The Mysterious Pass, which opens beyond space and time, is inconceivable by means of discursive thought and has, by definition, no fixed position.

In neidan, the Mysterious Pass represents the time and place in which an alchemist joins the complimentary antinomies on which he or she works, such as inner nature and vital force (xing and ming), Dragon and Tiger (longhu), lead and mercury, Fire and Water, heart and kidneys, or kăn 坎 (Yang within Yin) and lí 離 (Yin within Yang).

The Mysterious Pass, which opens beyond space and time, is inconceivable by means of discursive thought and has, by definition, no fixed position.

Only here [in the Center] a new union can occur, as the Mysterious Pass is the ideal space and time to experience the interpenetrating fluctuations of Yin and Yang. The Mysterious Pass is therefore the primordial Chaos (hundun) containing the germ of life-the pre-cosmic sparkle of Original Yang and Original Yin-which is the prime mover and the materia prima of the alchemical work.

Better known as the Secret of the Golden Flower, this is a famous neidan text that the Western world came to know through Richard Wilhelm's 1929 translation. The Chinese text used by Wilhelm was edited by Zhanran Huizhen zi in 1921. Besides this, at least five more versions are available, all of which date to the late Qing dynasty (1644-1911) and are ascribed to Lu Dongbin, who revealed them through spirit writing.