Yan Yong-he (1200~1268), styled Zili, was a pragmatic medical practitioner. At the age of 12, he began studying medicine under Liu Kai, and by 17, he started practicing medicine. He practiced for over 50 years, believing that the changes of the world differ between ancient and modern times, that climates vary in dryness and humidity, and that human constitutions differ in strength and weakness. He argued that using ancient prescriptions to treat modern diseases without distinction would yield unsatisfactory results and was therefore inadvisable. Guided by the principle of "Shichuan's practical intent to save lives," he combined the theoretical experiences of his predecessors with clinical practice. After more than 30 years of observation and experience, he authored the Jisheng Fang, a 10-volume work rich in content and truly a masterpiece. After another 15 years of further practice, he refined his work and authored the Jisheng Xufang. In summary, he placed great emphasis on practice, sought practical results, and maintained a rigorous and serious scholarly attitude, which is commendable.

Additionally, his academic thoughts are mainly reflected in his book Jisheng Fang. This book integrates theories with prescriptions, categorizing external afflictions and internal injuries, and also covering diseases related to external, gynecological, and the five sense organs, with a particular focus on miscellaneous diseases. Below, we will discuss his academic thoughts in detail.

1. He emphasized zang-fu organ differentiation, meticulously studying the pulse, cause, syndrome, and treatment. Based on the zang-fu organ theory, he analyzed the etiology and pathogenesis from the physiological and pathological characteristics of the human body, using the patterns of deficiency, excess, cold, and heat as the basis for establishing treatment principles and prescriptions. Particularly in the sections on "Treatment of Deficiency and Excess of zang-fu Organs," he extensively elaborated on differentiation and treatment methods. For example, in the section on "Treatment of Deficiency and Excess of the Liver and Gallbladder," he first discussed the physiological characteristics and etiology of the liver and gallbladder, then described the manifestations and treatment methods for cold, heat, deficiency, and excess syndromes of the liver and gallbladder. Starting from the functions of the zang-fu organs, he explained the normal while recognizing the abnormal, providing a concise and insightful discussion on the occurrence, development, outcome, and treatment of zang-fu organ pathologies. His discussions were clear and systematic, earning him the later evaluation that his prescriptions were "subtle yet essential, truly the guiding principles for life-saving," which is quite fitting. Additionally, in the process of differentiation and treatment, he emphasized the completeness of pulse, cause, syndrome, and treatment. For example, in the section on "Treatment of Bloody Stool," he pointed out, "A floating and weak pulse with a hollow feeling upon pressing indicates bloody stool." The cause was often "overeating, excessive drinking, sexual overexertion, deficiency of defensive qi, susceptibility to wind-cold, and accumulation of pathogenic heat, which lodges in the large intestine, leading to bloody stool." The syndrome was described as "bright red blood indicating wind; blood like red bean juice indicating cold; turbid and dark blood indicating heat." The treatment generally followed the principles of "dispelling wind, clearing heat, warming cold, and tonifying deficiency." This perspective of "not missing any of the four" in pulse, cause, syndrome, and treatment reflects the characteristics of Chinese medicine in "differentiating syndromes to seek causes" and "examining causes to determine treatment."
2. He valued the source of generation and transformation, emphasizing spleen supplementation while also focusing on kidney supplementation. He stressed the importance of protecting the spleen and stomach's healthy qi. He said that humans are born between heaven and earth, and none can do without the stomach as the primary organ. The stomach receives water and food, and the spleen governs transportation and transformation, generating qi from blood to nourish the body. He believed that the spleen and stomach's "peaceful" state should not be harmed, as harm would lead to illness, and he earnestly advised caution in using medications to "not damage the spleen and stomach." While emphasizing the spleen and stomach, he also valued the role of the kidneys. For example, when discussing the "Buzhen Pill," he said, "Excessive sexual activity depletes true yang, the fire of the kidney is not warm, unable to steam the spleen earth, leading to the loss of peaceful distribution and the middle jiao's dysfunction... The ancients said that tonifying the kidney is not as good as tonifying the spleen, but I say that tonifying the spleen is not as good as tonifying the kidney. If the kidney qi is strong, the fire of the dantian will steam the spleen earth, warming it, and the middle jiao will self-regulate, the diaphragm will open, and one will be able to eat." This is an inheritance and development of the Mingmen (GV4) theory of true fire in the kidney, which had a certain influence on the development of Mingmen (GV4) theory in the Ming Dynasty. Another example is his discussion on "seminal emission and turbidity," where he said, "Diseases of the heart can cause seminal emission and turbidity, and diseases of the kidney can also cause seminal emission and turbidity. This is all due to the lack of communication between the heart and kidney and the lack of firmness in the key." This is the first mention of the "heart-kidney non-communication" theory, also reflecting his emphasis on the importance of the innate foundation.
3. The airway is valued for its smoothness, and treating diseases requires skill in regulating qi. He believed that "the human airway is valued for its smoothness, as smoothness ensures the circulation of body fluids," so treating Zabing should prioritize regulating qi. For example, in treating apoplexy, he pointed out that "if it is caused by internal factors such as the seven emotions, the treatment should focus on regulating qi, not on treating wind. If it is caused by external factors such as the six excesses, one should also first regulate qi, and then treat according to the specific symptoms caused by the qi. This is a better approach." For phlegm-fluid retention, he also advocated "regulating qi first, followed by drainage." Similarly, in treating diarrhea, he proposed that "if it is caused by emotional distress, one should use qi-regulating medicinals in combination, treating according to the specific symptoms, so as not to miss the key points." At the same time, he was adept at selecting aromatic qi-regulating medicinals such as Clove, Aucklandia Root, Aquilaria, Sandalwood, and Villous Amomum Fruit, which not only promote the movement of qi but also invigorate the spleen and stomach, which are their strengths.
4. Contributions in pattern identification, formula formulation, and medication usage. He also had many innovative insights in pattern identification and treatment. For example, he pointed out that edema must be distinguished from tympanites, stating, "Tympanites is characterized by abdominal distension and fullness without swelling, while edema involves swelling of the face and limbs. Treating tympanites with water-draining medications and edema with tympanites medications is inappropriate." For treating edema, he advocated focusing on strengthening the spleen and warming the kidneys. Even for yang edema with excess patterns, he recommended using mild and balanced medications, opposing the use of drastic formulas for aggressive purging. For instance, in treating yang edema with Duck Head Pill, he only used Pepperweed Seed without strong purgation, combining it with Polyporus and Stephania Tetrandra for their mild and harmonizing effects. In the Pepperweed Seed Pill, he used lobedleaf pharbitis seed, half raw and half fried, to moderate its properties, demonstrating his meticulous approach to medication. Similarly, in treating dysentery, he opposed the early use of astringent and stopping medications, stating, "Many people, upon initial infection, often use Poppy Capsule, Pomegranate Rind, and Chebula Fruit to stop and astringent, not realizing that dysentery is mostly caused by food stagnation in the intestines and stomach... It is essential to first clear and cleanse the intestines and stomach, then address the root cause, and finally differentiate wind-cold, summerheat, and dampness for treatment." Likewise, for treating cough, he strongly cautioned against early use of astringent and cough-suppressing medications like Smoked Plum and Poppy Capsule to avoid trapping pathogens. Additionally, he paid great attention to the processing of materia medica. For example, besides using Croton Fruit frost, he often stir-fried Croton Fruit with other herbs to modify its properties. In the Toosendan Fruit Powder for hernia, he stir-fried Croton Fruit with Chinaberry Fruit and then removed the Croton Fruit, using only the Chinaberry Fruit. In the Xiang Leng Pill for abdominal mass, he stir-fried Croton Fruit with Zedoary Rhizome and used the Zedoary Rhizome. In the Que Tong Powder, he stir-fried Croton Fruit with Galangal and used the Galangal, showing his precise and refined approach to medication and the art of formula formulation.

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At the same time, he also emphasized the principles of formula formulation, stating, "When using a group of medications, it is essential to balance the strong and mild, and ensure the assistants and envoys are appropriate." "When using a strong formula, which is straightforward and effective, it is necessary to use a mild formula to moderate its strength, achieving a balance between strong and mild. Even if it does not produce immediate results, it can prevent future complications." Therefore, his formulas often balanced strength and mildness, combined movement and stillness, and reinforced yin and yang. For example, in the Returning to Spleen Decoction, he used Aucklandia Root to tonify without causing stagnation, and in the Turtle Carapace and Rehmannia Decoction, he used Cassia Bark to nourish without causing sliminess. Similarly, in treating hemorrhagic syndrome, he did not solely focus on stopping bleeding but often incorporated warming and blood-activating ingredients, such as ginger juice, Rhubarb, scallion juice, or frankincense, to stop bleeding without leaving stasis, demonstrating the rigor of his formula formulation.

In summary, Yan Yong-he is indeed a masterful and pragmatic medical scholar of his generation. His work Jisheng Fang presents precise theories, concise pattern identification, and meticulous medication usage. His formula formulation is neither excessive nor overly complex, embodying both inheritance and creativity, making it a highly practical formulary.