三十六难曰∶脏各有一耳，肾独有两者，何也？
然∶肾两者，非皆肾也。其左
者为肾。右者为命门。命门者，谓精神之所舍，原气之所系也；男子以藏精，女子以系胞。故知肾
有二也。
肾有两枚，左右各一，一主水，一主火，应乎升降之机也。命门者，以其为三焦之根，十二
经元气之海，藏精施化之具，系胞受孕之处，为人生命之原，故曰命门也。《灵枢·根结篇》、
《素问·阴阳离合论》所谓太阳根起于至阴，结于命门。命门者，目也。此指太阳经终于睛明，
睛明所夹之处为脑心，乃至命之穴，故曰命门，与此义不同。然实指右肾为命门，恐未尽是，
以气脉论之，水升于左，火降于右，左右者，阴阳之道路，升降之枢机，越人诊脉独取寸口，
以左尺候水，右尺候火，故左名肾，右名命门，其义或取乎此。
按∶西医言肾形如豆，色紫质坚，颇类猪羊之肾，左右两枚，长约三寸，阔约寸半，浓约七
八分，其重约三两至四两。人高肾大，人矮肾小，位在脊骨十二节间，周遭三焦脂膜包裹，肾
中有油膜一条，贯于脊骨，名为肾系，下通网膜，又有气管由肺而下，附脊循行，下入肾系，
而透入网膜，达于丹田下焦之原。夫两肾属水，中间肾系属火，即命门也。《素问·刺禁论》云∶
七节之旁，中有小心者，即指命门言也。人与天地参，命门与太极相似，太极生两仪，两仪生
四象，四象生八卦，八卦生六十四卦，自命门生两肾，两肾生六脏六腑，六脏六腑生四肢百骸
之类。故人之交媾，未有精聚，先有火会，是火为先天之本始，水为天一之真元，肾中之火，
名曰相火，即坎中龙雷之火也。是一阳陷于二阴之中，乃成乎离，而位乎坎，即两肾有命门之
义也。命门乃三焦之根，为相火之宅，相火布于三焦，即由命门始也。陈无择谓有脂状如手大，
正与膀胱相对，有白脉自中出，夹脊而上贯于脑者，亦指三焦肾系而言也。越人独取寸口诊候，
此相火生脾土，命门寄夫右尺，故作左为肾，右为命门以解之，亦水升于左，火降于右之义也。
第三十六难问道：五脏中其他脏器都只有一个，唯独肾脏有两个，这是为什么呢？
回答说：肾脏有两个，但并非两个都是通常所说的“肾”。左边那个称为肾，右边那个称为命门。命门，是精神藏舍的地方，也是原气系连之处；男子依靠它来贮藏精气，女子依靠它来维系子宫（胞宫）。因此知道肾有两个。
肾脏有两枚，左右各一，一个主水，一个主火，对应着气机升降的机理。命门，因为它是三焦的根本，十二经脉元气的海洋，是贮藏精气并施行化生的器具，是维系子宫和受孕的场所，是人的生命本源，所以称为命门。《灵枢·根结篇》和《素问·阴阳离合论》中说太阳经根起于至阴穴，结于命门。这里的命门，指的是眼睛。这是因为太阳经终于睛明穴，睛明穴所夹之处为脑心，是关系到生命的穴位，所以称为命门，这与本难的含义不同。然而，实际上指右肾为命门，恐怕并不完全正确，从气脉理论来看，水升于左，火降于右，左右是阴阳的道路，升降的枢纽，越人（秦越人，即扁鹊）诊脉独取寸口，以左尺部候水，右尺部候火，所以左名为肾，右名为命门，其含义或许取于此理。
按语：西医说肾脏形状像豆子，颜色紫黑，质地坚硬，很像猪羊的肾脏，左右两枚，长约三寸，宽约一寸半，厚约七八分，重量约三两到四两。人高则肾大，人矮则肾小，位置在脊柱第十二节之间，周围被三焦脂膜包裹，肾中有一条油膜，贯穿脊柱，称为肾系，向下连通网膜，又有气管由肺部下行，附着脊柱循行，向下进入肾系，并透入网膜，到达丹田和下焦的原气所在。两肾属于水，中间的肾系属于火，也就是命门。《素问·刺禁论》说：“第七节旁边，中间有小心”，就是指命门而言。人与天地相参，命门与太极相似，太极生两仪，两仪生四象，四象生八卦，八卦生六十四卦，从命门生出两肾，两肾生出六脏六腑，六脏六腑生出四肢百骸之类。所以人的交媾，在没有精气聚合之前，先有火的会合，这火是先天之本始，水是天一的真元，肾中的火，称为相火，也就是坎卦中龙雷之火。这是一阳陷于二阴之中，从而形成离卦，而位于坎位，这就是两肾中有命门的意义。命门是三焦的根本，是相火的宅舍，相火布散于三焦，就是从命门开始的。陈无择说有一块脂肪状如手掌大小，正对着膀胱，有白色脉络从中发出，夹着脊柱向上贯通于脑部，这也是指三焦和肾系而言。越人独取寸口诊候，认为此相火生脾土，命门寄居在右尺部，所以解释为左为肾，右为命门，这也是水升于左、火降于右的意义。
Difficulty 36 asks: The viscera each have one, but the Kidneys uniquely have two. Why is this?
The response states: The two Kidneys are not both "Kidney" in the ordinary sense. The left one is called the Kidney (Shen), and the right one is called the Gate of Life (Mingmen). The Gate of Life is where the spirit and essence reside and where the Original Qi (Yuanqi) is anchored; in men, it stores essence (Jing), and in women, it anchors the uterus (Baoguo). Thus, it is known that there are two Kidneys.
There are two Kidneys, one on the left and one on the right. One governs Water, and the other governs Fire, corresponding to the mechanism of ascending and descending. The Gate of Life is so named because it is the root of the Sanjiao (Triple Burner), the sea of Yuan Qi for the twelve meridians, the vessel for storing essence and facilitating transformation, the place where the uterus is anchored and conception occurs, and the origin of human life. The *Lingshu · Genjie Chapter* and the *Suwen · Yin Yang Lihe Lun* state that the Taiyang meridian originates at Zhiyin (BL67) and terminates at the Mingmen. Here, Mingmen refers to the eyes. This is because the Taiyang meridian ends at Jingming (BL1), and the area between the orbits of Jingming relates to the brain and heart, being an acupoint vital to life; thus it is called Mingmen. This meaning differs from the present difficulty. However, strictly identifying the right Kidney as the Gate of Life may not be entirely accurate. From the perspective of Qi and pulse theory, Water ascends on the left, and Fire descends on the right. The left and right are the pathways of Yin and Yang and the pivot of ascending and descending. Yue Ren (Qin Yueren, i.e., Bian Que) takes the pulse exclusively at the Cun Kou (Radial artery at the wrist), using the Left Chi position to assess Water and the Right Chi position to assess Fire. Therefore, the left is named Kidney, and the right is named Gate of Life; its rationale likely derives from this principle.
Commentary: Western medicine describes the kidneys as bean-shaped, purple-black in color, and firm in texture, resembling the kidneys of pigs or sheep. There are two, approximately three cun long, one and a half cun wide, and seven to eight fen thick, weighing about three to four liang. Taller individuals have larger kidneys, while shorter individuals have smaller ones. They are located between the twelfth vertebrae of the spine, surrounded by the fascia of the Sanjiao. Inside the kidney, there is a fatty membrane running through the spine, called the Kidney Meridian (Shenxi), which connects downward to the omentum. Additionally, the ureters descend from the lungs, run along the spine, enter the Kidney Meridian, penetrate the omentum, and reach the Dantian and the source of the Lower Jiao. The two Kidneys belong to Water, while the central Kidney Meridian belongs to Fire, which is the Gate of Life. The *Suwen · Cijin Lun* states: "Beside the seventh vertebra, there is the 'Small Heart'," referring to the Gate of Life. Humans correspond with Heaven and Earth; the Gate of Life is analogous to the Supreme Ultimate (Taiji). The Supreme Ultimate generates the Two Modes (Liangyi), the Two Modes generate the Four Images (Sixiang), the Four Images generate the Eight Trigrams (Bagua), and the Eight Trigrams generate the sixty-four hexagrams. From the Gate of Life arise the two Kidneys; from the two Kidneys arise the six Zang and six Fu organs; from the six Zang and six Fu organs arise the limbs and hundred bones. Therefore, in sexual intercourse, before essence gathers, fire meets first. This fire is the primordial beginning, and water is the true yuan of Tianyi. The fire within the Kidneys is called Ministerial Fire (Xianghuo), also known as the Dragon-Thunder Fire within Kan (Water trigram). This represents one Yang trapped within two Yins, forming the Li (Fire trigram) and residing in the Kan position, which signifies the presence of the Gate of Life within the two Kidneys. The Gate of Life is the root of the Sanjiao and the residence of Ministerial Fire. Ministerial Fire spreads throughout the Sanjiao, starting from the Gate of Life. Chen Wuyue described a fatty mass the size of a hand, directly opposite the bladder, with white vessels emerging from the center, running alongside the spine up to the brain; this also refers to the Sanjiao and Kidney Meridian. Yue Ren, taking the pulse exclusively at the Cun Kou, posited that this Ministerial Fire generates Spleen Earth, and the Gate of Life is housed in the Right Chi position. Thus, he explained it as the left being the Kidney and the right being the Gate of Life, consistent with the principle that Water ascends on the left and Fire descends on the right.