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The *Jingui Yaolue* (Essentials of the Golden Cabinet) states: Fever accompanied by exterior syndrome is called exterior heat; fever without exterior syndrome is called interior heat. Sudden onset of persistent fever is caused by long-standing stagnation and lack of circulation.

Fever resulting from the administration of warm herbs, or fever accompanied by aversion to cold and shivering, should be treated as follows: For mild heat, use cool-natured substances to harmonize it; for severe heat, use cold-natured substances to eliminate it; for extreme heat, induce sweating to disperse it. If sweating fails to completely resolve the pathogen, employ reverse restriction methods; if restriction fails, seek the underlying root cause to weaken the disease. Bitter substances treat the Zang organs (Yin organs located internally), while pungent substances treat the Fu organs (Yang organs located externally). Therefore, diseases in the interior are treated with purgation, and diseases in the exterior are treated with diaphoresis. It is also advisable to nourish blood and benefit Yin, after which the heat will naturally subside.

Commentary: This passage generally discusses the concepts of exterior and interior but does not distinguish between the origin (Ben) and manifestation (Biao) of the pathogenic factors. However, by observing the preceding and subsequent discussions, the pathogenesis becomes clear.

The *Neijing* (Inner Canon) states: "Yin deficiency generates interior heat" occurs due to overexertion, leading to a decline in physical form and Qi. Grain Qi becomes excessive in the Upper Jiao, failing to circulate downwards, causing blockage at the lower epigastrium. Stomach Qi becomes hot, and the heat steams upward into the chest, thus generating interior heat. "Yang excess generates exterior heat" occurs because the Upper Jiao is obstructed, leading to dense skin, closed acupoints (Couli), and blocked sweat pores (Xuanfu), preventing the diffusion of Wei Qi (Defensive Qi), thus generating exterior heat. "Yang deficiency generates exterior cold," and "Yin excess generates interior cold" (see section on Cold).

Zhao Sizhen notes: The discussion of four syndromes of Yin-Yang Deficiency-Excess in the *Suwen* (Basic Questions) pertains to miscellaneous diseases. The text in Chapter 6 of the *Nanjing* (Classic of Difficult Issues) discusses pulse diagnosis. The text cited in the *Waitai Miyao* (Essential Prescriptions Worth a Thousand Gold for Emergencies) discusses exterior-interior patterns in Shanghan (Cold Damage). However, the principles of Yin-Yang deficiency-excess advocated by Zhang Zhongjing are inherently included therein. The classic states: "When pathogenic factors are excessive, it is Excess; when vital essence is depleted, it is Deficiency." Because the righteous Qi (Zheng Qi) is initially deficient, allowing pathogenic factors to invade and appear excessive, two distinct syndromes arise: Yang deficiency with Yin excess, and Yin deficiency with Yang excess.

The *Huo Ren Shu* (Book that Revives People), however, merges the four syndromes of Yin-Yang deficiency-excess discussed in the *Suwen* regarding miscellaneous diseases to validate Zhang Zhongjing’s two syndromes in Shanghan. It alters "Yang excess [causing] exterior heat" to "interior heat" and "Yin excess [causing] interior cold" to "exterior cold." This interpretation initially lacks coherence but expands upon Zhang Zhongjing’s principles with detailed explanation. Generally, "Excess" refers to pathogenic factors, and "Deficiency" refers to righteous Qi, illustrating the dynamic balance between Yin-Yang deficiency-excess and the waxing-waning of pathogens versus righteousness.

Furthermore, within the human body, Yang governs the exterior and Yin governs the interior. When pathogens invade, the exterior is considered Yin and the interior Yang. If the True Yang of the exterior is initially deficient, Yin pathogens invade and become excessive; this is Yang deficiency affecting the exterior. A floating and tight pulse indicates Yin pathogens excessive in the exterior, termed Yang deficiency with Yin excess. Therefore, pungent-sweet warm formulas like Guizhi Tang (Cinnamon Twig Decoction) and Mahuang Tang (Ephedra Decoction) are used to induce sweating, eliminating Yin pathogens and restoring True Yang, thereby achieving health through the removal of pathogens and preservation of righteousness.

Conversely, if the True Qi of the interior is initially deficient, Yang pathogens enter and become excessive; this is Yin deficiency affecting the interior. A deep and forceful pulse indicates Yang pathogens excessive in the interior, termed Yin deficiency with Yang excess. Therefore, sour-bitter cold formulas like Chengqi Tang (Order the Qi Decoction) are used to purge, eliminating Yang pathogens and restoring True Yin. If one acts contrary to this—for example, using Guizhi Tang for Yang excess—death ensues upon ingestion; using Chengqi Tang for Yin excess leads to demise. These errors result in "making Excess more Excess and Deficiency more Deficient," causing pathogens to prevail and righteousness to fail. Thus, the profound meaning of Zhang Zhongjing’s Yang deficiency/Yin excess and Yin deficiency/Yang excess lies in distinguishing between exterior and interior patterns of pathogenic-righteous interaction, rather than merely discussing superficial vs. internal diseases. "Harmony" refers to areas without disease, while "Deficiency" refers to areas affected by disease. This discourse captures the essence of Zhang Zhongjing’s methodology.

Regarding "Yin deficiency generates interior heat," Dongyuan and Danxi further discuss "Yin deficiency generating exterior heat." Regarding "Yang excess generating exterior heat," Hejian often cites examples of "Yang excess generating interior heat." These are all derivations of pathogenesis. The *Jing* (Classics) refer to endogenous syndromes and fundamental diseases, whereas Hejian, Dongyuan, and Danxi describe extreme variations and branch manifestations, following the principle of "Hyperactivity causes harm, and control restores balance."

Additionally, Zhang Zhongjing discusses Yin-Yang interplay, naming it "Moving Yang." "Movement" indicates Yang deficiency, leading to sweating; "Moving Yin" indicates Yin deficiency, leading to fever. If there is no sweating but fever accompanied by cold limbs and aversion to cold, the Sanjiao (Triple Burner) is injured. If doctors mistakenly apply sweating or purging, causing aversion to cold and fever, these may stem from Yin-Yang disharmony or sinking of Yin-Yang. Such cases go beyond simple Yin-Yang deficiency-excess, differing only in etiology and extreme variations.