Despite its brevity, the Yinfu jing, or Book of the Hidden Agreement, is one of the most obscure and difficult Taoist texts. Traditionally attributed to the Yellow Emperor (Huangdi, one of mythical founders of Chinese civilization, and the foremost in the eyes of the Taoists), it dates from between the late sixth and the eighth century.
Later texts of Neidan (Internal Alchemy) have often placed the Yinfu jing with the Daode jing (Book of the Way and Its Virtue) and the Cantong qi (The Seal of the Unity of the Three) at the origins of their teachings. Within Neidan, the text is especially well-known for its idea of “stealing the mechanism” (daoji), which Neidan adepts understand as meaning the inversion of the process that leads from the precelestial to the postcelestial domains.
The Yinfu jing is translated here with the commentary by Yu Yan (1258–1314), a learned and prolific author of independent works and commentaries to earlier texts. Part of his works examine the Book of Changes (Yijing) and Chinese cosmology, while others are concerned with Neidan (including a major commentary to the Cantong qi). While Yu Yan is one of the authors who incorporate key concepts of Neo-Confucianism in their Neidan, his writings display a remarkable knowledge of both Nanzong (Southern Lineage) and Beizong (Northern Lineage) literature.
