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Er Yang (Two Yangs), refers to the Foot-Yangming Stomach Meridian and the Hand-Yangming Large Intestine Meridian. Diseases of these two meridians originate from the Heart and Spleen, because "pensive thinking" (Si) is the emotional manifestation of the Spleen, yet its root actually lies in the Heart. Initially, it stems from frustration regarding matters that are difficult to disclose or resolve privately. Consequently, pensive thinking causes Qi stagnation, which condenses into Fire, leading to the secret consumption of Ying Blood (Nutritive Blood) of the Heart. This prevents the Heart Fire from descending to interact with the Kidney Water. Meanwhile, Spleen Earth becomes stagnant and counter-attacks Kidney Water. Thus, men suffer from scanty essence, and women experience amenorrhea (absence of menstruation). This is essentially due to Kidney dryness and the desiccation of blood and fluids. Furthermore, if the Spleen has stagnant Fire, it transmits between the interior and exterior, causing Stomach Jin Ye (Body Fluids) to dry up. Since the Large Intestine serves as the conduit for the Stomach's transmission of waste, it also becomes diseased. These Er Yang diseases should be discussed in terms of Dryness-Fire syndromes: in the Stomach, they manifest as Xiao Ke (Wasting-Thirst/Consumption) and Ge Ju (Regurgitation/Inability to swallow); in the Intestines, they manifest as Bi Mi (Constipation) and Bian Bian (Hard stools). When the Stomach Fu organ is already dry, and the Spleen cannot transport and distribute Jin Ye, Feng Xiao (Wind Consumption) occurs. Feng Xiao means that excessive Fire generates Wind, exhausting the Spleen and causing muscle emaciation. If the Dryness of the Large Intestine transmits to the Lung, Xi Ben (Panting with Rushing Breath) occurs. Xi Ben refers to breathing accompanied by sounds and upward rushing of Qi that fails to descend. With four Zang organs and two Fu organs burning each other mutually, Yin fluids are completely exhausted; therefore, it is directly judged as "incurable death." In the past, Wang Andao argued that diseases of the intestines and stomach extended to the Heart and Spleen, reversing the causal relationship, and omitted an explanation for the cause of "frustration regarding private matters." Yu Jiayan elucidated this somewhat clearly but only stated what must happen, without exploring the underlying reasons; thus, I elaborate further here.

The *Nan Jing* (Classic of Difficult Issues), Difficulty 29, states: "Diseases of the Yang Wei Mai (Yang Linking Vessel) cause aversion to cold and fever; diseases of the Yin Wei Mai (Yin Linking Vessel) cause heart pain." Yue Ren (Bian Que) provided only this description without treatment methods. Later practitioners used Gui Zhi Tang (Cinnamon Twig Decoction) for treatment, which can be considered hitting the mark accurately. Because "Yang Wei links with Yang," it belongs to the Wei Qi (Defensive Qi) level, hence presenting with chills and fever; "Yin Wei links with Yin," it belongs to the Ying Qi (Nutritive Qi) level, hence presenting with heart pain. Gui Zhi Tang possesses the power to harmonize Ying and Wei Qi and regulate Yin and Yang, making it suitable for treating this condition when used in appropriate proportions.

Since the times of Xuanyuan (Yellow Emperor) and Qibo, there have been hundreds of authors writing books and establishing theories. Long revered as the Four Great Masters are Zhang Zhongjing, Liu Wansu, Li Dongyuan, and Zhu Danxi. This is largely because Zhongjing first discussed Shang Han (Cold Damage), establishing laws and formulas, truly serving as the Ancestor of Medicine. Shouzhen (Liu Wansu) further established theories based on Spring Warmth and Summer Heat, supplementing what Zhongjing left incomplete. Dongyuan (Li Gao) detailed the distinction between Pi Wei (Spleen and Stomach) internal injury and external contraction, supplementing what Zhang and Liu left incomplete. Danxi (Zhu Zhenheng) specialized in tonifying Yin, again supplementing what Dongyuan left incomplete. Each had their own insights, complementing each other's omissions, a point already clearly explained by Li Shicai. However, I argue that while saying Zhang and Liu discuss external contractions, and Li and Zhu discuss internal injuries, seems comprehensive, it is not entirely so. Xue Lizhai of the Ming Dynasty asserted that the principles of thirteen medical departments are unified. He lamented that surgeons stubbornly adhered to fixed prescriptions without investigating the root of diseases, specifically proposing the theory of unity between internal and external medicine. Furthermore, noting that conditions vary and modern people are physically weak and deficient, he strongly criticized cooling drugs and advocated warm tonification. This supplemented what Danxi left incomplete and broadened perspectives. Consequently, works by Zhang Jingyue and Li Shicai became popular, being essentially extensions of Xue Lizhai's legacy. Although internal injury syndromes were detailed uniquely by Li regarding the Spleen and Stomach, and by Zhu and Xue regarding Yin-Yang differentiation, leaving little unexplored, there remains the issue of direct summer heat pathogenic factors contracted in summer. Although Jiegu (Zhang Yuansu) and Dongyuan distinguished Yin and Yang via movement and stillness, which seemed reasonable, it was Zhang Fengkui who clarified the principles of summer heat damage, specifically authoring the *Quan Shu*, demonstrating profound insight. Additionally, Yu Jiayan not only deduced winter warmth, spring warmth, and latent qi diseases from Zhongjing's texts, revealing hidden meanings in books untouched for millennia like a torch in the dark, but also identified the category of Autumn Dryness, pointing out the error in the *Nei Jing*'s statement "Autumn damages with Dampness." This can be called a unique vision through the ages; thus, the theory of Heaven's Six Qi became fully complete. Moreover, beyond the Six Qi, there are epidemic pathogens contracted externally. While various books elaborated on these, none explored their transformations as thoroughly and explicitly as Wu Keyou, thereby supplementing the deficiencies of previous scholars studying external contractions. Other literature is piled high like beams; although each contains valuable theories and effective prescriptions, no work is perfect without flaws elsewhere. Even those with vast knowledge and numerous volumes do not surpass the four masters—Xue, Zhang, Wu, and Yu—who each opened new avenues to fill the gaps of Zhang, Liu, Li, and Zhu. Reading their works, one dare not overlook their merits; therefore, I specifically promote them as the Eight Great Masters.