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).