凡伤寒治法，必当先知经络次序，如一日在太阳，则为发寒、头痛等证；二日在阳明，则为目
痛、鼻
干、不眠等证；三日在少阳，则为耳聋、胁痛、寒热、口苦等证；四日在太阴，则为腹满自利等证
；五日在少阴，则为舌干口燥等证；六日在厥阴，则为烦满囊缩等证，此伤寒传经之大概也。然病
有不同，证有多变，故不可以一定之法，凿凿为拘。今人有不知察变者，每按日按经，执方求治，
则证多不合，益见其难矣。即如发热，无汗，头痛者，宜于发汗，本太阳经之证治也，然仲景曰∶
阳明病，外证云何？曰∶身热，汗自出，不恶寒，反恶热，此阳明之发热也；曰∶阳明病，反无汗
，而小便利，呕而咳，手足厥者，必苦头痛，此阳明之无汗头痛也；曰∶伤寒，脉弦细，头痛发热
者，属少阳，此少阳之头痛发热也。凡三阳皆为表证，而惟少阳则曰半表半里，不可发汗。然法曰
∶尺寸俱浮者，太阳受病也；尺寸俱长者，阳明受病也；尺寸俱弦者，少阳受病也，此三经皆受病
，未入于府者，可汗而已，岂非少阳亦所当汗乎？此三阳之治，宜乎若此。至于三阴，则亦有若此
者，如曰∶太阴病，脉浮者，可发汗，宜桂枝汤；曰∶少阴病，始得之，反发热，脉沉者，宜麻黄
附子细辛汤；曰∶厥阴证，下痢，腹胀满，身体疼痛者，先温其里，乃攻其表，温里四逆汤，攻表
桂枝汤，凡此皆三阴之发热，三阴之当汗者也。至于下证，则惟独少阳为半表半里之经，若不之，
恐邪气乘虚内陷，故不可攻，其他五经，皆有下证。由此观之，则三阳何尝无里证，三阴何尝无表
证。故善治者，但见表邪未解，即当解表，若表证未解，不可攻里也；但见里证已具，即当攻里，
若里证未实，尚宜和解也。或汗、或和、或下，但当随证缓急，而用得其宜，即古今画一之法也。
凡是治疗伤寒的方法，必须首先了解经络传变的次序。例如：第一天病在太阳经，就会出现恶寒、头痛等症状；第二天病在阳明经，就会出现目痛、鼻干、失眠等症状；第三天病在少阳经，就会出现耳聋、胁痛、寒热往来、口苦等症状；第四天病在太阴经，就会出现腹部胀满、自发性腹泻等症状；第五天病在少阴经，就会出现舌干、口燥等症状；第六天病在厥阴经，就会出现心烦腹满、阴囊收缩等症状，这是伤寒传变的大致规律。然而病情各不相同，症状变化多端，因此不能拘泥于固定的法则，死板地套用。现在有些人不懂得观察病情变化，往往机械地按照天数和经络来执方求治，结果症状与方药大多不相符合，更加显得困难重重。
就拿发热、无汗、头痛来说，适宜用发汗法，这本来是太阳经的证治原则。但是张仲景说：阳明病的表证表现是什么？回答是：身体发热，自然出汗，不恶寒，反而怕热，这是阳明病的发热；又说：阳明病，反而没有汗，小便通畅，呕吐且咳嗽，手脚冰凉的人，一定痛苦于头痛，这是阳明病的无汗头痛；又说：伤寒病，脉象弦细，头痛发热的，属于少阳病，这是少阳病的头痛发热。大凡三阳经（太阳、阳明、少阳）都属于表证，但只有少阳经称为“半表半里”，不可使用发汗法。然而医法又说：寸关尺三部脉象都浮的，是太阳经受邪；脉象都长的，是阳明经受邪；脉象都弦的，是少阳经受邪，这三条经脉都受邪气侵袭，如果病邪尚未传入腑脏，可以通过发汗治愈，这难道不是说明少阳病也应当发汗吗？这就是三阳经治疗的道理。
至于三阴经（太阴、少阴、厥阴），也有类似的情况。例如说：太阴病，脉象浮的，可以发汗，适宜用桂枝汤；又说：少阴病，刚得病时，反而发热，脉象沉的，适宜用麻黄附子细辛汤；又说：厥阴证，出现下痢、腹部胀满、身体疼痛的，应当先温补里虚，然后再解表邪，温里用四逆汤，解表用桂枝汤。凡此种种，都是三阴经的发热，以及三阴经应当使用汗法的例子。
至于攻下法，唯独少阳经因为是半表半里的经络，如果不谨慎对待，恐怕邪气乘虚内陷，所以不可使用攻下法，其他五条经脉，都有可以使用攻下法的证候。由此看来，三阳经何尝没有里证，三阴经何尝没有表证。所以善于治病的人，只要见到表邪未解，就应当解表，如果表证未解，不可攻里；只要见到里证已经具备，就应当攻里，如果里证尚未结实，还应当采用和解法。或者用汗法、或者用和法、或者用下法，只应当根据病情的缓急，选用适宜的方法，这就是古今统一的治疗法则。
In general, the treatment of Shanghan (Cold Damage) requires prior knowledge of the sequence of meridian transmission. For instance, on the first day, when the pathogen is in the Taiyang Meridian (Taiyang Channel Disease), symptoms such as aversion to cold and headache manifest; on the second day, when it enters the Yangming Meridian (Yangming Channel Disease), symptoms include eye pain, nasal dryness, and insomnia; on the third day, in the Shaoyang Meridian (Shaoyang Channel Disease), symptoms comprise deafness, hypochondriac pain, alternating chills and fever, and bitter taste in the mouth; on the fourth day, in the Taiyin Meridian (Taiyin Channel Disease), symptoms involve abdominal fullness and spontaneous diarrhea; on the fifth day, in the Shaoyin Meridian (Shaoyin Channel Disease), symptoms present as dry tongue and parched mouth; on the sixth day, in the Jueyin Meridian (Jueyin Channel Disease), symptoms include vexation, abdominal fullness, and retraction of the testicles. This constitutes the general pattern of meridian transmission in Shanghan. However, diseases vary, and symptoms are multifarious; therefore, one cannot rigidly adhere to fixed rules or apply them mechanically.
Currently, some practitioners fail to observe pathological changes, mechanically applying prescriptions based solely on days and meridians. Consequently, the symptoms often do not match the treatment, highlighting the difficulty. Take fever, absence of sweating, and headache as an example: these are typically treated with diaphoresis (sweating method), which aligns with the treatment principle for the Taiyang Meridian. Yet, Zhang Zhongjing stated: "What are the external manifestations of Yangming disease?" The answer is: body heat, spontaneous sweating, aversion to neither cold nor preference for heat; this represents fever in Yangming disease. He further stated: "In Yangming disease, if there is no sweating, but urination is clear and profuse, accompanied by vomiting and coughing, and cold extremities, the patient will suffer from severe headache"; this represents headache without sweating in Yangming disease. He also stated: "In Shanghan, if the pulse is wiry and fine, accompanied by headache and fever, it belongs to Shaoyang"; this represents headache and fever in Shaoyang disease.
Generally, the three Yang meridians (Taiyang, Yangming, Shaoyang) are considered exterior syndromes, but only Shaoyang is described as "half-exterior, half-interior," where diaphoresis is contraindicated. However, therapeutic principles also state: "If the pulse at the cun, guan, and chi positions are all floating, the disease is in Taiyang; if all are long, it is in Yangming; if all are wiry, it is in Shaoyang." If these three meridians are affected, and the pathogen has not yet entered the fu organs, it can be resolved through diaphoresis. Does this not imply that Shaoyang disease may also require diaphoresis? Such is the logic regarding the treatment of the three Yang meridians.
Similarly, the three Yin meridians (Taiyin, Shaoyin, Jueyin) have analogous cases. For instance: "In Taiyin disease, if the pulse is floating, diaphoresis may be used, preferably Guizhi Tang (Cinnamon Twig Decoction)." Also: "In Shaoyin disease, upon initial onset, if there is contrary fever and a deep pulse, Mahuang Fuzi Xixin Tang (Ephedra, Aconite, and Asarum Decoction) is indicated." Furthermore: "In Jueyin syndrome, if there is dysentery, abdominal fullness, and body pain, one should first warm the interior, then attack the exterior; use Sini Tang (Frigid Extremities Decoction) to warm the interior, and Guizhi Tang (Cinnamon Twig Decoction) to resolve the exterior." These examples illustrate fever in the three Yin meridians and instances where sweating methods are appropriate for the three Yin meridians.
Regarding purgation (downward draining), it is uniquely contraindicated for Shaoyang because it is a half-exterior, half-interior channel; failing to recognize this risks the pathogen sinking inward due to deficiency. Thus, purgation is forbidden for Shaoyang, whereas the other five meridians may have indications for purgation. From this perspective, do the three Yang meridians never present interior syndromes? Do the three Yin meridians never present exterior syndromes? Therefore, a skilled practitioner, seeing that exterior pathogens remain unresolved, must resolve the exterior; if the exterior syndrome is unresolved, one must not purge the interior. Conversely, if interior syndromes are fully established, one must purge the interior; if the interior condition is not yet solid, harmonization (hejie) is appropriate. Whether using diaphoresis, harmonization, or purgation, one must adapt to the urgency of the symptoms and apply the suitable method. This constitutes the unified therapeutic principle of ancient and modern times.