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Who can recognize the yin and yang (male and female principles) within Kan (Kidney, Water element) and Li (Heart, Fire element)? The vital mechanism of the Xuan Guan (Mysterious Pass, a key aperture for essence, qi, and spirit) holds the true secret. To say "Kan is empty and Li is solid" is incorrect; to say "Kan is solid and Li is empty" is also incorrect. Heaven establishes its form through utmost rigidity; Earth becomes mud without the sun's warmth. Within the triad of Heaven, Earth, and Humanity, the Golden Elixir (Jin Dan, the result of internal alchemy) exists in all; let us ask if the Immortals know this?

Regarding the methods for conceiving offspring, ancients have spoken extensively, yet I consider some aspects not entirely perfect, hence my doubts and explanations. Hereby, I list them for辨析 (analysis) and correction by those with profound understanding.

1. *Guang Si Jue* (Secrets for Progeny) states: "Thirty hours plus two and a half days, totaling twenty-eight or twenty-nine, you must calculate carefully. When menstrual blood stops and red petals fall everywhere, it is the auspicious time; passing the 'Jin Shui' (metaphor for menstruation) period leads to futile confusion. Wasting effort during confusion is like searching for residual flowers under trees. If one understands that blooming leads to fruiting, why worry about cassia not forming clusters?" Annotation: This implies that when a woman's menstruation just ends, the uterus is open, making it the time for sowing (conception). After this period, the uterus closes and cannot conceive. However, conception occurs after half a month or twenty days in some cases. Why is this so? Another wise woman remarked: "If we follow this theory, anyone acting improperly could simply wait until the second half of the cycle, avoiding any concerns." How excellent these words are! But are they truly credible?

2. *Dao Zang Jing* (Canon of the Dao) states: "Intercourse on the first, third, or fifth day after menstruation ceases results in a male child (Yang); intercourse on the second, fourth, or sixth day results in a female child (Yin)." Annotation: This attributes odd numbers to Yang (male) and even numbers to Yin (female). If this were true, everyone would know it, and having children would be easy. In reality, this is not necessarily the case.

3. *Chu Shi Yi Shu* (Remnant Book of Master Chu) states: "When man and woman unite, their emotions flow smoothly. If Yin Blood arrives first and Yang Essence follows, the blood opens to wrap the essence; the essence enters to become the foundation, forming a male. If Yang Essence arrives first and Yin Blood follows, the essence opens to wrap the blood; the blood enters to become the foundation, forming a female." Annotation: Initially, I found this reasoning appealing. However, upon closer examination, it seems flawed. During union, how can blood be moved? Only after conception does the essence lay the foundation and the blood nourish, gradually forming the fetus. Even if we change "blood" to "essence," saying "Yin Essence arrives first," it might seem plausible. Yet, it is common for young women to conceive from a single encounter. At that time, they are busy avoiding impropriety, so how can it be said that essence is discharged first? Rather, when desire stirs, Qi arrives; when Qi arrives, the Yin orifice opens; when the orifice opens, it absorbs; once absorbed, pregnancy invariably occurs. This is the natural law. Chu's theory seems forced and far-fetched.

4. Li Dongyuan stated: "One or two days after menstruation ends, the Sea of Blood is clean; Essence surpasses Blood, resulting in a male. Four or five days later, the blood vessels are vigorous; Blood surpasses Essence, resulting in a female." Annotation: This is not a definitive conclusion. Today, many couples who strive for conception immediately after menstruation still only have daughters. Is it because Blood prevails?

5. Zhu Danxi stated: "Through the union of Yin and Yang, the fetus condenses. The storage place is called the Uterus (Zi Gong), which hangs down and has two branches above, divided into two, shaped like a joined bowl, one reaching left and one right. If Essence surpasses Blood, Yang dominates, Qi is received in the left uterus, forming a male. If Essence does not surpass Blood, Yin dominates, Qi is received in the right uterus, forming a female." Annotation: This aligns with the *Sheng Ji Jing* (Holy Grace Canon) stating "Left movement forms male, right movement forms female." Examining seed grains, both have two lobes; thus, men have two testes, and the structure of the uterus is likely similar, so this is not erroneous. However, the idea that "left reception forms male, right reception forms female" cannot be predicted beforehand. Even with techniques for aiming left or right, the opening and closing of the vagina have their own mechanisms; wanting to aim left may not result in going left. The principle of Yin-Yang dominance seems to lie between heavenly timing and human affairs, following another path. Knowing this theory ultimately offers no benefit.