The syndrome of Shanghan (Cold Damage) was called a major disease in ancient times. Among the meridians, the Taiyang (Greater Yang) meridian is characterized by its numerous complexities and complicated treatment methods, surpassing other meridians. Therefore, distinguishing it is not easy. However, it is not indistinguishable.

Generally speaking, the primary pathological manifestations and standard treatment principles of the Taiyang meridian amount to no more than twenty-plus clauses. The rest are all adaptive methods (Quan Bian Fa), mediating methods (Wo Xuan Fa), rescue methods for adverse reactions (Jiu Ni Fa), and differential diagnosis methods for similar diseases (Lei Bing Fa).

If a practitioner treating Shanghan examines whether the pulse is slow or rapid, and distinguishes whether there is sweating or not, they can then apply diaphoretic and resolving methods. By using formulas such as Guizhi Tang (Cinnamon Twig Decoction) and Mahuang Tang (Ephedra Decoction), the pathogenic factor is expelled and the disease resolves.

In cases where the disease combines with the Yangming (Yang Brightness) meridian, or the Shaoyang (Lesser Yang) meridian, or involves all three Yang meridians simultaneously, one should employ methods to resolve the exterior and clear internal heat. Formulas like Gegen Tang (Puerariae Decoction), Huangqin Tang (Scutellariae Decoction), and Baihu Tang (White Tiger Decoction) also lead to the dispersion of pathogens and resolution of the disease. These constitute the standard treatment methods (Zheng Zhi Zhi Fa).

However, human constitutions vary in deficiency and excess, and zang-fu organs differ in yin and yang. Some patients may have pre-existing conditions such as phlegm-fluid retention, epigastric fullness (Pi Qi), dry throat, stranguria, sores, spontaneous sweating, or epistaxis. Others may be in a debilitated state due to sexual exhaustion, trauma from metal or blades, or postpartum blood loss. Although these present as Shanghan syndromes, one cannot simply adhere to the methods of Mahuang Tang or Guizhi Tang. Thus, formulas such as Xiaojianzhong Tang (Minor Construct the Middle Decoction), Zhigancao Tang (Honey-Processed Licorice Decoction), Daqinglong Tang (Major Green Dragon Decoction), Xiaoqinglong Tang (Minor Green Dragon Decoction), and Guizhi Er Mahuang Yi Tang (Guizhi-Two-Mahuang-One Decoction) are employed. This represents the adaptive method (Quan Bian Zhi Fa).

Nevertheless, when using Guizhi Tang or Mahuang Tang, one cannot guarantee the absence of errors due to excess or insufficiency. If sweating is incomplete and the pathogen does not disperse outward, the disease may transmit to other meridians, leading to jaundice or blood stasis accumulation. If sweating is excessive, damaging the yang qi, symptoms such as tremors, falling prone, muscle twitching, and tendon spasms may occur. Consequently, methods to induce further sweating or use specific medicinals for sweating, along with Zhenwu Tang (True Warrior Decoction) and Sini Tang (Four Reversal Decoction), are utilized. This is the mediating method (Wo Xuan Zhi Fa).

Moreover, medical practice has long been neglected, and treatment methods are rarely mastered. Practitioners might mistakenly use purgatives instead of diaphoretics when appropriate, or apply diaphoretics after purgation. They might also indiscriminately combine warm needle therapy, moxibustion, and water therapies. This leads to complications such as chest binding (Jie Xiong), epigastric fullness, diarrhea with heat, irritability and insomnia, inner irritability with hunger but lack of appetite, fright and mania, or cutaneous gooseflesh. In response, formulas like Daxiexiong Tang (Major Xiexia Decoction), Xiaoxiexiong Tang (Minor Xiexia Decoction), various Xiexin Tangs (Xiexin Decoctions), and Wen Ge San (Clam Shell Powder) are prescribed. This constitutes the rescue method for adverse reactions (Jiu Ni Zhi Fa).

Regarding pathogenic factors from heaven, there are six types of exogenous evils (Six Yin: Wind, Cold, Summerheat, Dampness, Dryness, Fire). The Taiyang meridian is susceptible to more than just cold. Besides Shanghan, there are syndromes such as Fengwen (Wind-Warmth), Wenbing (Warm Disease), Fengshi (Wind-Dampness), Zhongshi (Dampness Strike), Shiwen (Damp-Warmth), Zhongshu (Heat Stroke), and Huoluan (Cholera-like disorder). Their clinical presentations resemble Shanghan, but their treatments differ. Thus, formulas containing Guizhi and Fuzi, Shu Fu Tang (Atractylodes and Aconite Decoction), Mahuang Baizhu Tang (Ephedra and Atractylodes Decoction), Gouti San (Melon Seed Powder), Renshen Tang (Ginseng Decoction), and Baihu Tang (White Tiger Decoction) are used. This is the method for differentiating similar diseases (Shanghan Lei Bing Fa).

To shake a fur coat, one must grasp the collar; to organize a net, one must lift the main rope. Without grasping this key principle, merely engaging in trivial distinctions, even if extremely clear, would be of little practical use. Here, the main outlines are briefly presented first, followed by detailed categorization. The formulas and methods of Zhang Zhongjing are comprehensively listed. Thus, the myriad complexities of the Taiyang meridian are unified into a coherent system, akin to holding every bead of a hundred-and-eight-bead rosary in hand. The same logic applies to the Six Meridians, as detailed in subsequent chapters.