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Mr. Yuan Liaofan said: The heaven and earth, in generating all things, must have a time of *yinyun* (the state of yin-yang interaction and qi blending); the transformation and birth of all things must have a time of *leyu* (vitality flourishing and nurturing joy). Even cats and dogs, which are minute creatures, when about to conceive, will cry wildly and run and jump because they are touched by the qi of *yinyun* and *leyu* and cannot restrain themselves. This is the natural rhythm and the true mechanism of biological generation.

People discussing seed planting (conception) say: "Thirty hours plus two and a half days, you must calculate twenty-eight or twenty-nine." This merely states the general rule, not an exact theory. The *Dan Jing* (Scripture on Alchemy) says: In a month, there is only one day; in a day, there is only one hour. Whenever a woman menstruates once a month, there must be one day of *yinyun* condition, within one hour, where qi steams and heats, causing dizziness and stuffiness, with an unbearable desire for sexual intercourse. This is the exact timing. If one acts against this flow at this time, one can refine the elixir (one must obtain the method of reverse extraction; waiting is building the foundation, this is when pre-heaven qi generates); if one acts in accordance with it, conception occurs. The phrases "the third moon appears at Geng" and "warm lead cauldron, light penetrating the curtains" both describe these phenomena.

When sexual desire is strongly aroused, inside the uterus, there is something like a lotus stamen that naturally protrudes from the vagina, regardless of how many days have passed since menstruation ended, resembling a newly opened lotus. Women wash their lower body and can know this by probing with their hand, though they may be shy to speak of it. Men should secretly inform them beforehand, prompting them to speak of it themselves, ensuring immediate success.

The Shangshu Chu's discourse on seeking sons states: The concubines of the Princess Dowager of Jianping were beautiful but childless. They selected unmarried girls from common families to serve in the palace, yet still had no children. The Princess asked, "Is there a way to seek sons?" Cheng replied, "Union between men and women must occur at appropriate ages. Although a man's essence may flow freely at sixteen, he must wait until thirty to marry; although a woman's *tian gui* (menstrual blood/sexual maturity) arrives at fourteen, she must wait until twenty to marry (though some now nurse infants at twenty, this is a variation of events). All this is so that yin and yang are complete and solid before intercourse leads to pregnancy, pregnancy leads to childbirth, and childbirth yields strong, healthy, and long-lived offspring. Now, girls who have not come of age, whose *tian gui* has just begun, approach men too early, causing premature leakage of yin qi. Their bodies are incomplete and injured, their essence un-solidified yet moving recklessly. Thus, intercourse fails to produce pregnancy; if pregnant, it fails to nurture; if born, the child is fragile and short-lived. This is why the Prince had no sons. However, some women bear only daughters, others only sons. If Your Highness could seek out women who have borne many sons into the palace, there would be a way to obtain male heirs (this is one method of seeking children). The Prince said, 'Good.' Within a year, six sons were born. The meeting of old yang with young yin, and old yin with young yang, is also a way to have children."