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I have said that Zhang Zhongjing’s (the Grand Commandant of Changsha) discussions, while rigorous, contain flexible and adaptable methods. The atmospheric changes of heaven are not uniform; there is the Principal Qi (the fixed climatic pattern of each year) and the Guest Qi (the rotating, abnormal climatic patterns). The nature of human internal organs also varies; there are Yin Zang (constitutions prone to internal cold due to deficient Yang) and Yang Zang (constitutions prone to internal heat due to excess Yang). Spring warmth, summer heat, autumn dryness, and winter cold are the fixed positions of the four seasons; Jueyin Wind transforms into the First Qi, Shaoyin Heat transforms into the Second Qi, Shaoyang Fire transforms into the Third Qi, Taiyin Dampness transforms into the Fourth Qi, Yangming Dryness transforms into the Fifth Qi, and Taiyang Cold transforms into the Final Qi; this is the constant measure of the Six Steps. Therefore, Cold must injure in Winter, while Warmth and Heat must occur in Spring and Summer; this is the general rule.

However, speaking from the perspective of the Five Movements (Wu Yun), the succession of Earth Movement and Metal Movement is irregular; speaking from the perspective of the Six Qi, the influences of Si Tian (Qi governing the first half of the year), Zai Quan (Qi governing the second half of the year), Zuo Jian (Left Intercalary Qi), and You Jian (Right Intercalary Qi) differ individually. Added to this are the principles of Sheng and Fu (excess and deficiency leading to counter-reaction), and the numbers of Tai Guo (excess) and Bu Ji (deficiency). Thus, there can be Shang Han (Cold Damage) in Spring and Summer, and Wen Bing (Warm Diseases) in deep Winter. This is what is meant by "having the Qi of one season at the time of another," and one cannot rigidly adhere to the norms of solar terms and seasons. *Su Wen* states: "In places of extreme height, Winter Qi is always present; in places of extreme depth, Spring Qi is always present." *Ling Shu* states: "Dividing a day into four times, sunrise is Spring, noon is Summer, sunset is Autumn, and midnight is Winter." All these are discussed from the perspective of flexible methods.

Those who interpret the matter insist that Shang Han must be treated in Winter, and that Guizhi Tang (Cinnamon Twig Decoction), Mahuang Tang (Ephedra Decoction), and various acrid-warm agents for the Three Yin diseases must only be used for Winter Shang Han, and cannot be applied in Spring and Summer. Being rigid about seasonal timing, do they not lack completeness regarding the principle of change? *Tongtian Pian* states that there are Ta Yin persons, Shao Yin persons, Tai Yang persons, Shao Yang persons, and Yin-Yang Peaceful persons, along with the governance of twenty-five types of people derived from five groups of five. *Ben Zang Pian* further discusses twenty-five variations. In the past, Xia Yinggong (Xia Song) had an exceptional constitution; when sleeping, his body was as cold as if stiffened. He constantly consumed Xian Mao (Curculiginis Rhizoma), Zhong Ru (Gypsum Fibrosum), and Liu Huang (Sulfur) without limit. A minor clerk secretly ate some and died from developing a carbuncle. Conversely, Tai Yang persons (those with abundant Yang) consume raw and cold foods without restraint and indulge in sexual activity without fatigue. The reversal of Yin and Yang between them is like heaven and earth. Therefore, there is the theory of Cong Hua (following transformation). "Cong" refers to following the macro-climate; "Hua" refers to the nature of the Zang organs. Those with Yin Zang tend toward cold; those with Yang Zang tend toward heat. This is called "Yin follows Yang transformation, Yang follows Yin transformation, heat follows cold transformation, and cold follows heat transformation." These are all discussed as flexible methods, and one should especially not be bound by the norms of warm, cool, cold, and hot treatments.

Due to the differences in Principal and Guest Qi, combined with the distinctions of Zang-Fu Yin and Yang, diseases manifest in thousands of forms, impossible to fully exhaust or interrogate. Master Zhong (Zhang Zhongjing) expounded the profound meaning of Xuanyuan (Yellow Emperor) and Qibo, broadening the use of Tang Ye (decoctions), summarizing them into 397 methods and 113 formulas (excluding *Jingui Yaolue*). The subtlety of his established methods and the flexibility of treatment integrate seamlessly with the changes of Principal, Guest, Yin, and Yang. This depends on whether the practitioner can divine and clarify it. Later scholars debated the Foot meridians, Cu Bing (acute diseases), the Three Guidelines (Wind injuring Wei, Cold injuring Ying, Wind-Cold injuring both Ying and Wei), Six Qi, and the idea that Shang Han is merely one type of miscellaneous disease, engaging in endless disputes. Returning to seek answers in Master Zhong’s books, the vibrant vitality within them inherently encompasses everything. Therefore, treating diseases requires flexible methods.