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The song of Caoyizi states: Collecting and refining Lead and Mercury (Qian Gong, 铅汞) must be done before noon; when the first Yin arises, one should break its rigidity. Therefore, those who cultivate life take in Qi (vital energy) during Yang hours, and after noon, they only practice breath retention (breath-holding). This is to refine Yin. As stated in *Chicheng Ji*, refining Qi refers to breath retention. After practicing breath retention for a thousand days, the Five Zang organs will not interact, the Six Fu organs will not transmit, the hundred vessels will not flow, the hundred gates will naturally converge, Shen (spirit) and Qi will self-govern, and all transformations will naturally settle. Heaven and Earth cannot steal his body, and Yin and Yang cannot decay his form. From one to ten, from ten to a hundred, from a hundred to a thousand, retaining breath for 120 times is the Lesser Vehicle (Xiao Cheng), for 1,200 times is the Middle Vehicle (Zhong Cheng), and for 12,000 times is the Greater Vehicle (Da Cheng). Calculating day and night, there are 13,500 breaths; counting 540 movements of respiration as one breath brings one close to immortality. The technique is as follows: Be clear and thoughtless, purify the mind and sit upright, raise the head, close the eyes, and retain breath. Use the second interphalangeal joint of the right hand's index and middle fingers, known as the Gate of Life and Death (Sheng Si Men), which is the place where Shen receives Qi. Press this area with a rosary bead and count silently in the heart. If the breath becomes urgent, rotate the throat to the right; do not let it become too urgent, for urgency causes external air to escape slightly, and upon inhalation, the Qi rises again. This is recorded in the oral instructions.

Fetal breathing (Tai Xi) is the method of embracing the One and guarding the Center. When an infant is in the womb, it draws Qi from the umbilical cord. After ten months of shedding the fetal state, it draws Qi from the Shen Lu (Spirit Loft, i.e., the nasal cavity). A True Person has no inhalation or exhalation through the nose; their breath is deep and subtle. What Laozi referred to as "returning to the state of an infant" denotes fetal breathing. The *Fetal Breathing Song* states: "When the nose and mouth cease ordinary respiration, then the work of fetal breathing begins. Though dwelling within the universe, one is as if within the amniotic sac." Ordinary people have their birth gate blocked by the navel cover, so Qi cannot exit from there. Once the navel becomes empty/virtual, seeking Qi allows it to naturally emerge from this point, while there is no Qi movement in the nose. Sitting cross-legged and upright, with hands crossed above the navel constitutes making the navel virtual. After preserving the One, Qi should emerge from this point.

The Sacred Embryo forms internally, with fists clenched tightly and demeanor serene. The essence of health preservation has been half understood here. From Zi hour to Wu hour is the refining of Yang; from Wu hour to You hour is the refining of Yin. Yang governs movement, and Yin governs stillness. Yang should not overflow, and Yin should not cover excessively. When Yin is peaceful and Yang is secure, Spirit and Qi are regulated. However, knowing this is not difficult; the difficulty lies in persisting without interruption for a long time. Aligning with the Dao leads to simultaneous progress inside and out, and the matter of returning elixir can then be discussed.

*Gushen Lun* states: Containing saliva and refining Qi, expelling the old and absorbing the new, with Qi ascending to Niwan Palace (Mud Pill, located at the top of the head) and descending to Dantian (Elixir Field), is called Internal Elixir (Nei Dan). Yang Dragon and Yin Tiger, Wood Fluid and Metal Essence, intersecting and refining each other, is called External Elixir (Wai Dan). Those who cultivate the Dao first achieve the Internal Elixir, then refine External Medicine; when internal and external correspond, one attains immortality. Yanluozi said: "When true Qi truly corresponds, the Elixir arrives naturally." Jinhua Shansheng said: "If one only cultivates true Qi and does not cultivate the Elixir, changes at the end will often be difficult." Dingjizi also said: "Although one is a guest of longevity, one has not yet become an immortal who lifts the entire house to heaven. One must further cultivate the Great Medicine to begin flying to immortality."