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The study of medicine is broad in scope, profound in meaning, and vast in literature, yet its essence lies merely in Yin-Yang and the Five Elements. Heaven generates all things through Yin-Yang and the Five Elements. In the human body, the Qi of Yin-Yang forms the structure of blood-Qi, exterior-interior, and upper-lower; the Qi of the Five Elements constitutes the substance of the Zang-Fu organs. Thus, endowed with these elements, life exists. However, as Yin-Yang interweave and the Five Elements cycle, there are inevitably differences in density and quantity. Those who receive dense Qi of Yin-Yang and the Five Elements possess robust blood-Qi and Zang-Fu organs and remain disease-free; those who receive thin Qi suffer from weakness in blood-Qi and Zang-Fu organs and fall ill. Excess Yang implies excess Fire, leading to an irritable temperament and emaciated physique; excess Yin implies excess Dampness, leading to a sluggish temperament and obesity. Deficiency of Yang results in Qi deficiency, Exterior deficiency, and Upper deficiency, making one susceptible to external pathogens; deficiency of Yin results in Blood deficiency, Interior deficiency, and Lower deficiency, making one susceptible to internal injury. Moreover, with such a vulnerable constitution, people indulge their desires and fail to adapt to cold and heat. Consequently, healthy Qi is damaged and pathogenic factors invade, causing the Yin-Yang and Zang-Fu organs to become increasingly deficient or stagnant, alternating between Cold and Heat, thereby giving rise to myriad diseases. Therefore, when diagnosing illness, one must first distinguish the domains of blood-Qi, exterior-interior, upper-lower, and Zang-Fu to locate the pathology; then observe the pathogenic factors of deficiency-excess and Cold-Heat to treat them. The goal is to ensure Yin-Yang do not deviate, Zang-Fu do not overcome each other excessively, and tonification or purgation is applied appropriately to suit the condition, aiming solely for recovery. This lineage traces back to Huangdi (Yellow Emperor) and Qibo. Zhang Zhongjing detailed the differentiation of external contraction diseases regarding exterior-interior and Yin-Yang; Zhu Danxi focused on internal injury regarding blood-Qi and deficiency-excess; Li Dongyuan supported and protected Middle Qi; Liu Wanshu promoted eliminating the old and generating the new; Qian Yi clearly differentiated Zang organ diseases; Zhang Congzheng skillfully applied the three methods (sweating, vomiting, purging). Numerous medical texts from past dynasties exist, each possessing its own strengths. Hence, when later generations use formulas from various dynasties to treat diseases, efficacy varies because while disease names may be identical, treatment methods differ; sometimes the strength is utilized, sometimes it is not. Taking "heat disease" as an example: the name is the same, but treatments vary. Sijunzi Tang (Four Gentlemen Decoction) treats heat due to Blood excess; Siwu Tang (Four Substance Decoction) treats heat due to Blood deficiency; Baihu Tang (White Tiger Decoction) treats heat due to Qi excess; Buzhong Yiqi Tang (Tonify the Middle and Augment the Qi Decoction) treats heat due to Qi deficiency; Mahuang Tang (Ephedra Decoction) treats Exterior heat; Chengqi Tang (Descending Qi Decoction) treats Interior heat; Sini Tang (Frigid Extremities Decoction) treats false heat; Chaihu preparations treat true heat; Xieqing Wan (Drain Green Pill), Daochi San (Direct to the Heart Powder), Xiebai San (Drain White Powder), Zishen Wan (Nourish Kidney Pill), and Xiehuang San (Drain Yellow Powder) treat heat in the five Zang organs differently. If one can penetrate the pulse and symptoms and hit the critical point, all will be effective. However, using deficiency-methods for excess conditions, excess-methods for deficiency conditions, exterior-methods for interior conditions, interior-methods for exterior conditions, true-methods for false conditions, or false-methods for true conditions, life and death hang in the balance; how could one expect efficacy? Ignorant practitioners fail to grasp this principle, indiscriminately using ancient and modern formulas on similar-looking diseases, often resulting in premature death. Such individuals steal the reputation of saving lives but actually harm life and distort the truth of medicine. As Confucius stated that the "man of good character" (Xiangyuan) corrupts virtue and is the thief of virtue, these individuals are the thieves of medicine. They secretly damage their moral merit, and spirits will not bless them; can they not be cautious? Since childhood, I, Lou Ying, have devoted myself to this path. From the *Neijing* (Inner Canon) down to the writings of sages from all dynasties and famous formulas from various schools, I read by day and pondered by night, neglecting meals and sleep for over thirty years. Only then did I realize that the thousands of variations in pathological states do not exceed Yin-Yang and the Five Elements. Blood-Qi, exterior-interior, upper-lower, deficiency-excess, and Cold-Heat are all manifestations of one Yin and one Yang; the five Zang organs, six Fu organs, twelve meridians, and the Five Movements and Six Qi are all manifestations of the Five Elements. Like fish scales gathering or spokes converging on a hub, the art of medicine is complete. Therefore, without presuming upon my humble abilities, I have collected canonical texts and medical books, classifying and grouping diseases and methods based on Yin-Yang and Zang-Fu. Diseases are divided into chapters, with the Yin-Yang and Zang-Fu categories defined at the beginning of each volume to establish the main outline. Methods are listed as subheadings, with key points of Yin-Yang and Zang-Fu summarized above each item to clarify the details. For diseases sharing a chapter, branch chapters are established to attach them; for methods sharing a subheading, finer subheadings are created to order them. Where classics contain redundant text, misplaced slips, or missing slips, I verify and correct them according to medical principles. Where commentaries lose the original intent of the classics or theories contradict each other, I deduce and clarify them based on the classics. Thus, the similarities, differences, merits, and faults of various schools can be thoroughly understood, connecting fine and coarse aspects, covering major and minor points comprehensively, making different treatments for the same disease clear as palm reading. I named this work *Yixue Gangmu* (Outline of Medical Science). It is stored in a book box for easy reference, ensuring that when facing illness, one naturally has rules to follow, avoiding misprescription of decoctions that would harm life, distort medicine, and incur the wrath of the spirits. Although I dare not compare myself to the profound mind-methods of Huangdi and Qibo, this effort serves as a small contribution to the creative heart of Heaven and Earth.
Lou Ying Quanshan of Xianshan Xianju Yan wrote this.