病有定体。故立六经而分司之。病有变迁。更求合病并病而互参之。此仲景立法之尽善也。夫阴阳互根。气虽
分而神自合。三阳之底。便是三阴。三阴之表。即是三阳矣。如太阳病。而脉反沉。便合少阴。少阴病而反发热。
便合太阳。阳明脉迟即合太阴。太阴脉缓即合阳明。少阳脉细小。是合厥阴。厥阴微浮是合少阳。虽无合并之名。
而有合并之实。或阳得阴而解。阴得阳而解。或阳入阴而危。阴亡阳而逆。种种脉症。不可枚举。学人当于阴阳两
症中。察病势合不合。更于三阳三阴中。审其症之并与不并。于此阴病治阳。阳病治阴。扶阳抑阴。泻阳补阴等法。
用之恰当矣。三阳皆有发热症三阴皆有下利症。如发热而下利。是阴阳合病也。阴阳合病。阳盛者属阳经。则下利
为实热。如太阳阳明合病。少阳阳明合病。太阳少阳合病。必自下利。用葛根黄芩等汤者是也。阴盛者属阴经。则
下利属虚寒。如少阴病吐利。反发热者不死。少阴病。下利清谷。里寒外热。不恶寒。而面色赤。用通脉四逆者是
也。若阳与汤合。不合于阴。即是三阳合病。则不下利。而自汗出。为白虎症也。阴与阴合。不合于阳。即是三阴
合病。则不发热。而吐利厥逆。为四逆症也。并病与合病稍异者。合则一时并见。并则以次相乘。如太阳之头项强
痛未罢。递见脉弦眩冒。心下痞硬等症。是与少阳并病。更见 语。即是三阳并病矣。太阳与阳明并病。太阳症未
罢者。从太阳而小发汗。太阳症已罢者。从阳明而下之。其机在恶寒恶热而分也。然阳明之病在胃家实。太阳阳明
合病。喘而胸满者不可下。恐胃家未实耳。如阳明与太少合病。必自下利。何以得称阳明。要知夹热下利。即胃实
之始。内经所云暴注下迫。皆属乎热。其脉必浮大弦大。故得属之阳明。而不经于太阴也。若下利清谷。里寒外热。
脉浮而迟者。则浮不得属之于表。而迟则为在脏。若见脉微欲绝。即身不恶寒。而面色赤者。又当属之少阴。盖太阳
阳明下利之辨。在清谷不清谷。而太阴少阴之清谷。又在脉之迟与微为辨也。夫阳明主胃实。而有夹热利。太阴主
下利清谷。又因脉微细。而属少阴少阴脉微下利。反见阳明之不恶寒。而面色赤。若不于合并病参之。安知病情之
变迁如此。而为之施治哉。然此为六经言耳。若六经之合并。与内伤外感之合并。神而明之。不可胜极。以阴阳互
根之体。见阴阳离合之用。是知六经之准绳。更属定不定法矣。何漫云三阴无合病并病也哉。
疾病有固定的证候本质，因此设立六经来分别统领。疾病会有变化发展，更要综合考察合病与并病的情况互相参照。这是张仲景确立治疗法则的尽善尽美之处。阴阳互为根本，气虽然分为三阳三阴，但神机自然合一。三阳经的深层，就是三阴经；三阴经的表层，即是三阳经了。例如太阳病（太阳经受邪所致的病证），如果脉象反而沉，便属于合并少阴病（少阴经受邪所致的病证）；少阴病而反而发热，便属于合并太阳病。阳明病（阳明经受邪所致的病证）脉迟，即合并太阴病（太阴经受邪所致的病证）；太阴病脉缓，即合并阳明病。少阳病（少阳经受邪所致的病证）脉细小，是合并厥阴病（厥阴经受邪所致的病证）；厥阴病脉微浮，是合并少阳病。虽然没有合并之名，却有合并之实。或者阳气得阴气而解，阴气得阳气而解；或者阳气入阴分而危重，阴气亡失阳气而逆乱。种种脉症，不可枚举。学习者应当在阴阳两症中，观察病势是否合并；更要在三阳三阴中，审察症状是并见还是不并见。在此基础之上，运用阴病治阳、阳病治阴、扶阳抑阴、泻阳补阴等方法，使用得当即可。
三阳经都有发热的症状，三阴经都有下利的症状。如既发热又下利，这是阴阳合病。阴阳合病中，阳盛者归属于阳经，则下利属于实热证。如太阳阳明合病、少阳阳明合病、太阳少阳合病，必然自下利，使用葛根黄芩黄连汤等方剂的就是这类情况。阴盛者归属于阴经，则下利属于虚寒证。如少阴病出现呕吐下利，反而发热者不死；少阴病，下利清谷（大便中有未消化的食物），里寒外热，不恶寒，而面色赤红，使用通脉四逆汤的就是这类情况。若阳经与阳经合并，不与阴经合并，这就是三阳合病，则不下利，而是自汗出，属于白虎汤证。阴经与阴经合并，不与阳经合并，这就是三阴合病，则不发热，而是呕吐下利、四肢厥冷，属于四逆汤证。
并病与合病稍有不同：合病是同时并见，并病则是依次相乘。如太阳病的头项强痛尚未罢休，陆续出现脉弦、眩晕冒昧、心下痞硬等症状，这是与少阳并病；如果再出现谵语，就是三阳并病了。太阳与阳明并病，太阳症状未罢的，从太阳经用小发汗法；太阳症状已罢的，从阳明经用攻下法。其关键区别在于恶寒与恶热的表现。然而阳明病的病机在于胃家实（胃肠燥热结实）。太阳阳明合病，出现气喘且胸部满闷的，不可攻下，恐怕胃家尚未结实。如阳明与太阴、少阳合病，必然自下利，为什么能称为阳明病？要知道夹热下利，就是胃实证的开始。《内经》所说暴注下迫（急骤腹泻，肛门灼热紧迫），都属于热证。其脉象必定浮大或弦大，所以可以归属于阳明经，而不经过太阴经。若下利清谷，里寒外热，脉象浮而迟的，则浮脉不得归属于表证，而迟脉则表示病在脏。若见到脉微欲绝，即使身体不恶寒而面色赤红，又应当归属于少阴病。大概太阳阳明下利与太阴少阴下利的辨别，关键在于是否有清谷；而太阴少阴的清谷，又在脉象是迟还是微来进行辨别。
阳明主胃实，而有夹热下利；太阴主下利清谷，又因脉微细而归属于少阴。少阴脉微而下利，反而出现阳明病的不恶寒而面色赤红的症状。如果不参考合并病的情况，怎么知道病情的变迁如此复杂，从而为之施治呢？然而这仅仅是针对六经而言。如果是六经的合并，以及内伤外感的合并，若能神而明之，是不可穷尽的。以阴阳互为根本的本体，显现阴阳离合的作用，这就知道六经的准则，更是确定不定之法了。何必随意说三阴没有合病和并病呢？
Diseases have fixed pathological entities; therefore, the Six Channels (Liu Jing) are established to govern them respectively. Diseases undergo changes and transformations; thus, one must further investigate combined diseases (He Bing) and concurrent diseases (Bing Bing) for mutual reference. This represents the perfection of Zhang Zhongjing's legislative principles. Yin and Yang are mutually rooted; although Qi is divided, the Shen (spirit/mind) naturally unites. The root of the Three Yang channels is the Three Yin channels, and the exterior of the Three Yin channels is the Three Yang channels. For instance, in Taiyang disease (disease of the Taiyang channel), if the pulse is unexpectedly deep, it combines with Shaoyin disease (disease of the Shaoyin channel). In Shaoyin disease, if fever appears unexpectedly, it combines with Taiyang disease. In Yangming disease (disease of the Yangming channel), a slow pulse indicates combination with Taiyin disease (disease of the Taiyin channel); in Taiyin disease, a moderate pulse indicates combination with Yangming disease. In Shaoyang disease (disease of the Shaoyang channel), a fine and small pulse indicates combination with Jueyin disease (disease of the Jueyin channel); in Jueyin disease, a faint and floating pulse indicates combination with Shaoyang disease. Although there may be no name for "combination," there is the reality of combination. Sometimes Yang resolves through Yin, or Yin resolves through Yang; sometimes Yang enters Yin leading to danger, or Yin loses Yang leading to reversal. Such various pulse and symptom patterns cannot be enumerated. Students should observe whether the pathological trend is combined within the context of Yin and Yang symptoms, and examine whether symptoms are concurrent or not among the Three Yang and Three Yin channels. Based on this, methods such as treating Yang diseases by addressing Yin, treating Yin diseases by addressing Yang, supporting Yang to suppress Yin, and draining Yang to tonify Yin can be applied appropriately.
The Three Yang channels all present with fever, and the Three Yin channels all present with diarrhea. If both fever and diarrhea occur, it is a combined disease of Yin and Yang. In combined diseases of Yin and Yang, if Yang is excessive, it belongs to the Yang channels, and the diarrhea is due to excess heat. For example, in combined diseases of Taiyang and Yangming, Shaoyang and Yangming, or Taiyang and Shaoyang, spontaneous diarrhea inevitably occurs; this is treated with formulas like Gegen Qinlian Tang (Kudzu and Scutellaria Decoction). If Yin is excessive, it belongs to the Yin channels, and the diarrhea is due to deficiency cold. For example, in Shaoyin disease with vomiting and diarrhea, if fever appears unexpectedly, death is not inevitable; in Shaoyin disease with diarrhea containing undigested food (Qing Gu), interior cold with exterior heat, aversion to cold absent but facial redness present, Tongmai Sini Tang (Pulse-Returning Four Reversals Decoction) is used. If Yang channels combine with Yang channels, without combining with Yin, it is a combined disease of the Three Yang channels; then there is no diarrhea, but spontaneous sweating, which constitutes the Baihu Tang syndrome (White Tiger Syndrome). If Yin channels combine with Yin channels, without combining with Yang, it is a combined disease of the Three Yin channels; then there is no fever, but vomiting, diarrhea, and cold extremities, which constitutes the Sini Tang syndrome (Four Reversals Syndrome).
Concurrent diseases (Bing Bing) differ slightly from combined diseases (He Bing): He Bing presents simultaneously, whereas Bing Bing progresses sequentially. For instance, if the headache and stiff neck of Taiyang disease have not yet ceased, and subsequently symptoms such as a wiry pulse, dizziness, vertigo, and epigastric fullness and hardness appear, it is a concurrent disease with Shaoyang. If delirium also appears, it becomes a concurrent disease of the Three Yang channels. In concurrent disease of Taiyang and Yangming, if Taiyang symptoms persist, use mild diaphoresis from the Taiyang perspective; if Taiyang symptoms have resolved, use purgation from the Yangming perspective. The key distinction lies in aversion to cold versus aversion to heat. However, Yangming disease involves "Wei Jia Shi" (Stomach House Excess/Intestinal燥热结实). In combined disease of Taiyang and Yangming, if there is wheezing and chest fullness, purgation is contraindicated, lest the Stomach House Excess has not yet fully formed. If Yangming combines with Taiyin and Shaoyang, spontaneous diarrhea inevitably occurs. Why is this still called Yangming? One must understand that diarrhea with internal heat is the beginning of Stomach House Excess. As stated in the *Neijing*, sudden violent diarrhea with tenesmus belongs to heat. The pulse will necessarily be floating, large, or wiry-large, so it can be attributed to the Yangming channel, bypassing the Taiyin channel. If there is diarrhea with undigested food, interior cold with exterior heat, and a floating and slow pulse, the floating pulse does not belong to the exterior, and the slow pulse indicates disease in the Zang organs. If a faint pulse nearly vanishing is seen, even if there is no aversion to cold but facial redness, it should be attributed to Shaoyin disease. The differentiation between diarrhea in Taiyang-Yangming and Taoyin-Shaoyin lies in the presence of undigested food; and the differentiation of undigested food in Taoyin-Shaoyin lies in whether the pulse is slow or faint.
Yangming governs Stomach House Excess, yet presents with diarrhea accompanied by heat; Taiyin governs diarrhea with undigested food, and due to a fine and faint pulse, is attributed to Shaoyin. Shaoyin presents with a faint pulse and diarrhea, yet unexpectedly shows symptoms of Yangming disease such as absence of aversion to cold and facial redness. Without referencing combined diseases, how could one know the complexity of pathological changes and apply treatment accordingly? This discussion pertains only to the Six Channels. Regarding the combination of the Six Channels, and the combination of internal injury and external contraction, if understood spiritually and clearly, they are inexhaustible. By perceiving the function of Yin-Yang separation and union through their mutually rooted nature, one realizes that the guidelines of the Six Channels are indeed flexible principles rather than rigid rules. How can one casually claim that there are no combined or concurrent diseases in the Three Yin channels?