先天的水火，所以只要用热药则寒消，用寒药则热退，随手立见成效，何必另作他求。如果不祛除病邪，就急于补益正气，难道不会延误时日，坐失治疗良机吗？
如今行医的人，拘泥于《医贯》的说法，不论新病还是久病，不是用六味地黄丸就是八味丸，不是用补中益气汤（见气虚证）就是用归脾汤（见血虚证），竟然觉得历代方书都可以删去，这也太迷惑了。有人问：气就是火，血就是水，胎儿在母腹中时，气血已经具备，这说明气血也是先天所生；六味地黄丸补水，八味丸补火，说明水火也是后天所养。现在说气血属于后天，水火属于先天，岂不是轻视气血而重视水火吗？回答说：气血，是水火的大纲。人身体的呼吸运动、知觉神明，都是这火的作用。气可以说涉及呼吸运动，但不能涵盖知觉神明，所以气虽然即是火，但不足以完全概括火。人身的血液精髓，都是这水的作用。血只是水中红色的部分罢了，不能以此概括其余部分，所以血虽然即是水，但不足以完全概括水。生命初始，胎孕刚形成，形状如露珠，是父母的精气结合而成。因此水火是先天的先天，经过数月形成形体之后，气血才完备，所以气血是先天的后天。至于出生后，通过饮食所长养的气血，它们属于后天，更是不言而喻的了。这就是水火、气血在先天后天方面的区分。先天实为后天的根本，所以水火是气血的源头，而下焦又是中焦、上焦的根本，所以肾与命门是水火的本源。它们轻重的区别，自然不能没有分别。（先天的水火，又互相为根，如嘘气成水，阴根于阳；蒸水成气，阳根于阴。）
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.)"