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The *Nan Jing* (Classic of Difficult Issues) is the essence of the *Ling Shu* and *Su Wen* (collectively known as the *Huang Di Nei Jing*, or Yellow Emperor's Inner Canon, foundational texts of Traditional Chinese Medicine). While the *Ling Shu* and *Su Wen* are often considered pseudonymous works, the *Nan Jing* is a book authored by hand by Bian Que. It inherits the wisdom of past sages and opens the way for future worthies, serving undoubtedly as the standard for the medical field. However, ancient texts have long been obscured, and versions circulating in the world contain many errors. Consequently, commentators have relied on subjective speculation, and readers have harbored doubts. Because the *Nan Jing* is obscure, the *Ling Shu* and *Su Wen* are not fully illuminated; and because the *Ling Shu* and *Su Wen* are not illuminated, the Way of Medicine might almost cease to exist.

Previously, I traveled to Wu Men (Suzhou) to seek medical knowledge, where I met a friend from She named Jin Zi, who presented me with an ancient edition of the *Nan Jing*. Upon reading it, the principles became clear and the details orderly; the pulse theories were connected, and the meridians integrated. Compared to the erroneous versions circulating in the world, it was like clearing away clouds and fog to behold the vast sky. Thus, I extensively examined various commentaries and proofread them item by item. No other volume possessed such clarity of text and precision of annotation as this one. I treasured it in my chest, brought it home, and planned to have it carved onto wooden blocks for printing, to share it with like-minded individuals. Fortunately, scholars who cherish antiquity enthusiastically supported this endeavor. Within a few months, the book was completed. This allowed a text that had been hidden and obscure for thousands of years to suddenly shine brightly throughout the realm. This is a fortune for the Way of Medicine, and indeed a great blessing for all those suffering from illness. Is this merit not magnificent? Therefore, I briefly outline its general history to record that I do not forget its source.

Preface by Zhang Ji of Jinxi, in the second month of the Gengshen year, the fifth year of the Jiaqing reign.

In my youth, I studied medicine and followed He Sizong, a gentleman from Yunjian, in my studies. Recently, having settled temporarily in Jin Chang (Suzhou), I frequently discussed medical classics with Wang Xiushen from Shangyang. The teachings of Nongxuan (referring to Shennong and Xuanyuan, symbolizing the origins of ancient medicine) are akin to the Six Classics (Confucian canonical texts); Bian Que and Zhang Zhongjing regarded Confucius as their spiritual master in the manner of Mengzi (Mencius). Therefore, the *Nan Jing* (Internal Difficulties) and the *Shang Han Lun* (Treatise on Cold Damage) constitute the three pillars of the medical gate. Since the Han dynasty, eminent scholars have emerged successively, relying solely on these three classics. Later authors, while making their own discoveries, gradually narrowed and shallowed their scope. This has led contemporary learners to prefer the superficial and fear the profound, pursuing trivialities while forgetting the root. Alas! If the meaning of the classics is not clarified, upon what can medical art rely? How can we find scholars who love antiquity to jointly discuss these three classics, citing the precedent of the Song Renzong era when medical books were collated, and petitioning the Sage Emperor to order Confucian officials and experts to consult ancient sources and adapt to modern needs, compile a complete work, and distribute it throughout the realm for the benefit of all generations?

Just then, a guest informed me: "The person previously thought to be hard to find has now been found." His surname is Ding, his style name Shilu. He brought an ancient edition of the *Nan Jing* from Rongcheng, added elucidations and annotations, and came to Wu to have it printed. Upon hearing this, I was initially surprised, then skeptical. I recalled my late teacher mentioning that during the Song Dynasty, Ding Deyou had annotated the *Nan Jing*: Hua Tuohua obtained the book of Yue Ren (Bian Que) but was captured, and the book was burned. What circulates today was recompiled by Lü Guang, the Imperial Physician of Wu, containing many disordered bamboo slips, making the language and meaning difficult to understand. Of the seventeen schools of commentary, such as that of Hua Shi (Slao Shou), only doubts remain, making verification impossible. Perhaps there are ancient editions, but they have unfortunately not been revealed to the world. Is what you heard about perhaps the ancient edition of Lü Guang? The guest said no. So I accompanied Mr. Shen to visit him. We saw a man, ethereal and white-haired, unlike worldly folk. Reading his book, which was not something I had seen before, I realized that the original *Nan Jing* has clear language and coherent logic, with not a single superfluous character. Thus, Su Dongpo wrote in his preface to the *Lenga Sutra*: "Like the medical *Nan Jing*, every sentence contains principle, and every word contains method." Only someone with true insight would say so, further confirming that my teacher's words had basis. Alas! A book that had been hidden and nearly lost for over two thousand years has suddenly reappeared, allowing people everywhere to know that the Eighty-One Difficulties are Yue Ren's commentary on the *Nei Jing*, and that Mr. Ding's writing further elucidates Yue Ren's commentary. It is like the sun in mid-heaven, illuminating even the finest details. Once this edition is published, the editions of the seventeen schools can be discarded. Mr. Ding has achieved immortal merit; is this within the reach of ordinary people? Mr. Ding rose from his seat and replied: "Hetu and Luoshu appeared because of the Sage; they are books kept secret by the world, waiting for a sage dynasty to be revealed. This is also due to the laws of reason and number. Who am I to dare claim credit for this?" I therefore rejoiced at this encounter and recorded the event, respectfully stating this at the end of the manuscript.

Written by Yan Maoyuan, Layman Ju Po of Dongting, in the tenth month of the Wuwu year, third year of the Qianlong reign.