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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.