袁了凡先生云∶天地生物，必有氤氲之时，万物化生，必有乐育之时。猫犬至微，将受
娠也，其雌必狂呼而奔跳，以氤氲乐育之气触之而不能自止耳，此天然之节候，生化之真机
也。世人种子，有云∶三十时辰两日半，二十八九君须算。此特言其大概耳，非的论也。《
丹经》云∶一月止有一日，一日止有一时。凡妇人一月经行一度，必有一日氤氲之候，于一
时辰间，气蒸而热，昏而闷，有欲交接不可忍之状，此的候也。于此时逆而取之则成丹（须得逆取之道，待之便是筑基，
此先天气生时也），顺
而施之则成胎矣。其曰三日月出庚，又曰温温铅鼎，光透帘帏，皆言其景象也。当其欲情浓
动之时，子宫内有如莲花蕊者，不拘经净几日，自然挺出阴中，如莲蕊初开，内人洗下体，以
手探之自知也，但含羞不肯言耳，男子预密告之，令其自言，一举即中矣。
褚尚书求男论云∶建平孝王妃姬，皆丽无子，择民家未笄女子入御，又无子。问曰，求男有道乎？澄对曰∶
合
男女必当其年。男虽十六而精通，必三十而娶；女虽十四而天癸至，必二十而嫁（今有年二十而乳子者，亦事之变也），
皆欲阴阳完实，然后交而孕，孕而育，育而子坚壮强寿。今未笄之女，天
癸始至，已近男色，阴气早泄，未完而伤，未实而动，是以交而不孕，孕而不育，育而子脆
不寿，此王之所以无子也。然妇人有所产皆女者，有所产皆男者，大王诚能访求多男妇人至
宫府，有男之道也（方是求子一法）。王曰善。未再期，生六男。夫老阳遇少阴，老阴遇少阳，亦有子之道也
袁了凡先生说：天地化生万物，必定有氤氲（阴阳交感、气息交融）的时机；万物孕育生长，必定有乐育（生机勃发、愉悦滋养）的时刻。即使是猫狗这样微小的动物，在即将受孕时，雌性一定会狂叫奔跑跳跃，这是因为被氤氲乐育之气触动而无法自持罢了。这是天然的节律，也是生化产生的真正机理。
世人谈论求子，有人说：“三十个时辰加两天半，二十八九天您得算准。”这仅仅是说个大概而已，并不是精确的论述。《丹经》说：一个月中只有一天，一天中只有一个时辰。凡是妇女每月月经来潮一次，必定有一天是氤氲之候，在一个时辰之间，体内气蒸发热，头昏胸闷，有想要性交却难以忍受的状态，这才是确切的时机。在这个时机如果逆向操作（指房中术中的固精不泄），就能炼成金丹（必须掌握逆向取精的方法，等待时机就是筑基，这是先天之气生成的时候）；如果顺势而行（指正常交合射精），就能形成胎孕。文中所说的“三日月出庚”，又说“温温铅鼎，光透帘帏”，都是在描述这种景象。
当情欲浓烈发动之时，子宫内部会有像莲花花蕊一样的组织，不管月经干净几天，自然会挺出阴道口，就像莲花初开一样。女性清洗下身，用手探入即可自知，只是因害羞不肯说出来罢了。男子应预先秘密告知她，让她自己说出感觉，这样一举就能成功受孕。
褚尚书关于求子的论述说：建平孝王的妃嫔们，虽然容貌美丽却没有生育，于是挑选民间未行成年礼的女子纳入后宫侍奉，依然没有生育。孝王问：“求取男丁有办法吗？”褚澄回答说：“男女结合必须年龄相当。男子虽然十六岁精气已通，但一定要等到三十岁才娶妻；女子虽然十四岁天癸（月经）已至，但一定要等到二十岁才出嫁（现在也有二十岁就哺乳孩子的，这是事物的变例）。这都是为了让人体的阴阳之气完备充实，然后交合才能受孕，受孕后能顺利养育，养育出的孩子才能坚固强壮且长寿。如今那些未行成年礼的女子，天癸刚刚到来，就接近男子，导致阴气过早泄漏，身体尚未完善就已受伤，元气尚未充实就已妄动，所以交合不能受孕，即使受孕也不能顺利生产，即使生下孩子也脆弱短命，这就是孝王没有儿子的原因。然而有些妇人生的全是女儿，有些妇人生的全是儿子，大王如果能访求多子多的妇人到宫中府邸，就有求得男丁的办法了（这是求子的一个方法）。孝王说：“好。”不到一年，生了六个男孩。所谓老阳遇到少阴，老阴遇到少阳，也是有子女的道理啊。”
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."