《伤寒》一书，所谓阴阳虚盛，则精微之义，不无辩析于其间。四十八难曰∶病之虚实，出者
为虚，入者为实。盖表之真阳既虚，故阴邪以盛，出而乘阳，是以脉浮于外，其病在表，法当汗之。
当其阴邪出表，脉浮于外之时，不可自惑以为阳
脉盛也。里之真阴既虚，故阳邪以盛，入而乘阴，是以脉入于内，其病在里，法当下之。当其阳邪
入里，脉实于内之时，不可自惑以为阴脉盛也。是说非古人之立言也，盖使人知如此之为阴盛，则
抑阴而助阳；如彼之为阳盛，则抑阳而助阴。阴盛，则邪出于外者，发表之药当性温，以助阳气，
如桂枝汤之类是也。阳盛，则邪入于内者，攻里之药当性寒，以抑阳气，如承气汤之类是也。或曰∶
阴出而乘于外，是阳之不足也。阳病，则当有以发表而汗之，何哉？是大不然，阴邪传于外，不汗
之，则邪何由而去？桂枝之性温，温之，乃所以助阳，阳有所助而长，则阴邪之所由以消，辛甘发
散为阳者，此也。张氏所谓承气入胃，阴盛乃亡者，正恐阴盛出外，而误以承气下之，安得而不亡。
或者有阳入而乘于内，是阴之不足也。阴病，则当有以温养而下之，何哉？是又不然，阳邪入于内，
不下之，则邪从何而出？承气之性寒，寒之，乃所以抑阳，阳受其抑则微，而真阴之所由以长，酸
苦涌泄为阴者，此也。张氏所谓桂枝下咽，阳盛则毙，正恐阳盛入内，而误以桂枝汗之，又
安得而不死。观古人发表之药多温，攻里之药多寒，则知阴阳虚实之意微，非只于汗下设矣，所以
为用药寒温设也。
《伤寒论》这部书，其中关于阴阳虚盛的论述，其精微深奥的含义，并非没有辨析之处。《难经·四十八难》说：病证的虚实，邪气外出表现为虚证，邪气内入表现为实证。大概是因为体表的真阳之气已经虚弱，所以阴寒邪气得以亢盛，向外侵犯阳气，因此脉象浮于体表，病位在表，治疗法则应当采用发汗法。当阴寒邪气外达于表、脉象浮于体表之时，不可自我迷惑而认为是阳脉亢盛。
体内的真阴之气已经虚弱，所以阳热邪气得以亢盛，向内侵犯阴气，因此脉象沉入于内，病位在里，治疗法则应当采用攻下法。当阳热邪气内陷入里、脉象实于体内之时，不可自我迷惑而认为是阴脉亢盛。这种说法并非古人随意立论，而是旨在让人明白：如果是阴寒邪气亢盛，就应当抑制阴寒而扶助阳气；如果是阳热邪气亢盛，就应当抑制阳热而扶助阴液。
若是阴寒邪气亢盛，导致邪气向外发散，用于解表发汗的药物性质应当偏温，以助长阳气，例如桂枝汤之类的方剂就是如此。若是阳热邪气亢盛，导致邪气向内结聚，用于攻逐里实的药物性质应当偏寒，以抑制阳热，例如承气汤之类的方剂就是如此。
有人问道：阴寒邪气外出并乘袭体表，这是阳气不足的表现。既然是阳病，就应当有通过发汗来解除表邪的方法，为什么还要强调这一点呢？这完全不对。阴寒邪气传变于体表，如果不通过发汗，邪气靠什么途径去除？桂枝汤药性温和，用温性的药物，正是为了扶助阳气，阳气得到扶助而增长，那么阴寒邪气自然随之消减，辛味和甘味的药物具有发散作用且归属于阳，说的就是这个道理。张仲景所说的“承气汤进入胃中，阴寒邪气才会灭亡”，正是担心阴寒邪气外越时，误用承气汤攻下，从而导致病情恶化甚至死亡。
或者有人问：阳热邪气内入并乘袭体内，这是阴液不足的表现。既然是阴病，就应当有通过温养并攻下的方法，为什么还要强调这一点呢？这又是不对的。阳热邪气内陷入里，如果不通过攻下，邪气从哪里排出？承气汤药性寒凉，用寒性的药物，正是为了抑制阳热，阳热受到抑制就会减弱，那么真阴之气自然得以生长，酸味和苦味的药物具有涌吐泄下的作用且归属于阴，说的就是这个道理。张仲景所说的“桂枝汤咽下，阳热亢盛者就会毙命”，正是担心阳热邪气内盛时，误用桂枝汤发汗，又怎么能不导致死亡呢？
观察古人用于解表的药物大多性质温热，用于攻里的药物大多性质寒凉，就可以知道阴阳虚实含义的精微之处，不仅仅在于是否使用汗法这一层面，而是专门为了指导用药的寒温属性而设立的。
The *Shanghan Lun* (Treatise on Cold Damage) contains discussions on the deficiency and excess of Yin and Yang, where the subtle meanings are not without analytical clarification. The *Nan Jing* (Classic of Difficult Issues), Chapter 48, states: "In terms of deficiency and excess in diseases, outward movement indicates deficiency, while inward movement indicates excess." Generally, when the True Yang of the exterior is deficient, Yin pathogens become excessive, moving outward to attack Yang. Consequently, the pulse floats on the exterior, the disease location is in the exterior, and the treatment principle should be diaphoresis (sweating therapy). When Yin pathogens move to the exterior and the pulse floats, one must not be confused into thinking this indicates an Excess of Yang pulse.
When the True Yin within is deficient, Yang pathogens become excessive, moving inward to attack Yin. Consequently, the pulse sinks inward, the disease location is in the interior, and the treatment principle should be purgation. When Yang pathogens move into the interior and the pulse is solid inside, one must not be confused into thinking this indicates an Excess of Yin pulse. This statement is not merely arbitrary theory established by ancients; rather, it aims to teach that if there is Excess of Yin, one should suppress Yin and assist Yang; if there is Excess of Yang, one should suppress Yang and assist Yin.
If Yin is excessive, causing pathogens to manifest outwardly, external-releasing medicines should have a warm nature to assist Yang Qi, such as Guizhi Tang (Cinnamon Twig Decoction). If Yang is excessive, causing pathogens to accumulate inwardly, interior-purging medicines should have a cold nature to suppress Yang Qi, such as Chengqi Tang (Gut-Purging Decoction).
One might ask: "Yin pathogens moving outward to attack the exterior indicates a deficiency of Yang. Since this is a Yang disease, should it not be treated with diaphoresis? Why emphasize this?" This is entirely incorrect. If Yin pathogens transmit to the exterior, how can they leave without diaphoresis? The nature of Guizhi Tang is warm; using warmth is precisely to assist Yang. When Yang is assisted and grows, the Yin pathogens are thereby eliminated. Herbs with pungent and sweet flavors that disperse belong to Yang; this is what is meant here. Zhang Zhongjing's statement that "Chengqi Tang entering the stomach causes Yin excess to perish" is precisely to warn against mistakenly using Chengqi Tang for purgation when Yin excess manifests outwardly, which would inevitably lead to demise.
Alternatively, one might ask: "Yang pathogens moving inward to attack the interior indicates a deficiency of Yin. Since this is a Yin disease, should it not be treated with warming nourishment and purgation? Why emphasize this?" This is also incorrect. If Yang pathogens enter the interior, how can they exit without purgation? The nature of Chengqi Tang is cold; using coldness is precisely to suppress Yang. When Yang is suppressed, it becomes mild, allowing True Yin to grow. Herbs with sour and bitter flavors that induce vomiting or purging belong to Yin; this is what is meant here. Zhang Zhongjing's statement that "Guizhi Tang swallowed causes death when Yang is excessive" is precisely to warn against mistakenly using Guizhi Tang for diaphoresis when Yang excess moves inwardly, which would inevitably lead to death.
Observing that ancient external-releasing medicines were mostly warm and interior-purging medicines were mostly cold, one can understand that the subtle meaning of Yin-Yang deficiency and excess lies not merely in the application of sweating methods, but is specifically established to guide the choice between warm and cold medicinal properties.