经曰∶精气夺则虚。又曰∶邪之所凑，其气必虚。虚者，空也，无也。譬诸国内空
虚，民众离散，则盗贼蜂起，镇抚为难，若非委任贤智，安靖休养以生息之，未可保其
无事也。病之虚者，亦犹是已，医非明哲，孰能镇之，以收合散亡，克复故物之功哉！
经曰∶不能治其虚，安问其余？盖言虚为口病之本，宜其首举以冠诸证也。然充足空虚
者，气血也，化生气血者，水火也，水火者，主身之本，神明之用也。《灵枢》曰∶
水之精为志，火之精为神。然水火宜平不宜偏，宜交不宜分。火性炎上，故宜使之下∶
水性就下，故宜使之上，水上火下，名之曰交。交则为既济，不交则为末济，交者生之
象，不交者死之征也。如消渴证不交，火偏盛也，水气证不交，水偏盛也。故火者，阳
也，气也，与水为对待者也。水为阴精，火为阳气，二物匹配，名曰阴阳和平，亦名少
火。生气如是，则诸病不作，可得长生矣。倘不善摄养，以致阴亏水涸，则火偏胜，所
谓阴不足，则阳必奏之，是为阴盛阴虚，亦曰壮火蚀气，是知火即气也，气即火也，故
《仙经》谓药即火，火即药，一而二，二而一者也。东垣亦曰∶火与元气不两立。亦指
此也。譬诸水性，水流本寒，过极则凝而不流为层冰矣，解则复常，非二物也。盖平则
水火既济，火即为真阳之气，及其偏也，则即阳气而为火矣，始与元气不两立，而成乖
否之象焉。故戴人曰∶莫治风，莫治燥，治得火时风燥了。言苟能解此，则已达阴阳水
火之原，曲畅旁返，何施不可，正指火之变态多端，其为病也非一，明此则余皆可辨。
但重养阴者，谓人之一身，水一而已，火则二焉，阳常有余，阴常不足，自少至老，所
生疾病，靡不由于真阴不足。况节欲者少，嗜欲者多，以致阴水愈亏，阳火愈旺。奈阴
道难长，峻补则无旦夕之效，故补阴之品，自少至老，不可一日间断。其补阳之药，劝
戒谆谆，虽然性禀不同，阳盛人补阴固宜，阴盛人补阳尤要，况阴从阳长，单滋阴分，
徒伤胃气，反绝后天化生之源，要知纯阴之药，则得肃杀闭藏之气，何有阳和化育之功
哉！况天地以阳为生之根，人生以火为命之门，天开于子，而阳生焉，是子为阳之本，
而为先天，人生于寅，而火兆焉，是寅为火之母，而为后天火者，生之本也，阳者，火
之用也。故曰∶天非此人，不能化生万物∶人非此火，不能有生。天之阳气，能交于
下∶地之阴气，能交于上；人之真火，能藏于下，则真水能布于上，阳施阴化之象，克
昭气血平和之长日旺。盖阴阳之精，互藏其宅，阴中有阳，阳中有阴。故心火也，而含
赤液；肾水也，而藏白气。赤液为阴，白气为阳，循环往复，昼夜不息，此常度也。苟
不知摄养，纵恣情欲，亏损真阴，阳无所附，因而发越上升，此火空则发之义，是周
身之气并于阳也，并于阳，则阳愈盛，而阴愈亏，由是上焦发热，咳嗽生痰，迫血吐
衄，头痛烦躁，胸前骨痛，口干舌苦，五心烦热，潮热骨蒸，小便短赤，此其候也。久
则孤阳不能独旺，无根之火，岂能长明？《经》所谓“壮火食气”，气亦弱矣，而阳亦
虚焉。由是饮食不化，泻泄无度，丹田不暖，筋骨无力，梦遗精滑，眩晕自汗，卒倒
僵仆，此其候也。然少阴脏中，重在真阳，阳不回则邪不去，厥阴脏中，职司藏血，不
养血则脉不起，故治之者，阳甚虚者补阳以生阴，使阴从阳长也；阴甚虚者，补阴以配
阳，使阳从阴化也，阴阳调和，百病消解。若以偏重，或阴、或阳执见，则不惟设药以
救偏，而反增偏害之至矣。
《黄帝内经》说：精气耗损就会表现为虚证。又说：邪气聚集的地方，正气必然虚弱。所谓“虚”，就是空虚、没有东西。这就好比一个国家内部空虚，百姓离散，那么盗贼就会蜂拥而起，镇压安抚变得困难，如果不委任贤能智慧之人，安定社会、休养生息以恢复元气，就不能保证国家平安无事。身体的虚证，也是同样的道理，医生如果不是明哲睿智的人，谁能起到镇守正气、收合散失、恢复原有功能的作用呢！
《黄帝内经》说：如果不能治疗虚证，还谈什么其他的病证？这是说虚证是各种疾病的根本，所以应该首先列举它作为各类证候的开端。然而，决定身体充足还是空虚的，是气和血；而化生气血的，是水（肾阴）和火（心阳）。水和火，是主宰生命的根本，也是神明活动的物质基础。《灵枢经》说：水的精华表现为志，火的精华表现为神。然而，水火应当平衡而不宜偏颇，应当交融而不宜分离。火的性质是向上炎升，所以应当使其下行；水的性质是向下流注，所以应当使其上行，水在上、火在下，这就叫做“交”。相交就称为“既济”（成功、和谐），不交就称为“未济”（失败、混乱），相交是生的象征，不交是死的征兆。例如消渴证（糖尿病等以多饮多食多尿为特征的疾病）是因为水火不交，火偏盛；水气证（水肿等以水液代谢障碍为主的疾病）是因为水火不交，水偏盛。
所以，火属于阳，属于气，与水相对待。水是阴精，火是阳气，这两者匹配得当，就叫做“阴阳和平”，也叫做“少火”（生理性的温和之火）。如果生气如此调和，那么各种疾病就不会发生，就可以获得长寿。如果不善于调养，导致阴亏水涸，那么火就会偏胜，这就是所谓的“阴不足，则阳必凑之”，这被称为“阴盛阴虚”（此处指阴液亏损导致阳气相对亢盛，或真阴不足导致虚火妄动，原文语境偏向于阴亏阳亢导致的病理状态，即壮火蚀气），也称为“壮火蚀气”（过亢的火消耗元气）。由此可知，火就是气，气就是火，所以《仙经》说药就是火，火就是药，是一回事，又是两回事。李东垣也说：火与元气不两立。也是指这个意思。
打个比方，水的本性是寒凉的，水流本来寒冷，如果冷到极点就会凝结成冰不再流动，解冻后又恢复常态，并不是两种不同的物体。大概来说，平和的时候水火既济，此时的火就是真阳之气；等到它偏颇的时候，它就变成了病理性的火，这才与元气对立，形成乖戾否塞的现象。所以戴人（张从正）说：不要只治风，不要只治燥，把火治好了，风和燥也就好了。意思是如果能理解这一点，就已经达到了阴阳水火的根源，就能灵活变通，无所不能，这正是指出火的变态多种多样，它引起的疾病也不止一种，明白了这一点，其他的病证都可以辨别了。
但是，那些重视养阴的人认为，人的身体中，水只有一源，而火有两类（君火与相火），阳气常常有余，阴精常常不足，从少年到老年，所患的疾病，无不由于真阴不足。况且节制欲望的人少，沉溺欲望的人多，导致阴水更加亏损，阳火更加旺盛。无奈阴精难以滋生，峻补的方法没有朝夕见效的功效，所以补阴的药物，从少年到老年，不可一日间断。对于补阳的药物，劝诫非常恳切。虽然人的禀性不同，阳气盛的人确实应该补阴，阴气盛的人尤其需要补阳。况且阴随阳长，单纯滋阴，只会损伤胃气，反而断绝后天化生之源。要知道纯阴性质的药物，得到的是肃杀闭藏的气机，哪里会有阳和化育的功效呢！
况且天地以阳作为生命的根本，人生以火作为生命的门户。天开于子时，阳气由此而生，所以子是阳的根本，属于先天；人生于寅时，火兆由此显现，所以寅是火的母亲，属于后天。火，是生命的根本；阳，是火的作用。所以说：天如果没有这个人（指具备特定生命特质的人/或者指代阴阳互根的道理，此处语境略晦涩，通常解为天人相应，人需顺应天道），就不能化生万物；人如果没有这团火，就不能生存。天的阳气，能够交于下；地的阴气，能够交于上；人的真火，能够藏于下，那么真水就能布于上，这是阳施阴化的景象，可以彰显气血平和长久旺盛的状态。
大概阴阳的精微，互相藏在对方的宅邸里，阴中有阳，阳中有阴。所以心虽然是火，但含有赤色的液体（心血）；肾虽然是水，但藏着白色的气体（肾气/元阳）。赤色液体属阴，白色气体属阳，循环往复，昼夜不停，这是正常的规律。如果不知道调养，放纵情欲，亏损真阴，阳气就没有依附，因而发散上升，这就是“火空则发”的含义，这是周身的气都并聚于阳分。气并聚于阳分，那么阳就越盛，阴就越亏，因此出现上焦发热、咳嗽生痰、迫血妄行导致吐血衄血、头痛烦躁、胸前骨痛、口干舌苦、五心烦热、潮热骨蒸、小便短赤，这是这种证候的表现。
时间久了，孤阳不能独自旺盛，无根之火，怎么能长久燃烧？《内经》所说的“壮火食气”，气也变得衰弱了，阳也变得虚了。因此出现饮食不消化、泻泄无度、丹田不暖、筋骨无力、梦遗滑精、眩晕自汗、突然昏倒僵仆，这是另一种证候的表现。
然而，在少阴脏（心肾）中，重在真阳，阳气不回复，邪气就不能去除；在厥阴脏（肝）中，职责是藏血，不养血，脉象就不能起复。所以治疗时，阳气很虚的，要补阳以生阴，使阴随阳生长；阴精很虚的，要补阴以配阳，使阳随阴转化。阴阳调和，百病消解。如果偏执一端，或者固执于阴，或者固执于阳，那么不仅用药不能纠正偏颇，反而会增加偏颇带来的危害。
The *Yellow Emperor's Inner Canon* states: "When essence and qi are depleted, deficiency occurs." It also states: "Where pathogenic factors gather, the body's righteous qi must be deficient." Deficiency refers to emptiness and lack. This is analogous to a country that is internally empty and whose populace has scattered; in such a case, thieves and bandits will rise like bees, making suppression and pacification difficult. Unless wise and virtuous individuals are appointed to stabilize society, allow for rest and recuperation, and help the people recover, peace cannot be guaranteed. The deficiency of the body is exactly the same. If a physician is not wise and perceptive, who can stabilize the righteous qi, gather what has scattered, and restore the original state?
The *Inner Canon* says: "If one cannot treat deficiency, how can one ask about other conditions?" This implies that deficiency is the root of all diseases, so it should be listed first among various syndromes. However, what determines whether the body is sufficient or deficient is qi and blood; and what transforms into qi and blood is water (Kidney Yin) and fire (Heart Yang). Water and fire are the foundation of life and the basis for the activity of the spirit (*Shen*). The *Ling Shu* (Spiritual Pivot) states: "The essence of water manifests as Zhi (Will), and the essence of fire manifests as Shen (Spirit)." However, water and fire should be balanced rather than biased, and they should interact rather than separate. Fire has a nature of flaming upward, so it should be made to descend; water has a nature of flowing downward, so it should be made to ascend. When water is above and fire is below, this is called "interaction" (*Jiao*). Interaction leads to *Ji Ji* (Completion/Success), while non-interaction leads to *Wei Ji* (Incompletion/Failure). Interaction is the sign of life, and non-interaction is the sign of death. For example, in Xiao Ke syndrome (Thirst-Knowledge disease, characterized by excessive thirst and urination), the lack of interaction indicates an excess of fire; in Shui Qi syndrome (Water-Qi disorder, characterized by edema), the lack of interaction indicates an excess of water.
Therefore, fire belongs to Yang and Qi, standing in opposition to water. Water is Yin Essence, and fire is Yang Qi. When these two match properly, it is called "Harmony of Yin and Yang," also known as "Sha Huo" (Lesser Fire, referring to physiological warm fire). If vital energy is thus harmonious, various diseases will not arise, and longevity can be achieved. If one fails to nourish oneself properly, leading to Yin depletion and drying up of water, then fire will become excessive. This is what is meant by "when Yin is insufficient, Yang will encroach upon it," resulting in a state described here as "Yin Excess and Yang Deficiency" (contextually implying relative Yang hyperactivity due to absolute Yin deficiency, or pathological fire consuming Qi), also referred to as "Zhuang Huo Shi Qi" (Excessive Fire consumes Qi). From this, we know that fire is Qi, and Qi is fire. Thus, the *Immortal's Classic* states that medicine is fire, and fire is medicine; they are one yet two, and two yet one. Li Dongyuan also said: "Fire and Yuan Qi (Original Qi) cannot coexist." This refers to the same principle.
Consider water as an analogy: water is naturally cold. When flowing, it is cold; if it becomes extremely cold, it freezes into ice and stops flowing. Upon thawing, it returns to its normal state; it is not a different substance. Generally speaking, when in balance, water and fire achieve *Ji Ji*, and at this time, fire is the True Yang Qi. However, when it becomes biased, it turns into pathological fire, only then opposing Yuan Qi and creating a state of stagnation and obstruction. Therefore, Dai Ren (Zhang Congzheng) said: "Do not merely treat Wind, do not merely treat Dryness; when you treat Fire, Wind and Dryness will be resolved." This means that if one understands this, one has reached the origin of Yin-Yang and Water-Fire, allowing for flexible application in any situation. This specifically points out that Fire has many transformations and causes various diseases. Understanding this allows one to distinguish other conditions.
However, those who emphasize nourishing Yin argue that in the human body, there is only one source of Water, but two types of Fire (Ministerial Fire and Ministerial Fire/Yang), meaning Yang is often in excess and Yin is often insufficient. From youth to old age, diseases arise almost entirely from a deficiency of True Yin. Moreover, few people restrain their desires, while many indulge them, causing Yin Water to become more depleted and Yang Fire to become more vigorous. Unfortunately, Yin Essence is difficult to regenerate, and drastic tonification yields no immediate effect. Therefore, Yin-nourishing herbs should never be interrupted from youth to old age. Regarding Yang-tonifying herbs, the warnings are earnest. Although individual constitutions differ, those with Yang excess should indeed tonify Yin, and those with Yin excess particularly need to tonify Yang. Furthermore, Yin grows with Yang; solely nourishing Yin damages Stomach Qi and cuts off the source of postnatal transformation. Purely Yin-natured herbs possess the energy of desolation and closure, lacking the功 (merit) of Yang-like warmth and nurturing.
Moreover, Heaven takes Yang as the root of life, and humans take Fire as the gate of life. Heaven opens at Zi hour, where Yang is born; thus, Zi is the root of Yang and belongs to Pre-Heaven. Humans are born at Yin hour, where Fire appears; thus, Yin is the mother of Fire and belongs to Post-Heaven. Fire is the root of life; Yang is the function of Fire. Therefore, it is said: Without this [principle/person], Heaven cannot transform and generate all things; without this Fire, humans cannot have life. The Yang Qi of Heaven can interact downward; the Yin Qi of Earth can interact upward; the True Fire of humans can hide below, allowing True Water to spread above. This is the image of Yang imparting and Yin transforming, which can manifest the long-term prosperity of harmonious Qi and Blood.
Essentially, the essence of Yin and Yang hides within each other's dwellings; Yin contains Yang, and Yang contains Yin. Thus, although the Heart is Fire, it contains red fluid (Blood); although the Kidney is Water, it stores white Qi (Yuan Qi/True Yang). Red fluid is Yin, white Qi is Yang. They circulate continuously day and night; this is the normal measure. If one does not know how to nourish oneself and indulges in desires, depleting True Yin, Yang will have no attachment and will therefore flare up and ascend. This is the meaning of "Fire flares when empty." Consequently, Qi of the whole body converges on Yang. When Qi converges on Yang, Yang becomes even more excessive, and Yin becomes even more deficient. As a result, heat arises in the Upper Jiao, coughing produces phlegm, forced blood leads to hematemesis and epistaxis, headache and irritability occur, pain is felt in the chest bones, the mouth and tongue feel dry and bitter, the five palms feel hot, tidal fever and bone-steaming heat appear, and urine is short and red. These are the signs of this syndrome.
Over time, solitary Yang cannot flourish alone, and rootless fire cannot burn forever. As the *Canon* says, "Excessive Fire consumes Qi," Qi becomes weak, and Yang becomes deficient. Consequently, food is not digested, diarrhea is uncontrolled, the Dantian feels cold, muscles and bones feel weak, nocturnal emission and spermatorrhea occur, dizziness and spontaneous sweating happen, and sudden collapse may ensue. These are the signs of another syndrome.
However, in the Sha Yin organs (Heart and Kidney), the focus is on True Yang; if Yang does not return, pathogens cannot be expelled. In the Jue Yin organ (Liver), the function is storing Blood; if Blood is not nourished, the pulse cannot rise. Therefore, in treatment: if Yang is very deficient, tonify Yang to generate Yin, so that Yin grows with Yang; if Yin is very deficient, tonify Yin to accompany Yang, so that Yang transforms with Yin. When Yin and Yang are harmonized, hundred diseases dissolve. If one is biased, adhering strictly to either Yin or Yang, not only will the medication fail to correct the imbalance, but it will instead exacerbate the harm caused by the bias.