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Who can recognize that the essence of male and female lies within Kan (Water) and Li (Fire), where the true mechanism of life resides in the mysterious gate. To say "Kan is empty and Li is solid" is incorrect, and to say "Kan is solid and Li is empty" is also incorrect. Heaven maintains its form through utmost firmness, while Earth forms mud because it is never without moisture. All three existences possess the Golden Elixir; let us ask if the Immortals know this?

Regarding the methods for conceiving offspring, ancients have spoken extensively, yet I consider some of their views imperfect, arising from doubt. However, I carefully list these theories for辨析 (analysis) so that enlightened scholars may judge and correct them.

*Guangsi Jue* states: "Thirty hours plus two and a half days, you must calculate twenty-eight or twenty-nine days. When menstrual blood stops and falls like red petals, it is the auspicious time. If one waits until the 'Metal-Water' phase passes, efforts are as futile as treating cholera; searching for residual red under trees is wasted labor. If one understands that blooming flowers yield fruit, why worry that cassia will not grow in clusters?"

Commentary: This implies that immediately after menstruation ends, the uterus is open, making it the ideal time for conception. After this period, the uterus closes and cannot receive pregnancy. However, pregnancies occurring ten days, half a month, or even twenty days later contradict this. A wise woman remarked: "If this theory holds, those engaging in illicit relations could simply wait until the second half of the cycle to avoid consequences." While clever, is this truly credible?

The *Daozang Jing* states: "For women, intercourse on the 1st, 3rd, or 5th day after menstruation ceases results in a son (Yang path); intercourse on the 2nd, 4th, or 6th day results in a daughter (Yin path)."

Commentary: This attributes sons to odd numbers (Yang) and daughters to even numbers (Yin). If this were true, everyone would know it, and having a son would be easy. In reality, this is likely not the case.

*Zhushi Yishu* states: "When man and woman unite, their essences flow freely. If Yin Blood arrives first and Yang Essence follows, the blood opens to envelop the essence; the essence enters to form bone, resulting in a male fetus. If Yang Essence arrives first and Yin Blood follows, the essence opens to envelop the blood; the blood enters to form the root, resulting in a female fetus."

Commentary: At first glance, this seems plausible. However, upon closer inspection, it is flawed. How can intercourse disturb blood at the moment of union? Only after conception does the essence serve as the foundation and blood as nourishment for fetal development. Even if we replace 'blood' with 'essence' (Yin Essence arriving first), it remains problematic. Often, young women conceive after a single encounter; they are busy avoiding such situations, so how can they be described as releasing essence? Rather, when desire arises, Qi moves; when Qi moves, the Yin opens; when the Yin opens, it absorbs essence; absorption leads inevitably to pregnancy. This is natural law. Chu’s theory seems forced.

Li Dongyuan stated: "One or two days after menstruation ends, the Sea of Blood is just cleared; if Essence exceeds Blood, a male is conceived. Four or five days later, Blood vessels are vigorous; if Essence cannot exceed Blood, a female is conceived."

Commentary: This is not a definitive conclusion. Many couples who strive for conception immediately after menstruation still only have daughters. Is this due to Blood exceeding Essence?

Zhu Danxi stated: "When Yin and Yang intertwine, pregnancy condenses. The storage place is called the Uterus. It has a connection below and two branches above, divided into left and right, shaped like a joined bowl. One reaches the left, one to the right. If Essence exceeds Blood, Yang dominates; conception occurs in the left Uterus, forming a male. If Essence does not exceed Blood, Yin dominates; conception occurs in the right Uterus, forming a female."

Commentary: This aligns with the *Shengji Jing* saying "Left movement creates males, right movement creates females." Anatomically, the uterus indeed has two lobes, similar to the two testes in men. Thus, the concept of the uterus having two parts is not erroneous. However, the idea that left conception yields males and right yields females is speculative post-hoc reasoning. Even with techniques for aiming left or right, the opening and closing of the Yin have their own mechanisms; one cannot force conception to occur specifically on the left or right. The principle of Yin-Yang dominance seems to lie between heavenly timing and human circumstances, suggesting another way. Knowing this theory ultimately offers no practical benefit.