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The alchemical texts tell us that the One cavity has two openings and is empty in the center. This is why it is called the cavity with dual openings. My teacher told me that the One cavity emerges when mouth is linked with mouth and opening is connected to opening. These linkages are located where the ren and du meridians meet. Here, yin and yang interact, and the raven and rabbit play.
Special procedures have been designed to open the dual orifices of the One cavity. These procedures involve the method of action and are most subtle. The Triplex Unity states: “When the upper end is closed, the cavity comes into being; when the lower end is closed, it does not exist. The upper end must be absolutely empty because the virtuous spirit resides on top. To activate the two openings, the vapor of metal is required.” These statements capture the essence of the method of activating the two openings. By upper and lower, we mean sky and earth. By closing, we mean completing the circuit when the two openings are joined. By existing and nonexisting, we refer to the characteristics of the subtle cavity. This is how the form and qualities of the One cavity are typically described.
Both upper and lower ends are hidden in the cavity. As it is existing yet not existing, everything is enclosed within it. In absolute emptiness and stillness, sky and earth will join and we will experience the cavity’s subtleties. These subtleties, however, cannot be named; we can only say that somehow they exist. The internal alchemists describe this joining as “the ephemeral merging of the generative energies” and “activating the female generative power within the generative fluid.” The cavity will emerge only in a state of nothingness. It is said that the cavity of the absolute void has a structure and is yet without structure. It is also said that the cavity originates from nothingness because a substance that exists has returned to nonexistence.
While it is said that the upper end should be empty, this does not mean that there is nothing there. Rather, it means that what is there has neither structure nor form. This formless entity is the pre-celestial true lead. Try to encounter it from the front and you won’t be able see its head. Follow it from behind and you won’t see its tail. When internal alchemists talk about “the upper part being served by stillness,” they are referring to using nothingness to generate existence. What do we mean by serving? And whom are we serving? We serve the virtuous spirit with emptiness, and when the service is complete, the vapor will return naturally. The biggest problem facing practitioners is when there is no virtuous spirit above. By above, we mean above the Yellow Pavilion; by virtue, we mean humility and softness. This is summed up aptly by a phrase from the Triplex Unity: “The test of whether the Tao is attained and whether the pivot of virtue exists lies in the application of softness.”
Apply the principles of the Tao when you are in absolute emptiness. Use virtue to attain the lead. Virtue belongs to the path of action; the Tao belongs to the path of nonaction. The two openings of the One cavity make up the gate of the Mysterious Female. It is within the One cavity that the golden pill is born. If you know how to gather the precious substance into the dual openings of the One cavity, the two gates will open and the metal will return home to merge with original nature. This is the process of “returning the pill to the dantien,” and the key to this process lies in the production of the vapor of metal, which in turn requires the joint activities of gathering and absorbing.