李杲曰。四逆者。四肢不温也。厥者。手足逆冷也。伤寒邪在三阳。则手足必热。传到太阴。
则手足不热而温。至少阴。则邪热之入里渐深。故四肢逆而不温。及至厥阴。则又手足厥冷。更甚
于逆矣。其四逆散。以凉药而治四肢不温。其四逆汤。以热药而治寒极而成逆厥者也。四肢通冷。
比之手足独冷。则有间。夫死者。以四逆言之。可治者。以厥冷言之。则亦可见四逆与手足厥冷之
有轻重矣。盖四肢通冷。其病为重。手足独冷。其病为轻也。四肢与手足。却有所分。以
四字加于逆字之上。是通指手足臂胫以上言也。以手足二字加于厥逆厥冷之上。是独指手足言也。盖
以四逆为四肢通冷。而厥为手足独冷也。
陶华曰。阴阳二厥。治之一瘥。生死立判。夫阳厥者。先自三阳经气分。因感寒邪。于头疼发
热恶寒以后。传进三阴血分。变出四肢厥冷乍温。大便燥实。谵语发渴。扬手掷足。不恶寒反怕热。
脉沉有力。此见传经热症。谓之阳厥。阳极发厥者。即阳症似阴。外虽有厥冷。内有热邪耳。盖因
大便结实失下。使血气不通。故手足乍冷乍温也。如火炼金。热极金反化水。水寒极而成冰。反能
载物。厥微热亦微。四逆散。厥深热亦深。大承气汤。正谓亢则害其物。承乃制其极也。若误为阴
症。便进热药。如抱薪救火矣。夫阴厥者。因三阴经血分自受寒邪。初病。无身热。无头疼。就使
恶寒。四肢厥冷。直至臂胫以上。过乎肘膝。不温。引衣蜷卧。不渴。兼或腹满吐泻。或战栗。面
如刀刮。口吐涎沫。脉沉迟无力。此为阴经直中真阴寒症。不从阳经传入。谓之阴厥也。轻则理中
汤。重则四逆汤温之。勿令误也。又曰。人之手足。乃胃土之末。凡脾胃有热。手足必热。脾胃有
寒。手足必冷。理之常也。惟伤寒乃有厥深热亦深。厥微热亦微之论。何耶。曰。此为极则变。火
气亢极。反兼水化。故有此象耳。阴阳反复。病之逆从。未可以常理论也。凡言厥逆厥寒厥冷手足
寒冷。皆变文耳。不必分轻重。若言四肢。则有异也。亦未可纯为寒症。若厥冷直至臂胫以上。则为
真寒无疑。急用姜附等温之。少缓便难治。谓其冷上过乎肘。下过乎膝。非内有真寒。达于四肢而
何。然更当以脉与症参之。庶乎无误。凡看伤寒。不可以厥逆。便断为寒。必参脉与兼症。方知端的。
如手足厥冷。兼之腹痛腹满。泄利清白。小便亦清。口不渴。恶寒战栗。面如刀刮。皆寒症也。
若腹痛后重。泄利稠粘。小便赤涩。渴而好饮。皆热症也。宜详审之。
李杲说：所谓“四逆”，是指四肢不温暖；所谓“厥”，是指手足逆向寒冷。伤寒邪气在太阳、阳明、少阳三阳经时，手足必然发热；邪气传入太阴经时，手足就不热而表现为温和；传到少阴经时，邪热入里逐渐加深，所以四肢逆向寒冷且不温暖；到了厥阴经时，又出现手足厥冷，比“逆”更严重一些。四逆散，是用凉性药物来治疗四肢不温的；四逆汤，是用热性药物来治疗寒极而导致的逆厥的。全身四肢都冷，与仅手足单独发冷相比，是有区别的。对于死者，用“四逆”来形容；对于可治者，用“厥冷”来形容，由此也可以看出“四逆”与“手足厥冷”在病情轻重上的差异。大概来说，四肢全部发冷，病情较重；仅手足单独发冷，病情较轻。四肢与手足的概念有所区分，“四逆”中的“四”字加在“逆”字之上，是泛指包括手、足、臂、胫在内的整个肢体而言；“手足”二字加在“厥逆”或“厥冷”之上，是专指手和足而言。大概将“四逆”理解为四肢通冷，而将“厥”理解为手足独冷。
陶华说：阴阳两种厥证，治疗上稍有差错，生死立刻就能判定。所谓的阳厥，起初是三阳经的气分感受寒邪，在头痛、发热、恶寒之后，邪气传入三阴经的血分，变生出四肢厥冷但偶尔转温、大便干燥结实、胡言乱语、口渴、挥手踢脚、不怕冷反而怕热、脉象沉而有力的症状。这是看到传经的热证，称之为阳厥。阳极导致发厥，即阳证似阴证，外表虽然有厥冷，体内却有热邪。这是因为大便结实未及时攻下，导致气血不通，所以手足有时冷有时温。这就像火炼金属，热极则金反化为水；水寒极则结成冰，反而能承载物体。厥冷轻微，热邪也轻微，适用四逆散；厥冷深重，热邪也深重，适用大承气汤。这正是说亢盛则为害，顺承才能制约其极端。如果误认为是阴证，就进服热药，如同抱着柴草去救火。所谓的阴厥，是因为三阴经的血分直接感受寒邪，初起时没有身热，没有头痛，即使有恶寒，也是四肢厥冷，一直延伸到手臂和小腿以上，超过肘部和膝盖，不温暖，喜欢蜷缩着身体睡觉，口不渴，或者伴有腹部胀满、呕吐腹泻，或者战栗，面色像刀刮过一样苍白，口吐涎沫，脉象沉迟无力。这是阴经直中真阴寒证，不是从阳经传入的，称之为阴厥。轻症用理中汤，重症用四逆汤温补，不要弄错了。又说：人的手足，是胃土之末梢。凡是脾胃有热，手足必然发热；脾胃有寒，手足必然发冷，这是常理。只有伤寒才有“厥深热亦深，厥微热亦微”的论述，为什么呢？回答说：这是物极必反的变化，火气亢盛到极点，反而兼有水化的现象，所以出现这种表象。阴阳反复，病势的逆顺，不能用常理来衡量。凡是说厥逆、厥寒、厥冷、手足寒冷，都是变化的文辞罢了，不必区分轻重。如果说四肢，则有不同之处，也不能单纯认为是寒证。如果厥冷一直延伸到手臂和小腿以上，那就是真寒无疑，急需使用干姜、附子等温热药物治疗，稍微迟缓就很难治疗了。说它的冷超过了肘部，低于膝盖，如果不是内有真寒，达于四肢，还能是什么呢？然而更应当结合脉象和症状来参考，这样才能没有错误。凡是诊治伤寒，不能因为看到厥逆，就断定为寒证，必须参合脉象和伴随症状，才知道确切情况。比如手足厥冷，同时伴有腹痛、腹部胀满、排泄清稀白色粪便、小便也清长、口不渴、恶寒战栗、面色像刀刮过一样，这些都是寒证。如果腹痛且有里急后重感，排泄稠厚粘滞的粪便，小便赤涩，口渴喜欢饮水，这些都是热证。应当详细审察。
Li Gao stated: "Si Ni" (Four Reversals) refers to the limbs being not warm; "Jue" (Reversal/Cold Extremities) refers to the hands and feet being reversely cold. When pathogenic factors of Shanghan (Cold Damage) are in the Taiyang, Yangming, and Shaoyang (Three Yang) meridians, the hands and feet must be hot. When transmitted to the Taiyin meridian, the hands and feet become not hot but warm. When reaching the Shaoyin meridian, the pathogenic heat enters deeper into the interior, thus causing the four limbs to be reversely cold and not warm. Upon reaching the Jueyin meridian, there is again cold extremities of the hands and feet, which is even more severe than "Ni". Si Ni San (Four Reversals Powder) uses cooling herbs to treat the non-warmness of the four limbs. Si Ni Tang (Four Reversals Decoction) uses warming herbs to treat the reversal cold extremities caused by extreme cold. Generalized coldness of the four limbs differs from isolated coldness of the hands and feet. For deceased patients, "Si Ni" is used to describe the condition; for those who can be treated, "Jue Leng" (Cold Extremities) is used. This shows the difference in severity between "Si Ni" and "Shou Zu Jue Leng" (Cold Hands and Feet). Generally, generalized coldness of the four limbs indicates a heavier disease, while isolated coldness of the hands and feet indicates a lighter disease. There is a distinction between "Si Zhi" (four limbs) and "Shou Zu" (hands and feet). Adding the character "Si" (four) before "Ni" (reversal) generally refers to the entire limbs including arms, forearms, legs, and calves. Adding "Shou Zu" (hands and feet) before "Jue Ni" or "Jue Leng" specifically refers only to the hands and feet. Thus, "Si Ni" is understood as generalized coldness of the four limbs, while "Jue" is understood as isolated coldness of the hands and feet.
Tao Hua stated: The treatment of Yin Jue (Yin Reversal) and Yang Jue (Yang Reversal), if slightly mistaken, leads to immediate life or death outcomes. Yang Jue originates from the Qi level of the Three Yang meridians due to cold pathogen invasion. After symptoms such as headache, fever, and aversion to cold, it transmits to the Blood level of the Three Yin meridians, manifesting as cold extremities that occasionally become warm, dry and solid stool, delirium, thirst, waving hands and kicking feet, aversion to heat rather than cold, and a deep and forceful pulse. This presents as a heat syndrome transmitted through meridians, termed Yang Jue. Extreme Yang leading to Jue is essentially a Yang syndrome resembling Yin; although there is external coldness, there is internal heat pathogen. This occurs because dry stool was not purged in time, causing stagnation of Qi and Blood, resulting in alternating cold and warmth of the hands and feet. It is like fire refining metal; when heat is extreme, metal transforms into water. When water is extremely cold, it becomes ice, yet can still bear weight. Mild Jue implies mild heat, treated with Si Ni San (Four Reversals Powder); deep Jue implies deep heat, treated with Da Cheng Qi Tang (Major Order the Qi Decoction). This means that when亢盛 (hyperactivity/excess) causes harm, one must follow the principle to control its extremity. If mistakenly treated as a Yin syndrome with warming herbs, it is like adding firewood to save a fire. Yin Jue occurs when the Blood level of the Three Yin meridians directly suffers from cold pathogens. Initially, there is no body heat or headache. Even if there is aversion to cold, the limbs are cold, extending up to the arms and calves above the elbows and knees, remaining unwarmed. The patient prefers curling up, has no thirst, and may have abdominal fullness, vomiting, diarrhea, shivering, a face pale as if scraped by a knife, spitting saliva, and a deep, slow, and forceless pulse. This is a true Yin-cold syndrome directly attacking the Yin meridians, not transmitted from the Yang meridians, termed Yin Jue. Mild cases are treated with Li Zhong Tang (Regulate the Middle Decoction), severe cases with Si Ni Tang (Four Reversals Decoction) to warm them; do not make mistakes. Furthermore, the hands and feet are the extremities of the Stomach Earth. Generally, if the Spleen and Stomach have heat, the hands and feet are hot; if they have cold, the hands and feet are cold. This is the normal rule. Only in Shanghan is there the theory that "deep Jue implies deep heat, mild Jue implies mild heat." Why? This is a transformation at the extreme limit; when Fire Qi is excessively hyperactive, it assumes the characteristics of Water, hence this appearance. Yin and Yang reverse each other; the deviation and compliance of diseases cannot be judged by normal rules. Terms like Jue Ni, Jue Han, Jue Leng, and Shou Zu Leng are merely rhetorical variations and need not distinguish severity. However, mentioning "Si Zhi" (four limbs) is different and cannot be purely considered a cold syndrome. If coldness extends up to the arms and calves, it is undoubtedly true cold, requiring urgent use of warming herbs like Ginger (Sheng Jiang / Gan Jiang) and Aconite (Fu Zi). Delay makes it difficult to treat. If the cold exceeds the elbows and goes below the knees, unless there is true cold inside reaching the four limbs, what else could it be? However, one must further correlate the pulse and symptoms to avoid errors. In diagnosing Shanghan, one cannot conclude it is a cold syndrome solely based on Jue Ni; one must combine the pulse and accompanying symptoms to know the truth. For example, cold extremities accompanied by abdominal pain and fullness, clear white diarrhea, clear urine, no thirst, aversion to cold, shivering, and a face pale as if scraped by a knife are all cold syndromes. If abdominal pain is accompanied by tenesmus, thick and sticky diarrhea, red and scanty urine, thirst with desire to drink, these are heat syndromes. One should examine carefully.