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，此仲景二法之尽善也。夫阴阳互根，气
虽分而神自合。三阳之里，便是三阴，三阴之表，即是三阳。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，种种脉证，不可枚举。学人当
于阴阳两症中，察病势之合不合，更于三阳三阴中，审其症之并不并，予以阴病治阳，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，如发热而下利者，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、阳明少阳合病、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，如少阴病吐利及发热者不死，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。若阳与阳合，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，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，是与少阳并病，更见谵语，即三阳并病矣。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，其机在恶寒发热而分也。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。若阳明与太、少合病，必自下利，何以得称阳明？要
知协热下利，即胃实之始，《内经》所云“暴注下迫，皆属于热”，其脉必浮大弦
大，故得属之阳明，而不系太阴也。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。盖太阴阳明下利之辨，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，而有协热利；太阴主下利清谷，又因脉微细而属少阴。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。
何漫云三阴无合并病也哉？
疾病有固定的证候体系，因此设立六经来分别统领；疾病又有变化发展的过程，更要结合合病与并病的概念互相参照，这是张仲景两种诊疗方法的完美体现。阴阳互为根本，气机虽然分为三阴三阳，但神机自然合一。三阳经的里层，就是三阴经；三阴经的表层，即是三阳经。例如太阳病（太阳经受邪所致的病证）而脉象反而沉，便是合并了少阴病（少阴经受邪所致的病证）；少阴病而反而发热，便是合并了太阳病。阳明病（阳明经受邪所致的病证）脉迟，即合并了太阴病（太阴经受邪所致的病证）；太阴病脉缓，即合并了阳明病。少阳病（少阳经受邪所致的病证）脉细小，是合并了厥阴病（厥阴经受邪所致的病证）；厥阴病脉微浮，是合并了少阳病。虽然没有“合并”的名称，却有合并的实质。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，种种脉象和症状，无法一一列举。学习者应当在阴阳两症中，察看病势是否合并，更在三阳三阴中，审辨症状是并见还是兼夹，给予阴病治阳、阳病治阴、扶阳抑阴、泻阳补阴等方法，运用得当即可。
三阳经都有发热的症状，三阴经都有下利的症状，如果既发热又下利，属于阴阳合病。阴阳合病中，阳气盛者归属于阳经，则下利属于实热，如太阳病与阳明病合病、阳明病与少阳病合病、太阳病与少阳病合病，必然自下利，使用葛根汤、黄芩汤等方剂的就是这类情况。阴气盛者归属于阴经，则下利属于虚寒，如少阴病出现呕吐下利及发热者不死，少阴病下利完谷不化，表现为里寒外热，不恶寒而面色赤红，使用通脉四逆汤的就是这类情况。如果是阳经与阳经合并，不与阴经合并，这就是三阳合病，则不下利而自汗出，属于白虎汤证（阳明经热盛证）。阴经与阴经合并，不与阳经合并，这就是三阴合病，不发热而呕吐下利且四肢厥冷，属于四逆汤证（少阴病阳虚寒盛证）。
并病与合病稍有不同，合病是指症状一时并见，并病是指症状依次相乘出现。如太阳病的头项强痛尚未解除，陆续出现脉弦、头晕目眩、心下痞硬，这是与少阳病并病，如果再出现谵语，就是三阳并病了。太阳病与阳明病并病，太阳病症未解除的，从太阳经用小发汗法治疗；太阳病症已解除的，从阳明经用攻下法治疗，其关键在于通过恶寒发热的有无来区分。然而阳明病的本质在于胃家实（胃肠燥热结实），太阳病与阳明病合病，出现喘息且胸部胀满的，不可用攻下法，恐怕是胃家尚未完全结实。若是阳明病与太阴、少阳合病，必然自下利，这怎么能称为阳明病呢？要知道协热下利（伴随发热的下利），就是胃实证的开始，《内经》所说“暴注下迫，皆属于热”，其脉象必然浮大或弦大，所以可以归属于阳明，而不隶属于太阴。如果是下利完谷不化，里寒外热，脉象浮而迟缓的，那么浮脉不得归属于表证，而迟脉则表示病在脏。如果见到脉微欲绝，即使身体不恶寒而面色赤红，又应当归属于少阴病。大概太阴病与阳明病下利的辨别，在于是否下利完谷不化，而太阴病与少阴病都出现下利完谷不化时，又在脉象是迟还是微上进行辨别。阳明主胃实证，却伴有协热利；太阴主下利完谷不化，却又因脉微细而归属于少阴。脉微而下利，反而出现阳明病的不恶寒而面色赤红的症状，如果不结合合并病来参详，怎么知道病情变化如此复杂，从而为之施治呢？然而这是六经的合并与内伤外感的合并，若能神而明之，是不可穷尽的。以阴阳互为根本的本体，见到阴阳离合的作用，就知道六经的准则，更是确定而不拘泥于固定法则的了。何必随意说三阴没有合并病呢？
Diseases have fixed patterns, hence the Six Channels (Six Stages) are established to govern them respectively; diseases also undergo changes and transformations, thus one must further refer to combined diseases (Bingbing) and concurrent diseases (Hebing) for mutual reference. This represents the perfection of Zhang Zhongjing's two methods. Yin and Yang are mutually rooted; although Qi is divided into the Three Yang and Three Yin, the Shen (Spirit/Mind) naturally unites. The interior 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, if a Taiyang disease (disease of the Taiyang channel) presents with a sunken pulse, it combines with Shaoyin disease (disease of the Shaoyin channel); if Shaoyin disease presents with fever, it combines with Taiyang disease. If Yangming disease (disease of the Yangming channel) presents with a slow pulse, it combines with Taiyin disease (disease of the Taiyin channel); if Taiyin disease presents with a moderate pulse, it combines with Yangming disease. If Shaoyang disease (disease of the Shaoyang channel) presents with a fine and small pulse, it combines with Jueyin disease (disease of the Jueyin channel); if Jueyin disease presents with a faint and floating pulse, it combines with Shaoyang disease. Although there is 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 rebellion. Various pulses and symptoms cannot be enumerated. Students should examine whether the disease trend is combined within the Yin and Yang syndromes, and further scrutinize whether the symptoms are concurrent or overlapping among the Three Yang and Three Yin. By applying methods such as treating Yang diseases via Yin, treating Yin diseases via Yang, supporting Yang to suppress Yin, and draining Yang to tonify Yin appropriately, treatment can be effective.
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 Yin-Yang combined disease. In Yin-Yang combined diseases, if Yang is excessive, it belongs to the Yang channels, and the diarrhea is due to excess heat. Examples include Taiyang-Yangming combined disease, Yangming-Shaoyang combined disease, and Taiyang-Shaoyang combined disease, which inevitably cause spontaneous diarrhea; these are treated with formulas like Gegen Tang (Kudzu Decoction) and Huangqin Tang (Scutellaria Decoction). If Yin is excessive, it belongs to the Yin channels, and the diarrhea is due to deficiency cold. Examples include Shaoyin disease with vomiting, diarrhea, and fever (which is not fatal), or Shaoyin disease with diarrhea of undigested food, presenting with interior cold and exterior heat, aversion to cold absent but with a red complexion; these are treated with Tongmai Sini Tang (Pulse-Regulating Four Reversals Decoction). If Yang combines with Yang without involving Yin, it is a Three Yang combined disease, presenting with spontaneous sweating but no diarrhea, corresponding to the Baihu Tang syndrome (White Tiger Decoction pattern). If Yin combines with Yin without involving Yang, it is a Three Yin combined disease, presenting with no fever but with vomiting, diarrhea, and cold extremities, corresponding to the Sini Tang syndrome (Four Reversals Decoction pattern).
Concurrent disease (Bingbing) differs slightly from combined disease (Hebing): Hebing refers to symptoms appearing simultaneously, while Bingbing refers to symptoms succeeding one another. For example, if the headache and stiff neck of Taiyang disease have not yet resolved, and subsequently a wiry pulse, dizziness, and epigastric fullness/stiffness appear, this is a concurrent disease with Shaoyang disease. If delirium also appears, it becomes a concurrent disease of the Three Yang. In Taiyang-Yangming concurrent disease, if Taiyang symptoms persist, treat with mild diaphoresis from the Taiyang perspective; if Taiyang symptoms have resolved, treat with purgation from the Yangming perspective. The key distinction lies in the presence or absence of aversion to cold and fever. However, Yangming disease fundamentally involves stomach excess (Wei Jia Shi - gastric organ excess/heat accumulation). In Taiyang-Yangming combined disease, if there is wheezing and chest fullness, purgation is contraindicated, lest the stomach excess is not yet fully formed. If Yangming disease combines with Taiyin and Shaoyang, causing spontaneous diarrhea, how can it still be called Yangming disease? One must understand that "diarrhea accompanying heat" (Xie Re Xia Li) is the beginning of stomach excess. As stated in the *Neijing* (Inner Canon), "acute watery diarrhea with tenesmus belongs to heat." The pulse will necessarily be floating, large, or wiry-large, allowing it to be attributed to Yangming rather than Taiyin. Conversely, if there is diarrhea of undigested food, interior cold with exterior heat, and a floating yet slow pulse, the floating aspect does not belong to the exterior, and the slowness indicates disease in the Zang organs. If the pulse is faint and nearly imperceptible, even if there is no aversion to cold but a red complexion, it should be attributed to Shaoyin disease. The differentiation between Taiyin and Yangming diarrhea lies in whether undigested food is passed. Furthermore, when both Taiyin and Shaoyin present with diarrhea of undigested food, the differentiation relies on whether the pulse is slow or faint. Yangming governs stomach excess but may present with diarrhea accompanying heat; Taiyin governs diarrhea of undigested food but may be attributed to Shaoyin due to a faint and fine pulse. A faint pulse with diarrhea, yet presenting with Yangming signs of no aversion to cold and a red complexion, requires reference to combined diseases to understand such complex disease progression and apply appropriate treatment. Thus, the combinations of the Six Channels and the combinations of internal injury versus external contraction, if understood spiritually and clearly, are inexhaustible. By perceiving the function of Yin-Yang separation and union through their mutual rooting, one realizes that the principles of the Six Channels are definitive yet flexible, not rigidly fixed. How can one casually claim that there are no combined diseases in the Three Yin channels?