《难经》，非经也。以《灵》、《素》之微言奥旨引端未发者，设为问答之语，俾畅厥义也。古人书篇名义，非可苟称，
难者辩论之谓，天下岂有以难名为经者，故知《难经》非经也。自古言医者，皆祖《内经》，而《内经》之学，至汉而分∶
仓公氏以诊胜，仲景以方胜，华佗氏以针灸杂法胜；虽皆不离乎《内经》，而师承各别。逮晋、唐以后，则支流愈分，徒
讲乎医之术，而不讲乎医之道，则去圣远矣。惟《难经》则悉本《内经》之语，而敷畅其义，圣学之传，惟此为得其宗。
然窃有疑焉，其说有即以经文为释者，有悖经文而为释者，有颠倒经文以为释者。夫苟如他书之别有师承，则人自立说，
源流莫考，即使与古圣之说大悖，亦无从而证其是非，若即本《内经》之文以释《内经》，则《内经》具在也，以经证经
而是非显然矣。然此书之垂已二千余年，注者不下数十家，皆不敢有异议，其间有大可疑者，且多曲为解释，并他书之
是者反疑之，则岂前人皆无识乎？殆非也。盖经学之不讲久矣！惟知溯流以寻源，源不得，则中道而止，未尝从源以及
流也。故以《难经》视《难经》，则《难经》自无可议；以《内经》之义疏视《难经》，则《难经》正多疵也。余始也，
盖尝崇信而佩习之，习之久而渐疑其或非，更习之久而信己之必是，非信己也，信夫《难经》之必不可违乎《内经》也。
于是本其发难之情，先为申述《内经》本意，索其条理，随文诠释；既乃别其异同，辨其是否，其间有殊法异义，其说
不本于《内经》，而与《内经》相发明者，此则别有师承，又不得执《内经》而议其可否。惟夫遵《内经》之训而诠解未
洽者，则摘而证之于《经》，非以《难经》为可訾也，正所以彰《难经》于天下后世，使知《难经》之为《内经》羽翼，
其渊源如是也，因名之为《经释》。《难经》所以释《经》，今复以《经》释《难》，以《难》释《经》而《经》明，以《经》
释《难》而《难》明，此则所谓医之道也，而非术也。其曰秦越人着者，始见于《新唐书·艺文志》，盖不可定，然实两
汉以前书云！
雍正五年三月既望松陵徐大椿叙
《难经》，并非真正的“经”。它是引用《灵枢》、《素问》中精微深奥的旨意，针对其中尚未阐发或初露端倪的内容，通过设问与回答的形式，来充分阐述其义理。古人对于书籍篇章的名称和含义，是不会随意安加的。“难”字意为辩论、诘难，天下哪有以“难”字命名却称为“经”的书呢？因此可知《难经》并非“经”。自古以来谈论医学的人，都以《内经》为宗师，而《内经》的学问，到了汉代便分化为不同的流派：仓公（淳于意）以诊断技术著称，张仲景以方剂著称，华佗以针灸及其他杂法著称；虽然这些都不脱离《内经》的范围，但师承渊源各不相同。等到晋代、唐代以后，支流更加分化，人们只讲究医术的技巧，而不探究医道的根本，这就离圣人的学说越来越远了。只有《难经》完全依据《内经》的文句，并进一步阐发其义理，是圣人学术传承中唯一得其宗指的著作。
然而我私下里对此存有疑问：书中的解释，有的直接以经文作为注释，有的违背经文进行解释，有的颠倒经文顺序来进行解释。如果像其他书籍那样另有独立的师承渊源，那么每个人都可以自立学说，源流无法考证，即使与古代圣人的说法大相径庭，也无法从中证明其是非对错。如果是基于《内经》的文本来解释《内经》，那么《内经》原文就在那里，用经文来印证经文，是非曲直显然易见。但是这本书流传至今已经两千多年，注疏者不下几十家，都不敢提出异议，其中有很多值得怀疑的地方，并且大多进行牵强附会的解释，甚至对其它书籍中正确的内容反而产生怀疑，这难道是因为前人没有见识吗？大概不是这样。大概是经学（指对经典的研究）荒废已久！人们只知道顺着支流去寻找源头，如果找不到源头，就中途停止，从未曾从源头一直追溯到支流。所以，如果用《难经》本身的标准来看待《难经》，那么《难经》自然无可非议；但如果用《内经》的义理标准来审视《难经》，那么《难经》确实存在很多瑕疵。
起初，我也曾经崇信并研习它，研习久了逐渐怀疑它可能不对；再研习久了，便确信自己是对的，这并不是自信，而是确信《难经》绝不可违背《内经》的原则。于是，根据它提出疑难的情境，先申述《内经》的本意，梳理其条理，随文进行诠释；接着辨别其中的异同，判断其是否正确。其中如果有独特的方法和特殊的义理，其说法不源于《内经》，但与《内经》相互发明、补充的，这是另有师承，就不能拘泥于《内经》来议论其可否。只有那些遵循《内经》训诫但解释不够恰当的地方，才摘出来用《内经》原文加以证实，并不是要指责《难经》，正是为了向天下后世彰显《难经》，让人们知道《难经》是《内经》的羽翼，其渊源关系如此，因此命名为《经释》。《难经》用来解释《内经》，现在反过来用《内经》解释《难经》，用《难经》解释《内经》而使《内经》显明，用《内经》解释《难经》而使《难经》显明，这就是所谓的医道，而非单纯的医术。至于说该书由秦越人（扁鹊）所著，最早见于《新唐书·艺文志》，大概不能确定，但这确实是两汉以前的书籍！
雍正五年三月十六日，松陵徐大椿作序
"Nan Jing" (Classic of Difficult Issues) is not a "Jing" (Classic). It cites the subtle and profound principles from the "Ling Shu" (Spiritual Pivot) and "Su Wen" (Basic Questions), addressing those points that were merely hinted at or left undeveloped, employing a question-and-answer format to fully elucidate their meanings. The ancients did not casually assign names and titles to chapters of books. The character "Nan" (Difficult/Debating) implies debate or interrogation; how could there be a book named "Difficult" yet titled as a "Classic"? Thus, it is known that "Nan Jing" is not a Classic. Since ancient times, all who spoke of medicine regarded the "Nei Jing" (Inner Canon) as their ancestor. However, the study of the "Nei Jing" diverged into different schools by the Han Dynasty: Cang Gong (Chunyu Yi) was renowned for his diagnostic skills, Zhang Zhongjing for his prescriptions, and Hua Tuo for his acupuncture and miscellaneous methods. Although all these remained within the scope of the "Nei Jing", their lineages of transmission were distinct. By the Jin and Tang dynasties, these branches multiplied further, with people focusing solely on the techniques of medicine while neglecting its fundamental Dao (Way/Principles), thereby distancing themselves from the teachings of the Sages. Only the "Nan Jing" is entirely based on the wording of the "Nei Jing" and elaborates upon its meaning, representing the only work in the transmission of Sage learning that truly captures its essence.
Yet, I harbor private doubts regarding its interpretations: some explanations directly use the canonical text as commentary, some contradict the canonical text in their interpretation, and some invert the order of the canonical text to explain it. If, like other texts, it possessed an independent lineage of transmission, individuals could establish their own theories, making the origins untraceable; even if they greatly contradicted the words of ancient Sages, one could not verify their correctness or incorrectness. However, since it bases its explanations on the text of the "Nei Jing", the original text of the "Nei Jing" remains available; using the Canon to verify the Canon makes right and wrong obvious. Nevertheless, this book has been transmitted for over two thousand years, with dozens of commentators who dared not raise objections. There are many points worthy of suspicion, yet most commentators offered forced explanations, even doubting what was correct in other texts. Was it because predecessors lacked insight? Probably not. It is likely that the study of Classics had long been neglected! People knew only to trace back from the tributaries to find the source; if the source was not found, they stopped midway, never tracing from the source down to the tributaries. Therefore, viewing the "Nan Jing" through the lens of the "Nan Jing" itself leaves it open to no criticism; but viewing the "Nan Jing" through the hermeneutics of the "Nei Jing" reveals many flaws in the "Nan Jing".
Initially, I revered and studied it. After prolonged study, I gradually suspected it might be incorrect; after further prolonged study, I became convinced of my own correctness. This was not self-confidence, but confidence that the "Nan Jing" must not deviate from the "Nei Jing". Consequently, based on the context of its difficult questions, I first expounded the original intent of the "Nei Jing", traced its logical structure, and interpreted it passage by passage. Then, I distinguished differences and similarities, and judged correctness. Where there were unique methods and special meanings, whose theories did not originate from the "Nei Jing" but mutually illuminated and supplemented it, these represented separate lineages and could not be judged solely by adherence to the "Nei Jing". For those parts that followed the instructions of the "Nei Jing" but were inadequately explained, I extracted them and verified them against the "Nei Jing". This was not to criticize the "Nan Jing", but precisely to highlight the "Nan Jing" to the world and future generations, showing that the "Nan Jing" serves as a wing/support to the "Nei Jing", and thus its origin. Hence, I named this work "Jing Shi" (Explanation of the Classics). The "Nan Jing" explains the "Jing"; now, I use the "Jing" to explain the "Nan". Using the "Nan" to explain the "Jing" clarifies the "Jing"; using the "Jing" to explain the "Nan" clarifies the "Nan". This is what is meant by the Dao of medicine, rather than mere technique. As for the claim that it was authored by Qin Yue Ren (Bian Que), it first appears in the "Yi Wen Zhi" (Treatise on Literature) of the "Xin Tang Shu" (New Book of Tang); this cannot be definitively established, but it is indeed a book from before the Two Han Dynasties!
Preface by Xu Dachun of Songling, on the 16th day of the third month, in the fifth year of Yongzheng.