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[Chart of Si Tian and Zai Quan]

Si Tian (Heavenly Ruler), Zai Quan (Earthly Ruler), and the four interstitial Qi (Jian Qi) constitute the six steps of Ke Qi (Hostile/Periodic Qi). That which governs the year is called Si Tian, positioned at the third step (San Zhi Qi). Opposite to Si Tian is Zai Quan, positioned at the final step (Zhong Zhi Qi). To the left of Si Tian is the Left Interstitial Qi of Heaven; to the right is the Right Interstitial Qi of Heaven. The Ke Qi of each year begins at the Earth's Left Interstitial Qi, two positions preceding Si Tian, which is designated as the First Qi (Chu Qi), proceeding through the Second and Third Qi, finally reaching the Sixth Qi of Zai Quan. Each Qi governs its respective segment. However, Si Tian generally governs the first half of the year, and Zai Quan generally governs the second half of the year. Thus, it is said: before the mid-year, Heavenly Qi governs; after the mid-year, Earthly Qi governs.

In the Zi and Wu years, Shao Yin Jun Huo (Ministerial Fire) rules Si Tian; in Chou and Wei years, Tai Yin Shi Tu (Damp Earth) rules Si Tian; in Yin and Shen years, Shao Yang Xiang Huo (Ministerial Fire) rules Si Tian; in Mao and You years, Yang Ming Zao Jin (Dry Metal) rules Si Tian; in Chen and Xu years, Tai Yang Han Shui (Cold Water) rules Si Tian; in Si and Hai years, Jue Yin Feng Mu (Wind Wood) rules Si Tian. The First Qi starts from the Earth's Left Interstitial Qi, counted according to the opposing positions of Si Tian and Zai Quan.

The proverb says: "If one does not study the Five Movements and Six Qi, what use is it to search through all medical formularies?" Therefore, those who dabble in medical principles often take studying "Si Yun" (Ruling Movements) as their primary task. Little do they know that chapters such as *Tian Yuan Ji* were originally not part of the *Su Wen* (Basic Questions); Wang Bing selected content from the *Yin Yang Da Lun* (Great Treatise on Yin and Yang) to supplement the classic. Later generations regarded these as sayings of ancient sages, daring not to criticize them. In reality, they have little relevance to actual medical practice. Moreover, the text explicitly states that while seasons have fixed positions, Qi has no absolute certainty. Such detailed discussion serves only to exhaustively explore the theory. Even if there are regular patterns of Sheng (Overcoming) and Fu (Rebounding) Qi, policies differ between North and South, terrain varies in height and depth across the four directions, and seasonal transformations may occur out of time. Within a hundred paces, weather conditions may differ; a thousand miles away, cold and warmth may vary. How can one apply rigid methods to accommodate extraordinary changes? Memorizing this for consultation purposes is acceptable, but regarding it as a fixed method for treating diseases leads to rigidity and lack of flexibility.

Shang Han (Cold Damage) disease does not present solely with exterior syndromes. However, the Tai Yang (Greater Yang) channel belongs to Cold Water and governs the exterior of the body. Any disease originating from the Tai Yang channel is named Shang Han. Its symptoms include headache, neck stiffness, fever, and aversion to cold. If sweating is present at the onset, Gui Zhi Tang (Cinnamon Twig Decoction) is used; if no sweating is present, Ma Huang Tang (Ephedra Decoction) is used. If treated correctly, once the exterior syndrome is resolved, no further treatment is needed. However, Tai Yang governs the outermost layer of the exterior, while Yang Ming (Yang Brightness) governs the muscles, which also belongs to the exterior category. Its symptoms include body heat, eye pain, nasal dryness, and insomnia; typically, Sheng Ma Ge Gen Tang (Kudzu Root and Rhizoma Cimicifugae Decoction) is employed. Shao Yang (Lesser Yang) governs the chest and hypochondrium, also belonging to the exterior category. Its symptoms include chest and hypochondriac pain, vertigo, bitter taste in the mouth, and deafness; *Shang Han Lun* employs Xiao Chai Hu Tang (Minor Bupleurum Decoction) with modifications. Since the Three Yang channels all belong to the exterior, they are listed under exterior syndromes.

(Interpretation of Classic Text) The *Su Wen · Re Bing Lun* (Treatise on Febrile Diseases in the Basic Questions) states: "When humans are injured by Cold, they develop febrile diseases." This refers primarily to the transmission of the Six Qi, where the condition remains a febrile disease from beginning to end. Zhang Zhongjing followed the theory of Six Qi, deriving the main principles from opposite and lateral perspectives. He correlated the formless Qi of the Three Yang and Three Yin with the有形 (form/solid) skin, muscle meridians, skeletal structure, and the deficiency-excess and cold-heat conditions of the five zang and six fu organs, establishing his discourse based on standard procedures and adaptive variations. Due to space limitations, not all can be included here. I have authored *Shang Han Lun Qian Zhu* (Light Commentary on the Treatise on Cold Damage), which is highly suitable for careful study.