仲景深虑三阴经有伤寒传入之热病，有直中三阴之寒病，二条关系甚大。故作《伤寒论》注明
如是者乃传经之热病，如是者乃直中阴经之寒病，并立篇中，互相发明，相得益彰。奈后人
见同在篇中，不究何者为阴，何者为阳。不知伤寒热症，不独北方。南方亦有；中寒之症，
惟北方有之，南方甚少。故仲景以不发热，神气清，小便白，六脉迟，口中和，不消水，定
为三阴寒症，再无差误也。例如自利不渴者，属太阴，以其脏有寒故也，当温之，宜四逆汤
辈，少阴病下利脉微，与白通汤者；下利清谷、里寒外热、手足厥逆者，厥阴病手足厥寒、
脉细欲绝者，下利手足厥冷无脉者，俱用四逆汤。凡此皆三阴寒症也。
阴症者，不从阳经传入，寒邪直中三阴之症也。夫阴症而见阴象，人人知之；设阴症而反见
阳热之象，人则惑之矣。例如少阴病，下利清谷，里寒外热，手足厥逆，脉微欲绝，身反不
恶寒，其面赤色，或咽痛者；厥阴伤寒六七日，不下利便发热而利者；大汗出，热不去，内
拘急，四肢疼者；与夫阴极发燥，欲坐泥水井中者。凡此皆是阴症似阳之症。然究其实，则
大有分别者也。按下利清谷，手足厥逆，已是阴症；且得脉微欲绝，则阴症更有确据。此之
不恶寒，面赤色，乃是真阳外脱；此之咽痛，乃是虚阳上浮。又按三阳合病下利，初起即发
热而利。今不发热，不下利，即初起不发热之互词。直至六七日后发热下利，则发热乃真阳
外越，下利乃是真阴下竭矣。再按阳邪之症，若大汗出，热必去；若大汗出，不拘急，四肢
不疼。今大汗出，热反不去，反拘急，四肢反痛，则此之大汗出，非表邪之汗，乃是亡阳之
汗；此之热不去，非阳邪之热，乃是虚阳外脱之热；此之拘急身痛，非表症之候，乃是亡阳
身痛之候矣。
张仲景深深忧虑三阴经中既有由他经传入的热病，又有寒邪直接侵入三阴经的寒病，这两类病症关系重大。因此他在《伤寒论》中明确注明：像这种情况属于传经的热病，像那种情况属于直中阴经的寒病，并在篇中并列设立，互相阐发，相得益彰。无奈后人见到它们同列在篇中，不探究哪一个是阴证，哪一个是阳证。不知道伤寒热症，不仅北方有，南方也有；而中寒之症，只有北方有，南方很少。所以张仲景以不发热、神气清爽、小便色白、六脉迟缓、口中平和（无苦燥感）、不渴欲饮水，来定为三阴寒症，以此再无差误。例如自发性腹泻且不口渴的，属于太阴病，因为内脏有寒的缘故，应当用温法治疗，适宜使用四逆汤之类的方剂；少阴病出现下利且脉象微弱的，给予白通汤；出现下利完谷不化、内寒外热、手足厥冷的；厥阴病出现手足厥冷、脉细几乎摸不到的；以及下利、手足厥冷且无脉的，都使用四逆汤。凡此种种，都是三阴寒症。
阴症，是指不从阳经传入，而是寒邪直接侵犯三阴经的病症。凡是阴症出现阴证的征象，人人都知道；假如阴症反而出现阳热的征象，人们就会感到困惑了。例如少阴病，下利完谷不化，内寒外热，手足厥冷，脉微欲绝，身体反而不恶寒，面色发红，或者咽喉痛的；厥阴伤寒经过六七天，原本没有下利却突然发热并伴有下利的；大汗出，热势不退，体内拘急，四肢疼痛的；以及阴气极盛导致烦躁，想要坐在泥水井中的。凡此种种，都是阴症似阳的病症。然而究其实际，则有很大的区别。按上述下利完谷不化、手足厥冷，已经是阴症；再加上脉微欲绝，则阴症更有确凿依据。这种不恶寒、面色赤红，乃是真阳外脱；这种咽痛，乃是虚阳上浮。再按三阳合病导致的下利，初起即发热并伴有下利。现在不发热、不下利，这是指初起就不发热的另一种说法。直到六七日后才发热下利，那么此时的发热乃是真阳外越，下利乃是真阴下竭。再按阳邪引起的病症，如果大汗出，热势必退；如果大汗出，身体不会拘急，四肢也不会疼痛。现在大汗出，热势反而不退，反而出现拘急，四肢反而疼痛，那么这种大汗出，不是表邪的汗，而是亡阳的汗；这种热不退，不是阳邪的热，而是虚阳外脱的热；这种拘急身痛，不是表症的候象，而是亡阳身痛的候象了。
Zhang Zhongjing deeply worried that the Three Yin meridians (San Yin Jing, referring to the Taiyin, Shaoyin, and Jueyin meridians) would contain both heat diseases transmitted from other meridians and cold diseases directly invading the Three Yin meridians. The relationship between these two categories is significant. Therefore, in *Shanghan Lun* (Treatise on Cold Damage), he explicitly noted: such cases are heat diseases transmitted via meridians; such cases are cold diseases directly invading the Yin meridians. He established them side by side in the chapters to mutually elucidate each other, thereby enhancing their value. However, later generations, seeing them listed together in the same chapter, failed to investigate which was Yin and which was Yang. They did not realize that cold-damage heat syndromes exist not only in the North but also in the South; whereas cold-mid syndrome (Zhong Han, direct invasion of cold pathogen) exists primarily in the North and is rare in the South. Thus, Zhang Zhongjing defined the Three Yin cold syndromes based on the absence of fever, clear mental state, white urine, slow pulse in all six positions, a neutral sensation in the mouth, and no desire for water, ensuring no errors. For example, spontaneous diarrhea without thirst belongs to Taiyin disease due to internal cold; it should be treated with warming methods, suitable for formulas like Sini Tang (Four Reversal Decoction). In Shaoyin disease with diarrhea and a faint pulse, Baotong Tang (White Connecting Decoction) is administered. For cases involving diarrhea with undigested food, interior cold with exterior heat, and cold extremities; or Jueyin disease with cold extremities and a thin pulse nearly imperceptible; or diarrhea with cold extremities and absent pulse, Sini Tang is used universally. All these are Three Yin cold syndromes.
Yin syndrome refers to conditions where cold pathogens directly invade the Three Yin meridians without transmission from the Yang meridians. Everyone knows that Yin syndromes present with Yin manifestations; however, if a Yin syndrome paradoxically presents with Yang-heat manifestations, people become confused. For instance, in Shaoyin disease, there may be diarrhea with undigested food, interior cold with exterior heat, cold extremities, and a faint pulse nearly imperceptible, yet the body does not feel aversion to cold, the face appears red, or there is sore throat. In Jueyin cold damage, after six or seven days, one might develop fever accompanied by diarrhea despite previously having none. There may be profuse sweating with persistent heat, internal tension, and limb pain. Or, extreme Yin leading to restlessness, with a desire to sit in muddy wells. All these are examples of Yin syndrome resembling Yang. However, upon closer examination, there are significant differences. Diarrhea with undigested food and cold extremities already indicate a Yin syndrome; coupled with a faint, nearly imperceptible pulse, this diagnosis is confirmed. The lack of aversion to cold and red face here indicate the external collapse of True Yang (Zhen Yang); the sore throat indicates floating of Deficient Yang (Xu Yang). Regarding diarrhea caused by combined disease of the Three Yang meridians, fever and diarrhea typically begin simultaneously. Here, "no fever and no diarrhea" refers to the initial absence of fever. If fever and diarrhea appear only after six or seven days, the fever indicates the external wandering of True Yang, and the diarrhea indicates the exhaustion of True Yin (Zhen Yin). Furthermore, in symptoms caused by Yang pathogens, profuse sweating should lead to the resolution of heat; if sweating occurs, there should be no tension or limb pain. Here, profuse sweating with persistent heat, tension, and limb pain indicates that this sweating is not from exterior pathogens but from Yang collapse (Wang Yang); the persistent heat is not from Yang pathogens but from the external collapse of Deficient Yang; the tension and body pain are not signs of exterior syndromes but indicators of body pain due to Yang collapse.