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What are the Six Meridians? The Six Meridians refer to Taiyang, Yangming, Shaoyang, Taiyin, Shaoyin, and Jueyin. Within these six meridians, they are further divided into Hand and Foot meridians, thereby encompassing the twelve primary meridians and their associated Zang-Fu organs. Li believed that "historical physicians have explored the Six Meredians from perspectives such as Zang-Fu organs, meridians, Qi transformation, anatomical locations, and disease stages. Although these research methods have each made their own contributions, they also possess certain one-sidedness" (National Higher Medical Colleges Teaching Reference Book: Treatise on Cold Damage). Since Zang-Fu organs are the core of human physiological activities, their functions inevitably affect all parts of the body, while the physiological activities of all body parts must belong to or influence the Zang-Fu organs. From the perspective of pathological manifestations of Zang-Fu organs, during the development of a disease, lesions in each meridian often involve the related Zang-Fu organs, presenting as Zang-Fu syndromes. For instance, the Bladder is the Fu organ of Taiyang. If exterior pathogenic factors of Taiyang are not resolved and transmit into the Fu organ, causing dysfunction of the Bladder and retention of water-Qi, symptoms such as difficult urination, urgency and tension in the lower abdomen, irritability with thirst or desire to drink water, and vomiting immediately after drinking may appear. The Stomach and Large Intestine are the Fu organs of Yangming. When pathogens enter Yangming, excessive dry-heat in the Stomach injures Body Fluids, leading to high fever, spontaneous sweating, aversion to cold being absent but aversion to heat present, dry mouth and tongue, unrelenting irritability and thirst, and a surging pulse. If dry-heat accumulates and solidifies in the intestines and stomach, blocking the flow of Fu organ Qi, symptoms such as tidal fever, delirium, continuous sweating on the hands and feet, abdominal distension and pain with tenderness, and constipation may occur. The Gallbladder and Sanjiao (Triple Burner) are the Fu organs of Shaoyang. If Gallbladder fire flares upward, there will be bitter taste in the mouth, dry throat, and dizziness; if the Sanjiao loses its regulatory function, or water stagnates below the heart, palpitations below the heart and difficult urination may occur; if cold-water invades the Lungs, coughing results; or if the pivot mechanism of Shaoyang is impaired, leaving cold-fluid retained and untransformed in the middle, symptoms such as alternating chills and fever, irritability, slight fullness in the chest and hypochondrium, difficult urination, thirst without vomiting, and only sweating on the head may appear. The Spleen is the Zang organ of Taiyin. Disease involves failure of Spleen-Yang to rise and dysfunction of transportation and transformation, resulting in Spleen deficiency, internal cold, and stagnation of cold-dampness, which can manifest as abdominal fullness with vomiting, inability to eat, spontaneous diarrhea, and periodic abdominal pain. The Heart and Kidney are the Zang organs of Shaoyin. Disease involves decline of Heart and Kidney, insufficiency of Qi and Blood, presenting with a faint and thin pulse, desire to sleep, aversion to cold, curling up in bed, and even cold extremities, diarrhea, or vomiting—a series of symptoms indicating deficiency of Yang-Qi and excess of Yin-cold internally. If Heart-fire is too hyperactive and Kidney-Yang is insufficient, symptoms such as irritability in the heart, insomnia, dry throat, crimson tongue body, and fine and rapid pulse may appear, indicating Yin-deficiency with intense heat. The Liver is the Zang organ of Jueyin. Disease involves mixed cold and heat, with Liver-Qi rebelling upward, manifesting as excessive thirst, sensation of Qi rushing up to strike the heart, pain and heat in the heart, hunger without desire to eat, vomiting roundworms after eating, or diarrhea. Lesions of Zang-Fu organs should be studied from multiple factors.

Meridians originate from Zang-Fu organs, network the whole body, and circulate Qi and Blood. They have independent functions but also subordinate aspects to the Zang-Fu organs. Regarding pathological manifestations of meridians, for example, the Foot-Taiyang Meridian starts at the inner canthus, goes up to the forehead and vertex, descends behind the neck along the spine to the waist and foot, running along the back of the body. Thus, when Taiyang is attacked by pathogens, symptoms such as headache and stiffness of the neck and back appear. The Foot-Yangming Meridian starts on both sides of the nasal depression, connects with the eyes, and descends from the clavicular fossa through the chest and abdomen, running along the front of the body. Thus, when Yangming meridian is attacked by pathogens, symptoms such as eye pain and dry nose appear. The Foot-Shaoyang Meridian starts at the outer canthus, goes up to the temporal angle, descends behind the ear, enters the ear, and descends from the clavicular fossa through the chest and hypochondrium, running along the side of the body. Thus, when Shaoyang meridian is attacked by pathogens, symptoms such as deafness, red eyes, and distressing fullness in the chest and hypochondrium appear. The Three Yin meridians belong to interior syndromes. Although the syndromes reflected by their meridians are not as prominent as those of the Three Yang meridians, certain symptoms they manifest, such as abdominal fullness and pain in Taiyin disease, dry and painful throat in Shaoyang disease, and vertex headache in Jueyin disease, are all related to the course of their meridians.

Qi transformation refers to the changes of the Six Qi: Wind, Cold, Summer-heat, Dampness, Dryness, and Fire. The theory of Qi transformation originates from the Seven Chapters of Yunqi in *Su Wen* (Basic Questions). For instance, *Su Wen · Chapter 74: Da Liu Yuan* states: "The generation of all diseases arises from the transformations and variations caused by Wind, Cold, Summer-heat, Dampness, Dryness, and Fire." It also says, "Those who speak well of Heaven must have verification in humans; those who speak well of antiquity must align with the present; those who speak well of the present must be manifested in things." (*Su Wen · Chapter 23: Liu Wei Zhi Da Lun*) Due to the correspondence between Heaven and humanity, the Six Qi are used not only to explain the laws of climate-induced diseases ("transformations and variations") but also to illustrate changes in the functional activities of human Zang-Fu organs. Later commentators mostly used this to explain the pathological evolution of Six Meridian diseases. For example, in *Shanghan Lun*, Guizhi Tang (Cinnamon Twig Decoction) is used for Wind-stroke, and Mahuang Tang (Ephedra Decoction) for Taiyang Cold Damage, addressing exterior Wind-Cold. If Lizhong Tang (Poria Decoction) treats Taiyin Interior Cold, Fuzi Tang (Aconite Decoction) treats pure Yin without Yang, and Tongmai Bailong Tang (Pulse-Unblocking Bai-Ling Decoction) treats Floating Yang and Rebellating Yang, these belong to severe interior cold syndromes. Heat is relative to Cold. For example, Gegen Qinlian Tang (Kudzu and Coptis Decoction) treats interior and exterior heat; Baihu Tang (White Tiger Decoction) treats exclusive heat in Yangming; and the three Chengqi Decoctions use purgation as the method, treating the tangible dry-heat of Yangming. Dampness is relative to Dryness. Yinchen Hao Tang (Artemisia Capillaris Decoction), Zhizi Baipi Tang (Gardenia and Phellodendron Bark Decoction), and Mahlian Qiaoxiao Dou Tang (Ephedra, Forsythia, and Soybean Decoction) are the three formulas for treating Yangming damp-heat jaundice; if it is cold-damp jaundice, one can seek solutions among Lizhong and Sini Decoctions in Taiyin disease. However, this discussion of Six Qi focuses more on external pathogenic factors. If internal bodily functions are disordered, the transformation of Six Qi can also be generated. For example, mild Spleen and Kidney Yang can cause internal cold; Spleen deficiency failing to transport leads to internal dampness; dry fluids and withered Jin-Ye generate internal heat, so treatment principles can refer to warming and clearing methods of Taiyin and Yangming. As for Jueyin and Shaoyin Yin-deficiency with Yang-hyperactivity generating Wind and Fire, Zhang Zhongjing's methods seem incomplete. However, extending this, methods of softening, moistening, nourishing, and calming, such as Huanglian Ejiao Tang (Coptis and Donkey-Hide Gelatin Decoction) and Jiawei Fumai Tang (Modified Pulse-Nourishing Decoction), are commonly used by Warm Disease scholars, which must be known. Furthermore, Fire is considered the thief of Original Qi, and ancients stated that all Six Qi can transform into Fire. Clinical evidence shows that Fire diseases are common, which is a fact. In Zhang Zhongjing's formulas, Sanhuang Xiexin Tang (Three Yellows Heart-Draining Decoction) treats hyperactive Fire of the Heart and Stomach; Baihu Tang and Zhuye Tang (Bamboo Leaf Decoction) treat dry Fire of the Yangming Stomach; Chaihu Longgu Muli Tang (Bupleurum, Dragon Bone, and Mussel Shell Decoction) treats rebellious Fire of the Liver and Gallbladder; Huanglian Ejiao Tang treats Fire of Shaoyin Yin-deficiency with Yang-hyperactivity. All these not only establish legislation for Cold Damage but can also be referenced for internal injury miscellaneous diseases.

In recent times, epidemics (Li Qi) have relatively decreased. However, based on internal gastrointestinal diseases treated in the regions north and south of the Yangtze River and the Jianghan Plain, damp-heat syndromes are predominant; for Liver and Gallbladder diseases, Fire transformation is common. Therefore, using acrid-warm and drying-hot herbs easily causes the pathology to deteriorate. This coincides with the concept that Yangming and Jueyin do not follow the root and branch but transform according to the Middle Manifestation. Thus, the theory of Qi transformation, when sought in clinical practice, is credible and demonstrable by facts. Researching from this angle helps understand physiological and pathological conditions at different stages, but seeking the origin still attributes it to Zang-Fu organs and meridians. It is evident that Qi transformation loses its material basis without Zang-Fu organs and meridians; conversely, Zang-Fu organs and meridals cannot reflect functional activities without Qi transformation. As for the location and stage of diseases, they have significant characteristics in clinical practice and are indispensable parts of diagnostics. However, the externally reflected location and stage are mostly superficial phenomena, requiring integration with various factors to find the root cause. Therefore, starting from clinical reality, combining Six Meridian syndromes with Zang-Fu organs, meridians, Qi transformation, and location organically is necessary to correctly understand the significance of Six Meridian differentiation in *Shanghan Lun*.

(IV) Elucidating the Essence of Six Meridian Differentiation 1. Analyzing Ten Methods of Six Meridian Differentiation Differentiation of syndromes and treatment in Traditional Chinese Medicine is an important method for making correct judgments and deciding reasonable treatment measures based on patients' specific conditions through normal diagnostic procedures like examining illnesses and asking questions. It has been used by TCM practitioners for thousands of years. Differentiation of syndromes and treatment originates from *Shanghan Lun*. At the beginning of the Six Meridian chapters of this book, headings such as "Differentiating Taiyang Disease Pulse, Syndrome, and Treatment" and "Differentiating Yangming Disease Pulse, Syndrome, and Treatment" are listed. The term "Disease" in the context of Six Meridians means "Taiyang constitutes disease," "Yangming constitutes disease," or simply "Taiyang Disease," "Yangming Disease."

The term "Syndrome" (Zheng): For Taiyang Disease, the guideline is "floating pulse, stiffness and pain in the head and neck, and aversion to cold." If accompanied by fever, sweating, aversion to wind, and a slow pulse, it is named Wind-stroke, i.e., Guizhi Syndrome. If accompanied by fever or no fever, body ache, lumbar pain, absence of sweating, wheezing, and tight pulses at both Cun and Chi positions, it is named Cold Damage, i.e., Mahuang Syndrome.

The term "Pulse": For Taiyang exterior diseases, floating is the main pulse, but within floating pulses, there are tight, slow, rapid, or delayed variations. The term "Treatment": For exterior diseases, inducing sweating is the major principle. Within sweating methods, there are acrid-warm releasing muscle harmonizing Ying and Wei methods, aiming for slight perspiration. Deepening the discussion on treatment methods, there are primary treatment, adjunctive treatment, initial treatment, subsequent treatment, orthodox treatment, and counter-treatment. Under the premise of Six Meridians, *Shanghan Lun* applies the principles of syndrome differentiation and treatment to diagnose and treat complex pathogenic factors and pathological mechanisms, exploring general and special rules. Prominent points include: (1) Differentiating Yin and Yang: *Nei Jing* states, "Yin and Yang are the Way of Heaven and Earth, the net of all things, the parents of change, the beginning of life and death, the mansion of spirit. To treat diseases, one must seek the root" (*Su Wen · Yin Yang Ying Xiang Da Lun*). The basic rule of Six Meridian differentiation and treatment in *Shanghan Lun* mainly lies in observing the waxing and waning of the body's Yin and Yang, the advance and retreat of pathogens and righteous Qi, to judge the crux of the lesion and decide reasonable treatment measures. Thus, differentiating Yin and Yang is essential for seeking the root.

The meaning of Yin and Yang generally refers to: exterior and upper locations are Yang; interior and lower locations are Yin. Hot and Excess nature are Yang; Cold and Deficient nature are Yin. Broadly dividing the Six Meridians: Taiyang, Yangming, and Shaoyang are Three Yang diseases (Yang); Taiyin, Shaoyin, and Jueyin are Three Yin diseases (Yin). Yang diseases often show excitation and progression, so treatment focuses on expelling pathogens, suitable for sweating, vomiting, purging, and harmonizing methods. Yin diseases often show regression and decline, so treatment focuses on supporting righteousness, using warming interior and supporting Yang, nourishing Yin and clearing heat, or mixing cold and warm herbs to treat both pathogens and righteous Qi simultaneously. The concept of Yin and Yang in *Shanghan Lun*'s specific application of syndrome differentiation can be summarized in four aspects: diseases have Yin and Yang, syndromes have Yin and Yang, pulses have Yin and Yang, and treatments have Yin and Yang. For example, after differentiating the Taiyang guideline and Wind-Cold-Warm three syndromes, the text immediately proposes, "Diseases with fever and aversion to cold originate from Yin; diseases without fever and with aversion to cold originate from Yang" (cited from Song Edition Article 7). Generally, although Yang disease pathogens are strong, righteous Qi is still substantial, and defensive Yang reacts sensitively, so Three Yang diseases often have fever symptoms. Such as Taiyang disease with aversion to cold and fever, Shaoyang disease with alternating chills and fever, Yangming disease with high fever, spontaneous sweating, absence of aversion to cold but presence of aversion to heat. Yin diseases, where pathogens are strong and righteous Qi is deficient, have low resistance, so Three Yin diseases often lack fever symptoms.

For example, Three Yin deficiency-cold presents with aversion to cold, curling up in bed, and even cold extremities and reverse cold. This is the usual condition of Six Meridian onset. However, Taiyang Cold Damage may have cases without fever initially; Yangming disease may have deep heat with deep cold extremities; Shaoyang disease may have cases without alternating chills and fever. Regarding Three Yin, Taiyin disease may have warm hands and feet; Shaoyin disease may have counter-fever or interior cold with exterior heat; Jueyin deficiency-cold may first show cold extremities and diarrhea then turn to Yang return with fever. Why is this? Because during the development of Six Meridian diseases, slight differences in pathogenic factors affect pathological changes, often leading to syndromes appearing in another form. These are mostly variable situations, and one should not deny the validity of originating from Yang or Yin based on this. Thus, commentators like Qian Tianlai and Ke Yunbo placed this article at the head of the Six Meridians, providing a comprehensive overview, showing profound insight.

Another passage states, "Diseases originating from Yang, if wrongly purged, heat enters and forms Chest Binding; diseases originating from Yin, if wrongly purged, form Epigastric Fullness" (Article 131). This is because exterior syndrome wrongly purged allows Yang-heat to sink, binding with phlegm-fluid in the chest and heart to form excess, presenting with hardness, fullness, and pain in the epigastrium and hypochondrium, known as Heat-Excess Chest Binding, hence called "originating from Yang." If exterior syndrome wrongly purged allows pathogenic heat to enter internally without fluid binding, forming only Epigastric Fullness, it is called "originating from Yin." Thus, regarding Yin and Yang, comparing Chest Binding and Epigastric Fullness by syndromes allows understanding.

*Nei Jing* states, "Good diagnosticians examine color and press pulses, first distinguishing Yin and Yang" (*Su Wen · Yin Yang Ying Xiang Da Lun*). Therefore, Zhongjing's pulse method also uses Yin and Yang as the differentiation guideline. *Bian Mai Fa* says: "Large, floating, rapid, moving, and slippery pulses are named Yang; sinking, choppy, weak, wiry, and slow pulses are named Yin." Yang disease located in the exterior, lightly affected, with substantial righteous Qi and smooth circulation of Ying-Wei Qi and Blood, corresponds with pulses showing excess of Yang-heat. Conversely, sinking and choppy pulses indicate deep-seated pathogens, excess Yin and weak Yang, a sign of insufficiency. Although this article was written by Wang Shuhe, it reflects Zhongjing's core method.

Regarding treatment, *Shanghan Lun* emphasizes "self-harmonization of Yin and Yang" (see Article 58). When the body is affected by external pathogens or internal organ dysfunction, causing disharmony of Yin and Yang Qi, a lesion in one of the Six Meridians may appear. "Self-harmonization" does not mean sitting idle waiting for cure. For example, excess Yang-heat is cleared and purged; excess Yin with Yang decline is warmed and tonified. This is the meaning of *Nei Jing*'s "Treat Yin diseases by addressing Yang, treat Yang diseases by addressing Yin" (*Su Wen · Yin Yang Ying Xiang Da Lun*). *Jingui Yaolue* states, "What appears as Yin is saved by Yang methods; what appears as Yang is saved by Yin methods" (*Baihe Huhuo Yangdu Bing Mai Zheng Bing Zhi*), which is similar in meaning. Treatment always needs to address imbalance and save defects based on the waxing/waning of Yin/Yang and the strength/weakness of pathogens/righteous Qi, promoting Yin and Yang Qi to a relatively balanced normal state. This is what *Nei Jing* calls "Yin is calm and secret, Spirit is governed" (*Su Wen · Sheng Qi Tong Tian Lun*). Even if the disease is severe, recovery is possible. Thus, Yin and Yang are indeed the guideline for differentiation and criterion for treatment in the Six Meridians of *Shanghan Lun*.

(2) Differentiating Exterior and Interior: Exterior, interior, upper, and lower all refer to the location of the disease. *Shanghan Lun* considers Taiyang as the great exterior of the Six Meridians, while other meridians belong to the interior. However, the concept of exterior and interior is relative. For example, Taiyang governs exterior, Yangming governs interior, and Shaoyang governs half-exterior half-interior. Relative to Taiyin, Yangming governs exterior and Taiyin governs interior. Relative to Jueyin, Shaoyang governs exterior and Jueyin governs interior. In differentiation, exterior and interior must be analyzed combined with disease nature, pathogen strength, and righteous Qi strength. For instance, Taiyang governs exterior; Taiyang disease with spontaneous sweating and slow pulse is Exterior Deficiency Syndrome; without sweating and with tight pulse is Exterior Excess Syndrome. Shaoyin governs interior; Shaoyin disease with faint/thin pulse, desire to sleep, aversion to cold, curling up, and diarrhea is Interior Deficiency-Cold Syndrome; if seeing fine/rapid pulse, crimson tongue, irritability, insomnia, dry/painful throat, it is Interior Deficiency-Heat Syndrome.

*Shanghan Lun* also distinguishes exterior and interior within certain syndromes. For example, Taiyang Water Accumulation uses Wuling San as the main formula, mentioning "having exterior and interior syndromes" (Article 74). Its exterior refers to Wind-stroke fever unresolved for six or seven days; its interior refers to irritability/thirst, vomiting after drinking, and difficult urination. Similarly, Yangming dry-heat injuring fluids mentions "exterior and interior both hot" (Article 168). Its exterior refers to high fever, spontaneous sweating, aversion to heat; its interior refers to extreme thirst with desire to drink, dry tongue, and irritability. But the focus of the disease is "heat binding in the interior," so Baitu Jia Renshen Tang (White Tiger plus Ginseng Decoction) is used to directly clear Yangming interior heat and benefit Qi/nourish fluids.

Since diseases have exterior/interior distinctions, pulses have floating/sinking responses. Taiyang exterior diseases primarily feature floating pulses. If the pulse is floating and delayed, it indicates exterior disease with interior deficiency. Sinking indicates interior. However, Yangming dry-heat solidification presents with "sinking-excess pulse, treat by purging." Shaoyin Yang-deficiency with Yin-excess presents with "faint-thin-sinking pulse," treated by supporting Yang and suppressing Yin.

Regarding treatment methods, generally when exterior and interior syndromes appear together, the principle is to resolve the exterior first, then treat the interior. For example, Taiyang Blood Accumulation, if mild: "If the exterior is not resolved, attack is not yet appropriate; resolve the exterior first; once exterior is resolved, if only lower abdominal urgency/clumping exists, then attack it, using Taohe Chengqi Tang" (Article 106). If Blood Accumulation is severe and urgent, such as "Taiyang disease for six or seven days, exterior signs remain, pulse is faint and sinking... person becomes manic, heat is in the Lower Jiao, lower abdomen should be hard/full, urination is free, bleeding cures it... Di Dang Tang is the main formula" (Article 124). Here, exterior and interior signs exist, but interior signs are acute, so the principle of treating the interior urgently is applied. Additionally, when exterior and interior signs are equal in severity, simultaneous treatment can be used, such as Chaihu Guizhi Tang for Shaoyang disease with unresolved exterior, or Mahuang Fuzi Xixin Tang for Shaoyin disease with counter-fever and sinking pulse. Furthermore, simultaneous exterior-interior treatment can focus on exterior or interior depending on the condition. Former includes Da Qinglong Tang resolving both exterior and interior, releasing exterior and clearing interior, focusing on releasing exterior. Latter includes Guizhi Renshen Tang, also resolving exterior and warming interior, focusing on warming interior.