或曰∶人皆谓右肾属相火，相火即少火，观坎之象，则知肾具水火之道，一阳居二阴间为坎，水火并
而为肾，故惟坎加习也，子以右肾为属水，然则其说非与？余曰∶以惟坎加习，斯其说所以非也。夫坎
，水也。上下皆坎，《易》故曰习坎。观先天图，干南坤北。后天图，离南坎北。五行火高水下，故仙家
取坎填离，以水升火降，既济为道，谓采坎中之一阳，填离中之一阴，此还乾坤本源之意也。坎离，是兑
待之义，如彼谓一阳居二阴之间，无乃指一阳为火耶？然则离以一阴居二阳之间，又作何说也？夫物物具
五行，五行一阴阳，阴阳一太极，五脏均有此金木水火土，何乃指坎中之阳为火，指右肾为少火也。坎中
之阳，即两肾中间动气，五脏六腑之本，十二经脉之根，谓之阳则可，谓之火则不可，故谓坎中之阳，亦非
火也。二阴，即二肾也，肾既皆阴，则作一水一火并看者，亦非矣。不然，坎中之阳，尚不可以火目之，而
右肾又何可以属水哉？！或曰∶子是之言固矣，彼北极玄帝象
下有龟蛇者何说也？且昔沙随程可久曰∶北方常配二物，故惟坎加习，于物为龟为蛇。余曰∶此何可以证
水火并而为肾之谬也，盖龟蛇，乃道家寓意处，谓蛇属心火，龟属肾水，能降此二物，不使妄动，庶坎离
得以交 ，而身中之丹可成。若肾则封藏之本，精之处也，安可牵扯龟蛇而与之同类并观哉！断乎其不可矣。
有人问：人们都说右肾属于相火，相火就是少火，观察坎卦的卦象，就知道肾脏具备水火并存的道理。一阳爻居于两个阴爻之间构成坎卦，水与火合并成为肾的功能体现，所以只有“坎加习”（重坎）的说法。您认为右肾属于水，那么他们的说法难道不是错了吗？
我回答说：正是因为依据“坎加习”这种说法，才导致其错误。坎卦象征水。上下都是坎卦，《易经》因此称之为“习坎”。观察先天八卦图，乾在南，坤在北；后天八卦图中，离在南，坎在北。五行中火性炎上、水性润下，所以道家采取“取坎填离”的方法，使水上升、火下降，达到既济的状态作为修道之法。这指的是采集坎卦中间的一阳爻，填补离卦中间的一阴爻，这是返还乾坤本源的意思。坎和离，是兑卦对待的意义。如果像他们所说的一阳爻居于二阴爻之间，难道是指那一阳爻为火吗？那么离卦是一阴爻居于二阳爻之间，又该作何解释呢？万物都具备五行，五行归结为阴阳，阴阳归结为太极。五脏均具备金、木、水、火、土，为何要指坎卦中间的阳爻为火，指右肾为少火呢？坎卦中间的阳爻，其实就是两肾中间的动气，它是五脏六腑的根本，十二经脉的根源。称之为阳是可以的，称之为火就不可以了，所以说坎卦中间的阳爻也不是火。两个阴爻，就是指左右两个肾脏。既然肾脏都属于阴，那么将其看作一水一火并存的观点，也是错误的。否则，坎卦中间的阳爻尚且不可以用火来称呼，而右肾又怎么可以归属于水呢！
有人问：您的言论固然正确，但北方真武大帝（玄帝）神像下有龟和蛇，这又作何解释呢？况且从前沙随人程可久曾说：北方常配两种动物，所以只有“坎加习”，在动物中表现为龟和蛇。
我回答说：这怎么能用来证明水火并存于肾的错误观点呢？龟和蛇，是道家寓意所在的地方。说蛇属于心火，龟属于肾水，能够降伏这两种动物，不让它们妄动，从而使坎离得以交媾，身体中的金丹就可以结成。如果肾脏是封藏的根本，精液所在的处所，怎么可以牵扯龟和蛇而与肾脏同类并观呢！这显然是绝对不可能的。
A questioner asked: People all say that the right kidney belongs to Ministerial Fire (Xianghuo, 相火), and Ministerial Fire is Lesser Fire (Shaohuo, 少火). Observing the image of Kan (Kan, 坎), one can know that the kidneys possess the way of Water and Fire. One Yang line situated between two Yin lines forms Kan; Water and Fire combine to constitute the Kidney, hence only the concept of "Kan adding Xi" (Chong Kan, 重坎) applies. You argue that the right kidney belongs to Water; therefore, is their theory incorrect?
I replied: It is precisely because they rely on the concept of "Kan adding Xi" that their theory is flawed. Kan represents Water. When Kan appears above and below, the *Book of Changes* (*Yijing*, 易经) calls it "Xi Kan" (Repeating Kan, 习坎). Observing the Prior Heaven arrangement, Qian is in the South and Kun is in the North. In the Post-Heaven arrangement, Li is in the South and Kan is in the North. In the Five Elements (Wuxing, 五行), Fire rises high and Water flows low. Therefore, Daoists adopt the method of "extracting Kan to fill Li," causing Water to ascend and Fire to descend, achieving Ji Ji (Ji Ji, 既济, Already Fording/Crossed) as the Way. This refers to harvesting the single Yang within Kan to fill the single Yin within Li, which embodies the intention of returning to the original source of Qian and Kun. Kan and Li represent a mutual relationship akin to Dui (Dui, 兑). If, as they claim, the single Yang resides between two Yin lines, does this not imply that the single Yang is Fire? Then, how should we explain that Li consists of a single Yin residing between two Yang lines? All things possess the Five Elements; the Five Elements reduce to Yin and Yang; Yin and Yang reduce to Taiji. The Five Zang Organs (Zangfu, 脏腑) each contain Metal, Wood, Water, Fire, and Earth. Why point to the Yang within Kan as Fire, and the right kidney as Lesser Fire? The Yang within Kan is actually the Motile Qi (Dongqi, 动气) between the two kidneys; it is the root of the Five Zang and Six Fu organs and the root of the Twelve Meridians. Calling it Yang is acceptable, but calling it Fire is not. Thus, stating that the Yang within Kan is also Fire is incorrect. The two Yin lines refer to the two kidneys. Since both kidneys belong to Yin, viewing them as one Water and one Fire existing together is also incorrect. Otherwise, if the Yang within Kan cannot even be termed Fire, how can the right kidney be attributed to Water?!
The questioner asked: Your words are indeed sound, but what is the explanation for the image of the Black Emperor (Xuandi, 玄武/玄帝) under whom there are a turtle and a snake? Moreover, Cheng Kejiu from Shasuo once said: The North often pairs with two creatures, hence only "Kan adding Xi" manifests in animals as the turtle and the snake.
I replied: How can this prove the fallacy that Water and Fire coexist in the kidneys? The turtle and snake are merely symbolic meanings employed by Daoists. They assert that the snake belongs to Heart Fire (Xinhuo, 心火) and the turtle belongs to Kidney Water (Shenshui, 肾水). By subduing these two creatures and preventing them from moving recklessly, Kan and Li can interact and exchange, allowing the Golden Elixir (Dan, 丹) within the body to form. However, the Kidney is the foundation of sealing and storing, and the residence of Essence (Jing, 精). How can one drag in the turtle and snake to view them as similar to the Kidneys and observe them together? This is absolutely impossible.