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Wang Yutai stated: There are four principles in the art of conception. The first is selecting the soil; "soil" refers to the mother's blood. The second is nurturing the seed; "seed" refers to the father's essence (Jing). The third is seizing the timing; "timing" refers to the convergence when essence and blood interact and sense each other. The fourth is acting upon emptiness; "emptiness" refers to the initial stage of discarding the old and generating the new.

Yuan Liaofan stated: Heaven and Earth, in creating all things, must have specific seasons; for all things to transform and be born, there must be a favorable period for nurturing. Even in humble creatures like cats and dogs, before conception occurs, the female will cry out wildly and jump about because she is touched by the Qi of joyful nurturing and cannot restrain herself. This is the natural rhythm and the true mechanism of biological transformation. For any woman who menstruates once a month, there is certainly one day of fertile opportunity within a single two-hour period (Shichen), where Qi steams and generates heat, accompanied by dizziness, stuffiness, and an unbearable desire for intercourse. This is the precise fertile window (De Hou). If one acts against this trend to prevent conception, it results in [abnormal formation]; if one acts in accordance with this trend to engage in intercourse, pregnancy will be conceived.

Shen Zhai commented: The above four points outline the principles of conception, including methods for preserving and gathering essence and seizing the right time. The method of preserving essence and seizing the time lies in the regulation and care of the man. However, there are cases where a man fully applies these methods yet remains childless throughout his life. The fault does not lie in the man's failure to apply the methods correctly, but rather that the woman necessarily has an underlying cause. Therefore, the causes of female infertility are discussed subsequently.

The *Shengji Zonglu* states: Women fail to conceive due to insufficiency of the Chong and Ren vessels and deficiency-cold of Kidney Qi. The *Neijing* states: When a girl reaches fourteen years of age, Tian Gui arrives, the Ren vessel becomes unblocked, and the Tai Chong vessel becomes abundant. With Yin and Yang harmonized, she can conceive. If the Chong and Ren vessels are insufficient and Kidney Qi is deficient and cold, it cannot sustain the Bao Palace (uterus), thus causing infertility. There are also cases originating from illness in either the husband or wife, which require investigation of the root cause for treatment.

Miu Zhongchun stated: The Bao Palace in women is connected to the Kidneys and the Pericardium Collaterals, both of which are Yin organs. If they are deficient, Wind-Cold will invade the uterus, leading to sterility. Without warm-natured medicines, one cannot dispel Wind-Cold and facilitate the miraculous process of gestation. It is essential to use acrid-warm formulas, adding guiding herbs to direct the medicinal action to the Lower Jiao, targeting the Kidneys and Pericardium, to disperse Wind-Cold and warm the uterus.