Basu Zhenjing: Sun-Moon Ingestion in Shangqing Daoism

The Cavern of Truth: The Supreme Scripture of the Eight Essences

Referred to in abbreviated forms as The Scripture of the Eight Essences (Ba Su Zhen Jing) or The Classic of the Eight Essences (Ba Su Jing).

It is one of the early classics of the Shangqing School (a major school of Taoism), presumably compiled during the Eastern Jin Dynasty (317-420 CE).

In Volume 5 of Zhen Gao (The True Instructions, a core Shangqing text recording divine revelations), it is stated: "The Dao encompasses The Scripture of the Eight Essences, which is the hidden book of the Supreme [Dao]";
Shangqing Dadong Zhenjing Mu (Catalogue of the True Scripture of the Great Cavern of Shangqing) lists an entry titled "One Scroll of the Shangqing Scripture of the Eight Essences: Ingesting the Royal Radiance of the Sun and Moon" — both references denote this very scripture.

In the extant Daozang (Taoist Canon), two versions of The Scripture of the Eight Essences are preserved:

The first is Shangqing Tai Shang Ba Su Zhen Jing (The Shangqing Supreme Scripture of the Eight Essences), which is included in the "Yujue Category" (a section of the Daozang dedicated to commentaries and explanations of Taoist scriptures) under the "Dongxuan Division" (one of the major divisions of the Daozang, focusing on esoteric Shangqing teachings).
The second is Dong Zhen Tai Shang Ba Su Zhen Jing: Methods for Ingesting the Royal Radiance of the Sun and Moon (Dong Zhen Tai Shang Ba Su Zhen Jing Fu Shi Ri Yue Huang Hua Jue), which is included in the "Tongzihao" subcategory under the "Zhengyi Division" (the core division of the Daozang centered on orthodox Taoist rituals and doctrines).

According to quotations from Taoist texts such as Wushang Miyao (The Supreme Secret Essentials), the two aforementioned versions should be either different chapters or different transmission copies of the same original scripture. Their content and wording share both similarities and discrepancies. For further details, please refer to the respective entries of these two versions.