Yinfujing 陰符經 "Classic of the secret talisman" is a Daoist treatise traditionally attributed to the Yellow Emperor 黃帝, for which reason it is also called Huangdi yinfu jing 黃帝陰符經. The term yinfu is first seen in the history Zhanguoce 戰國策, where the coalition advisor Su Qin 蘇秦 obtains the "secret talisman" of Duke Taigong of Qi 齊太公 that included a military strategy. The book Yinfujing came into being during the early Tang period 唐 (618-907) in two versions, one owned by Li Quan 李筌, and one by Zhang Guo 張果. The main text is only 300 characters long and is divided into three paragraphs. Li Quan added a commentary to his text, which ends with the sentence Wo yi shi wu wen li zhe 我以時物丈理智 "I, by studying the science of Seasons and Things, become enlightened." (transl. Frederic Henry Balfour) Zhang Guo's version is 400 characters long yet not divided into chapters. Most scholars of the Song period 宋 (960-1279) preferred Zhang Guo's version.
Research about the origin of the text has been done a lot, but without a clear solution. It might be as old as from the Warring States period 戰國 (5th cent.-221 BCE), an assumption brought forward by the Song period scholars Shao Yong 邵雍 and Cheng Yi 程頤 as well as the late Qing period 清 (1644-1911) scholar Liang Qichao 梁啓超, yet other scholars see it as a product of the Jin period 晉 (265-420), or even as a compilation of the Northern Wei period 北魏 (386-534) Daoist master Kou Qianzhi 寇謙之, as assumed by Qing period scholars Yao Jiheng 姚際恒 and Quan Zuwang 全祖望. The Song period scholars Huang Tingjian 黄庭堅 and Zhu Xi 朱熹 (1130-1200) called it a forgery by Li Quan, while the Qing scholar Yu Jiaxi 余嘉錫 was of the opinion that it was a product of the Jin period masters Yang Xi 楊羲, Xu Mi 許謐 or Du Jingchan 杜京產. The version included in the Daoist Canon Daozang 道藏 is the 400-character text, divided into three chapters, yet without commentary.
The Yinfujing describes movement and processes of Heaven and Earth, the changes of Yin and Yang and their interrelation with the human world, with production and consumption of lives and vital energies. Only the Saint (shengren 聖人) is able to observe the Way (dao 道) of Heaven and to control the processes on Earth, so that he can master the secret relations between Heaven and the human world and reunite movement and silence in order to reach the Heavenly Way. Once on this path, he will be able to govern a state successfully or to nourish a long and healthy life.
Although traditional scholars disputed a lot about the origin of the Yinfujing, they all perceived it as a fundamental text of Daoism that can be seen as equally important as the books Laozi 老子 or Zhuangzi 莊子. There are a lot of commentaries to the Yinfujing. The bibliographical treatise in the encyclopedia Tongzhi 通志 alone includes 39 commentaries, the Daoist Canon includes 20 commentaries. Some of these commentaries interprete the Yinfujing as a text on military strategy, others as a recipe on Inner Alchemy (neidan 内丹), and others as a Confucian text describing the human nature.