《七十五难》曰∶经言东方实，西方虚，泻南方，补北方。何谓也？然∶金木水火土，当更相平。
东方，木也，西方，金也。木欲实，金当平之；火欲实，水当平之；土欲实，木当平之；金欲实，火当平
之；水欲实，土当平之。东方者，肝也，则知肝实；西方者，肺也，则知肺虚。泻南方火，补北方水。南
方火，火者木之子也；北方水，水者木之母也。水胜火，子能令母实，母能令子虚，故泻火补水，欲令金
不得平木也。经曰∶不能治其虚，何问其余。此之谓也。夫遽曰金欲实，火当平之，水欲实，土当平之。
是五行之气，皆有亢有制也。奚止东方实，而南方当泻乎？要知上二句乃启下之文，下二句乃承上之辞
也。意若曰∶假如东方实西方虚者，当泻南方而补北方也。泻南方者，泻东方之实，实则泻其子也；补北
方者，补西方之虚，子能令母实也。肺主呼吸，而肾为生气之原，故经言肾为本，肺为末。荀子曰∶未有
子富而父贫者。即此义也。然首末经义，重在补虚，（首末是经义，中段是越人释经。）故曰不能治其虚，何
问其余。当知泻南方，亦所以补西方也。何问其余者，言五行之
气，皆可推而论之。设使西方实东方虚，又当泻北方而补南方矣。虽然，东南主生长之令，其气多实；
西北主收藏之气，其气多虚。故曰岁半以上，胜之常也。夫金欲实，火当平之；水欲实，土当平之。分论
主时之气，而各有太过也。东实西虚，泻南补北，统论一岁之气，而有虚实也。（以岁气而兼论人身之五行，
皆可。）参论经义，可分可合，庶为得之。
《难经·七十五难》说：医经上讲“东方实，西方虚，泻南方，补北方”。这是什么意思呢？回答说：金、木、水、火、土五行之气，应当相互平衡制约。
东方属木，西方属金。木气如果偏盛（实），金气就应当制约它；火气如果偏盛，水气就应当制约它；土气如果偏盛，木气就应当制约它；金气如果偏盛，火气就应当制约它；水气如果偏盛，土气就应当制约它。东方对应肝脏，所以知道是肝气实证；西方对应肺脏，所以知道是肺气虚证。治疗上应泻南方之火，补北方之水。南方属火，火是木之子（子行）；北方属水，水是木之母（母行）。水能克火，根据“子能令母实，母能令子虚”的原理，所以通过泻火和补水，目的是让金气不再去克制木气。医经说：“不能治疗其虚证，还问其他什么呢？”说的就是这个道理。有人急促地说：“金欲实，火当平之；水欲实，土当平之。”这是说五行之气，既有亢盛的一面，也有被制约的一面。哪里仅仅是东方实而南方该泻呢？要知道前两句是启发下文的引语，后两句是承接上文的话。意思是说：假如出现东方实、西方虚的情况，就应当泻南方而补北方。所谓泻南方，就是泻东方（肝木）的实证，因为实则泻其子（心火）；所谓补北方，就是补西方（肺金）的虚证，因为子能令母实（肾水生肝木，进而助益肺金？此处原文逻辑需结合上下文理解，通常指补肾水以制心火，或滋水涵木，但原文解释为“子能令母实”，在五行相生中，水生木，木生火。若补北方水，水为木之母，木为火之子。原文逻辑稍显复杂，此处直译其意：补北方水，旨在补西方肺金之虚，依据是‘子能令母实’的理论变通或特定语境下的解释，但在标准五行中，金生水，水为金之子。补子（水）可令母（金）实，即“子能令母实”）。肺主呼吸，而肾为生气之源，所以医经说肾为本，肺为末。荀子说：“没有儿子富有而父亲贫穷的道理。”正是这个含义。然而首尾引用的经文义理，重在补虚（开头结尾是经文原义，中间段是秦越人解释经文），所以说“不能治疗其虚证，还问其他什么呢”。应当知道，泻南方之火，也是为了补西方之金。所谓“何问其余”，是说五行之气，都可以推演论述。假使出现西方实、东方虚的情况，又应当泻北方而补南方了。虽然如此，东南方主管生长之令，其气多实；西北方主管收藏之气，其气多虚。所以说上半年阳气胜，是常理。至于“金欲实，火当平之；水欲实，土当平之”，是分论主时之气，各有太过之处。“东实西虚，泻南补北”，是统论一岁之气，存在虚实变化。（将岁气与人身五行结合讨论，都是可以的。）参合经文的义理，既可分论也可合论，这样才算得其真谛。
Nan Jing (Classic of Difficult Issues), Chapter 75, states: The medical classics say, "The East is excess, the West is deficiency; drain the South, tonify the North." What does this mean? The answer is: The Five Elements (Wood, Fire, Earth, Metal, Water) should mutually balance and restrain each other.
The East corresponds to Wood, and the West to Metal. If Wood Qi is in excess, Metal Qi should restrain it; if Fire Qi is in excess, Water Qi should restrain it; if Earth Qi is in excess, Wood Qi should restrain it; if Metal Qi is in excess, Fire Qi should restrain it; if Water Qi is in excess, Earth Qi should restrain it. The East corresponds to the Liver (Gan), indicating Liver Excess (Shi); the West corresponds to the Lung (Fei), indicating Lung Deficiency (Xu). The treatment principle is to drain Fire in the South and tonify Water in the North. The South belongs to Fire, which is the Son (Zi) of Wood; the North belongs to Water, which is the Mother (Mu) of Wood. Water overcomes Fire. According to the principle that "the Son can make the Mother full, and the Mother can make the Son empty," draining Fire and tonifying Water aims to prevent Metal from restraining Wood. The classics state: "If you cannot treat the deficiency, what else is there to ask about?" This refers to this very concept. Some hastily argue: "If Metal desires fullness, Fire should restrain it; if Water desires fullness, Earth should restrain it." This implies that the Qi of the Five Elements has both hyperactive and restrained aspects. Why limit it only to the East being excess and the South needing drainage? It must be understood that the first two sentences are introductory phrases leading to the following text, while the latter two sentences are continuations of the preceding context. The meaning is: In the hypothetical scenario where the East is excess and the West is deficiency, one should drain the South and tonify the North. Draining the South means draining the excess of the East (Liver Wood), based on the principle "drain the Son when there is excess"; tonifying the North means tonifying the deficiency of the West (Lung Metal), based on the principle "the Son can make the Mother full." The Lung governs respiration, while the Kidney is the root of vital Qi generation. Therefore, the classics state that the Kidney is the Root (Ben) and the Lung is the Branch (Mo). Xunzi said: "There is no case where the son is wealthy while the father is poor." This illustrates the same logic. However, the core meaning at the beginning and end of the classic emphasizes tonifying deficiency (the beginning and end represent the original meaning of the classic, while the middle section represents Bian Que's explanation of the classic). Hence, it says, "If you cannot treat the deficiency, what else is there to ask about?" One should understand that draining the South (Fire) is also a method to tonify the West (Metal). The phrase "what else is there to ask about" implies that the Qi of the Five Elements can all be deduced and discussed. If, conversely, the West were excess and the East deficient, one would then drain the North and tonify the South. Nevertheless, the Southeast governs the command of growth, so its Qi tends to be excess; the Northwest governs the energy of storage and collection, so its Qi tends to be deficiency. Thus, it is said that in the first half of the year, Yang Qi prevailing is the norm. The statements "If Metal desires fullness, Fire should restrain it; if Water desires fullness, Earth should restrain it" discuss the Qi of the dominant seasons, each having excessive tendencies. "East excess, West deficiency, drain South, tonify North" discusses the Qi of the entire year, involving changes of excess and deficiency. (Combining annual Qi with the Five Elements of the human body allows for comprehensive discussion.) Analyzing the meaning of the classics, one can separate or combine them as appropriate, which is considered to grasp the true essence.