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San Yin and San Yang (Three Yin and Three Yang meridians): From the perspective of exterior meridians, Yang meridians are Yang, and Yin meridians are Yin; from the perspective of interior Zang-Fu organs, Zang organs are Yin, and Fu organs are Yang.

At the exterior level, cold pathogen injuring Ying Qi (nutritive Qi circulating within the vessels) involves blood within the vessels, which is Yin; wind pathogen injuring Wei Qi (defensive Qi circulating outside the vessels) involves Qi outside the vessels, which is Yang. At the interior level, among the Fu organs, the Stomach is Yang, Large Intestine is Yin, Bladder is Yang, Small Intestine is Yin, and Gallbladder is more Yin; among the Zang organs, Lung is Yang, Heart and Spleen are Yin, Liver and Kidney are "Zhi Yin" (Extreme Yin). *Huangdi Neijing* (Yellow Emperor's Inner Canon) refers to Spleen as Zhi Yin.

San Yang diseases also have interior syndromes, and San Yin diseases also have exterior syndromes. Syndromes at the exterior, regardless of Yin or Yang, mostly manifest in the Foot meridians; syndromes at the interior involve both Hand and Foot meridians. Chengqi Tang (Cathartic Decoction) for Yangming syndrome attacks the Large Intestine, not the Stomach; how could dry feces remain in the Stomach? Didang Tang (Attacking Stasis Decoction) for Taiyang syndrome attacks the Small Intestine, not the Bladder; if there were truly blood stasis in the Bladder, it would resemble hematuria with difficult urination, so how could urination be free while stool is black? Moreover, this syndrome includes symptoms like confusion and delirious mania, which are Heart syndromes; because the Heart and Small Intestine meridians are connected, their Qi mechanisms are also interconnected.

Tao Jian'an stated: In Shanghan (Cold Damage), the appearance of a Chen Mai (Deep Pulse) can distinguish Yin and Yang. He meant that when pathogens are in the San Yang meridians, the pulse is always Fu (Floating). When the pulse becomes Chen (Deep), it involves the interior of San Yang and the San Yin meridians. However, a Fu pulse that is weak and lacks spirit indicates extreme Yin deficiency, which is even more dangerous than pathogens sinking into the interior causing excess syndromes. Zhang Jingyue emphasized this point significantly, correctly asserting that one cannot say all Yin pulses are Deep, nor that Floating pulses exclude Yin conditions.

San Yin diseases all present with vomiting, diarrhea, and cold limbs. This is because when pathogens enter San Yin, they do not immediately invade the Zang organs but first disturb the Fu organs. Cold pathogens in the Fu organs cause various symptom changes; if they affect the Zang organs, death ensues. *Ling Shu* (Spiritual Pivot) states: "Pathogens striking Yin flow into the Fu organs," which is correct. Furthermore, vomiting relates to the Stomach, diarrhea to the Large Intestine, and limbs to the Spleen; thus, when pathogens enter San Yin, the Spleen and Stomach are most critical. If Spleen and Stomach function remains intact, even if pathogens enter the interior, treatment is easier.

The Gallbladder is a "Clear" Fu organ with no direct openings for entry or exit, so the three methods (sweating, emesis, purging) are contraindicated. However, the prohibition regarding Shaoyang (Lesser Yang) applies to its meridian; does meridian Qi lack movement? If pathogens move inward, they may not stay in the Gallbladder but move to the San Jiao (Triple Burner). San Jiao governs Qi transformation; although it lacks tangible excess like in Didang Tang or Chengqi Tang, methods regulating Qi ascent and descent are more relevant to San Jiao than Gallbladder. Therefore, Da Chaihu Tang (Major Bupleurum Decoction) includes purgatives specifically for San Jiao. Thus, Danxi in *Mai Yin Zheng Zhi* (Pulse, Etiology, Syndrome, and Treatment) noted that while Shaoyang prohibits the three methods, they are sometimes appropriate.

Diarrhea in San Yin should be contrasted with dryness-excess in Yangming. Cold-solid stool in San Yin is called "Yin Jie" (Yin Knot); diarrhea in San Yang is called "Xie Re Li" (Collateral Heat Diarrhea). Is there cold diarrhea? Yes, cold diarrhea is synonymous with San Yin disease.

There are six external pathogenic factors, yet Zhang Zhongjing titled his treatise *Shanghan Lun* (Treatise on Cold Damage). Fang Zhongxing and Zhang Yinan erroneously linked San Yin and San Yang strictly to the Six Qi (Six Climatic Factors). One should understand that mastering the principles of this book allows application to treating Six Qi disorders, though such comprehensive mastery is rare.

When Shanghan pathogens are at the exterior, they are differentiated by Six Meridians; upon entering the interior, they are differentiated by San Jiao. Wu Jutong stated that Warm Diseases are differentiated by San Jiao; why should Shanghan not also use San Jiao differentiation? This is correct, but failing to specify exterior vs. interior makes the explanation unclear.

Shaoyin (Lesser Yin) encompasses both kidneys, where Water and Fire reside together. Cold pathogens combine with Water Qi, suppressing Fire, resulting in a Chen Xi (Deep and Fine) pulse and somnolence, indicating suppressed Yang unable to extend. Suppressed Fire attempts to extend, causing irritability and nausea. Some ask: Since Shaoyin connects to the Heart, isn't irritability a Heart syndrome? If cold truly entered the Heart, coma would occur, not mere irritability. This irritability arises because Yuan Fire (True Fire) in the lower origin is suppressed by cold, unable to discharge, forcing a thread of Yang Qi upward to disturb Pericardium Qi.

The Heart does not accept pathogens directly; Shaoyin is spared, replaced by Shoujueyin Pericardium Meridian. The Pericardium is the outer palace of the Heart. Women's "Heat Entering Blood Chamber" is equivalent to men's "Heat Entering Pericardium." Zhang Zhongjing treated Heat Entering Blood Chamber with Xiao Chaihu Tang (Minor Bupleurum Decoction). Ye Tianshi contraindicated Chaihu for this condition, which was insightful. Xu Lingtai mocked him rashly. Careful analysis shows Xiao Chaihu Tang treats Shaoyang heat affecting the Pericardium. True Heat Entering Pericardium presents with stillness, coma, and delirium; excess heat may cause mania. Why no Cold Entering Pericardium? Although Pericardium substitutes for the Heart, it is a pure Fire organ closely linked to Spirit. If Cold/Water invades, it causes Fire decline and Spirit departure, leading to rapid death. Thus, Cold Damage rarely reaches Pericardium unless it transforms into Heat, attracting similar Qi. Hence, Heat enters Blood Chamber/Pericardium, but Cold does not (if it did, death would follow). Symptoms like vomiting, diarrhea, aversion to cold, curling up, cold limbs, and restlessness indicate fading Heart Yang.

Constipation with tidal fever, delirium, and confusion mirrors Heat invading Pericardium because both involve Heat in the Blood aspect. Differentiate by pulse: Pericardium Heat shows a Huan Hua (Slow and Slippery) Left Cun pulse; constipation shows a Chang Shi (Long and Excess) Right Chi pulse. Additionally, Shaoyin cough with diarrhea and delirium results from forced sweating via fire therapy, causing difficult urination. Shanghan with Fu pulse treated with fire therapy causing panic and restlessness requires Niujtang (Rescue Decoction). Both are cases of Yang collapse from forced sweating. Sweat is Heart fluid; deficiency fails to nourish Spirit. Delirium stems from Spirit confusion due to Pathogen affecting or disturbing Pericardium. For example, Liver overacting on Spleen causes abdominal fullness and delirium with Chen Jin (Deep and Tight) Cun Kou pulse, named "Zong"; puncturing Qimen acupoint works because Pathogen interacts with Heart.

Shanghan transmission involves pathogens extending from one meridian to another or shifting forward/backward. He Bing (Combined Disease) and Bing Bing (Concurrent Disease) involve excess pathogens reaching specific meridians. Sometimes pathogens in one meridian show symptoms of another; Yang meridian pathogens show Yin symptoms. How can symptoms appear without pathogen entry? Meridians connect, and Qi resonates; boundaries exist but are permeable. For instance, Shaoyang pulse Fu Da rising to Guan, somnolence, and sweating upon closing eyes reflect Shaoyin Heart syndrome, where Heart Qi cannot withstand Shaoyang dispersal. This is Qi resonance, not pathogen migration. Generally, Cold resonates with Cold (Lung/Kidney), Heat with Heat (Heart/Liver). This differs from transmission patterns. Some resonate before pathogen arrival; some resonate but pathogen doesn't arrive. Predecessors debated transmission endlessly without explaining this. Wang Xunchen criticized meridian differentiation, ignoring that tangible pathogens have boundaries.

*Shanghan Lun* and *Jingui Yaolue* prescribe formulas for fatal syndromes, a profound concept.

Shanghan has different syndromes with same treatments (e.g., non-thirsty diarrhea in Taiyin vs. thirsty diarrhea in Shaoyin, both treated with Sini Tang [Frigid Extremities Decoction]); and same syndromes with different treatments (e.g., Yangming diarrhea with abdominal pain requires purging; Taiyin diarrhea requires caution against purging to avoid chest stiffness). Similarities have reasons (Shaoyin thirst uses Sini Tang due to clear urine/lower jiao cold; Taiyin no-thirst due to organ cold); differences have reasons (appetite status). Readers must study these nuances after memorizing texts to avoid confusion.

Studying *Zhi Yao Da Lun*, "Victorious Qi arrives, Repaying Qi lies dormant." Treating Victorious Qi must consider Repaying Qi to avoid exacerbating latent Qi. Otherwise, cycles continue. Shanghan formulas combining cold/hot or salty/sour/bitter/pungent herbs reflect consideration of Repaying Qi. Repaying Qi is strongest in Shaoyang and Jueyin (*Neijing*: Fire/Dryness/Heat). Sweating injures Yang (Cold rises if Yin excess); purging injures Yin (Heat rises if Yang excess). Sweating may increase Yang heat; purging may deepen Yin cold due to herb properties. General rule: For Fire excess, use Salty taste (corrective), aided by Sweet (child Qi, guides outflow) and Sour (mother Qi, protects root). Fire's repaying Qi is Water; Sweet controls Water, Sour drains Water. So, for Fire excess, use Sour to repay. Wang Bing notes: Not repaying Qi leaves excess Qi empty, inviting damage. This subtlety must be understood by *Shanghan* readers.

Treating disease seeks the Root. There is a common Root for all diseases and specific Roots for each. Treaters should target specific Roots for precision. Xue Lizhai focused on True Water/Fire (Common Root), whereas *Shanghan* and *Jingui* treat specific roots precisely. Readers should analyze formula ingredients based on syndrome origin, meridian excess/deficiency, parent-child relationships, and root/branch dynamics to develop independent insight.

When studying established formulas, compare listed symptoms with ingredient actions. Modify or annotate if discrepancies arise. *Shanghan* and *Jingui* contain formulas hard to apply today; commentators often force interpretations. Shu Chiyuan questioned doubts but unfairly blamed ancient errors. Real delirium is excess; Zheng Sheng (repetitive speech) is deficiency. However, delirium has excess types (Yangming fu-excess, collateral heat diarrhea, heat entering blood chamber, Taiyang blood stasis) and deficiency types (excessive sweating/Yang collapse, excessive purging/Yin collapse, fluid exhaustion causing Spirit loss). Zheng Sheng is actually "Evil Sound" (weak Qi causing voice changes late in speech), not true deficiency.

Six Meridians chapters list Wind Strike pulse/syndromes to contrast Shanghan. Wind Strike may not involve Cold, but Shanghan always involves Wind carrying Cold. Severe is Zhong Han (Direct Cold), moderate is Shang Han, mild is Feng Zhong (Wind Strike). Thus, Six Meridians Wind Strike exterior syndromes imply Shanghan exterior syndromes. Tao Jian'an's "Direct Invasion" theory is valid, supported by *Neijing*. Initial Wind-Cold in meridians is exterior in nature, treated with warming dispersion. Sunyang chapter details initial injuries.

Shanghan begins similarly to Wind Strike; fatal cases resemble Direct Cold.

*Shanghan Lun* describes only Cold fatal syndromes, not Heat. Baihu Tang and Chengqi Tang are not inherently fatal. In Warm Diseases, it is reversed.

"Fan" (Reverse/Opposite) has multiple meanings: "Unexpectedly," "Yet," or "Again." E.g., "Weak pulse unexpectedly at Guan" means "just." "Unable to eat, yet able to eat" means unexpected. "Initial onset, yet fever with deep pulse" implies using Mahuang Xixin Fuzi Tang despite deep pulse because exterior signs persist. Readers must interpret contextually, not rigidly.

*Shanghan* discusses external infections; *Jingui* also covers external signs. No Qiang Huo (Notopterygium) is used. Even for Wind-Damp, only Mahuang, Yiyi Ren, Fuzi, Baizhu, Huangqi, and Fangji are used.

Commentators claim each meridian chapter has a guideline. Later mentions of "Meridian Disease" refer to these guidelines. However, Taiyin disease with Fu pulse suggests sweating with Guizhi Tang. If symptoms are purely internal cold (abdominal fullness, vomiting, diarrhea, pain), is Fu pulse alone sufficient for sweating? Such cases require careful analysis; reading must not be rigid.

Extreme Cold reverses to Heat; Extreme Heat reverses to Cold (Qi transformation). Predecessors distinguished true/false but didn't explain the mechanism. Saying "internal Cold forces external Heat" is insufficient. True reverse heat: No external heat signs, but dry mouth wanting water, rinsing without swallowing, scant urine, constipation with occasional loose stools. This is internal Cold binding, faint Yang failing to transform fluids ("cold water turning to ice"). True reverse cold: No external cold signs, but surging Heat drying fluids, obstructing Qi flow like a runner tripping. Predecessors vaguely suggested restoring Yang or clearing Heat. Actually, treat false heat by guiding Fire back to source; treat true reverse heat by warming and transforming fluids. Treat false cold by generating fluids/Qi; treat true reverse cold by generating fluids and venting Qi. False cold/heat are wandering deficient Qi; true reverse cold/heat are illusory extremes mirroring the opposite extreme.