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Shu Shangshu (Minister Shu) said: The five flavors of food and drink nourish the marrow, bones, flesh, blood, skin, hair, and nails. Men belong to Yang, and within Yang there must be Yin; the number associated with Yin is eight. Thus, at age eight, the Yang essence rises; at age sixteen, the Yang essence overflows. Women belong to Yin, and within Yin there must be Yang; the number associated with Yang is seven. Thus, at age seven, the Yin blood rises; at age fourteen, the Yin blood overflows. Both Yang essence and Yin blood are the refined essences derived from the five flavors of food and drink (The principles of decline and increase lie within the "Seven Damages and Eight Benefits"). When these energies rise, one's mental faculties become clear, teeth are replaced, yellow hair turns black, and weak sinews and tendons become strong. When they overflow, every part of the body—limbs, hands, feet, ears, eyes, and beyond—is filled. Even minute drops, like those from a needle or mustard seed, penetrate everywhere. A child resembles their parents because the parental essence and blood have permeated the parents' bodies entirely. Therefore, if a father has a disabled limb, the son will have a limb that does not resemble the father's; if a mother has a missing eye, the son will have an eye that does not resemble the mother's.

If sexual intercourse is practiced before the Jing (Essence) is fully developed, forcing its release, the five organs and physical forms will have deficiencies, leading to difficult-to-describe illnesses later in life. If one engages in sexual desire when the Yin organ is already flaccid, causing the Jing to be expelled, the Jing remains inside without exiting, leading to a涩 (astringent/stagnant) sensation in the urinary pathway, resulting in Lin syndrome (dysuria). If the Jing is exhausted and then further depleted, it causes pain in the pathways of urination and defecation; the more painful it becomes, the more one feels the urge to urinate or defecate, which in turn increases the pain.

When a woman's Tian Gui (Menstruation) arrives, if she has no sexual intercourse with men for over ten years, her cycle will become irregular. If she desires intercourse with men before ten years have passed since menarche, her cycle will also become irregular. Irregularity means old blood fails to exit, while new blood flows erratically; it may seep into the bones or transform into swelling. Even if intercourse occurs, conception may be difficult. Frequent intercourse with men depletes Jing, weakening the person; frequent childbirth and lactation deplete Blood, endangering life. Observing the state of Jing and Blood reveals half the truth. (When Blood is exhausted, white discharge appears; when Jing is exhausted, hemorrhage occurs. One case arises from Qi failing to transform into Blood; the other from Blood failing to transform into Jing. Both present similarly but their harms extend beyond this.)

Shu Shangshu said: In the union of man and woman, if both essences are smooth, and Yin Blood arrives first while Yang Essence follows, the Blood opens to envelop the Essence. The Essence enters and transforms into bone, forming a male fetus. If Yang Essence enters first and Yin Blood follows, the Essence opens to envelop the Blood. The Blood enters and transforms into bone, forming a female fetus. Yang Qi gathers in the face, so men have heavier faces; drowning men typically float prone. Yin Qi gathers on the back, so women have heavier backs; drowning women typically float supine. For quadrupeds drowning, both positions occur. If Yin and Yang arrive equally, a non-male/non-female body may form. If Jing and Blood are scattered, it indicates twins or multiple fetuses. If the father is young and the mother old, the daughter born will likely be frail. If the mother is robust and the father weak, the son born will likely be weak. Ancient skilled physicians examined this first. If Qi is unevenly damaged, they provide tonification. To tonify frail women, one nourishes Blood and strengthens the Spleen. To tonify weak men, one strengthens the Spleen and regulates sexual activity. Frail women should marry promptly; weak men should wait until they are robust. This is the fundamental principle outside of treating disease, which must not be overlooked.

Zhu Danxi said: The theory that the sequence of Jing and Blood determines the sex of the fetus is attributed to Chu Cheng. I have privately doubted it! Later, I read Li Dongyuan's treatise stating: One or two days after menstruation ends, the Sea of Blood is clean, and Jing surpasses Blood; conception results in a male. Four or five days later, the Blood vessels are vigorous, and Jing cannot surpass Blood; conception results in a female. This argument is also not sufficiently clear. Why? The *I Ching* (Book of Changes) states: "The Way of Qian creates Man, the Way of Kun creates Woman." Qian and Kun represent the nature of Yin and Yang; Left and Right represent the paths of Yin and Yang; Male and Female represent the manifestations of Yin and Yang. Father's Jing and Mother's Blood combine through mutual influence. The emission of Jing is the action of Yang; the retention of Blood by Jing is the transformation of Yin. Jing forms the bone, representing the beginning of all things from Qianyuan (Primordial Heaven); Blood forms the uterus/womb, representing the nurturing of all things from Kunyuan (Primordial Earth). Through the coupling of Yin and Yang, pregnancy condenses. The residence of the fetus is called the Zi Gong (Uterus), which has a single root below and bifurcates above, reaching left and right. If Jing surpasses Blood, and conception occurs on a Yang day/hour, Yang dominates, Qi is received in the Left Uterus, and the male form is created. If Jing does not surpass Blood, and conception occurs on a Yin day/hour, Yin dominates, Qi is received in the Right Uterus, and the female form is created.

Someone asked: I understand how sexes are distinguished, but what about twins? I replied: When Jing Qi is abundant, it divides, and Blood captures each portion separately. Thus, pregnancies containing both male and female fetuses occur when conception happens on a Yang day during a Yin hour, or a Yin day during a Yang hour, causing Yin and Yang to mix. They do not belong solely to the left or right, but receive Qi between the bifurcation. There can also be triplets, quadruplets, etc., following the same logic. Someone asked: What about males unable to be fathers, females unable to be mothers, or individuals with intersex characteristics? I said: Males unable to be fathers suffer from deficient Yang Qi; females unable to be mothers suffer from obstructed Yin Qi. Intersex conditions arise because the Yin body is invaded by mixed/impure Qi, resulting in varied appearances. There are two types of females with male characteristics: one who acts as a wife to men and a husband to women; another who can be a wife but not a husband. Another type has a female lower body but complete male upper features, caused by mixed Qi. Someone asked: Does mixed Qi invasion only affect the Yin body, yet produce such diverse forms? I said: The Yin body is deficient, making it susceptible to mixed Qi. When mixed Qi invades, Yin and Yang mix without a dominant force, neither belonging to left nor right, receiving Qi between the bifurcation. The resulting form depends on the severity of the mixed Qi, hence the variations in intersex characteristics.