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Tian Gui (Heavenly Water) originates from congenital essence and is generated by the vital Qi of life; Jing Xue (Essence and Blood) originates from acquired nature and is formed from the transformation of Gu Qi (Grain Qi). At the age of sixteen (2x8), Tian Gui arrives in males; belonging to Yang, it corresponds to the Sun's essence and becomes increasingly abundant. At the age of fourteen (2x7), Tian Gui arrives in females; belonging to Yin, it corresponds to the Moon's blood and flows monthly. When male and female combine their essence, offspring are conceived, following the Dao of Heaven for males and the Dao of Earth for females.

[Note] Tian Gui is endowed by parents and is the congenital True Qi that constitutes life. Essence and Blood are transformed from food and water and serve as the foundation for forming the physical body in acquired nature. At sixteen, a male's congenital Kidney Qi is vigorous; Tian Gui arrives and merges with the essence produced in acquired nature, becoming abundant. Males belong to Yang, and Yang corresponds to the Sun; thus, essence is abundant and erections occur daily. At fourteen, a female's congenital Qi is substantial; Tian Gui arrives and merges with the blood produced in acquired nature, becoming full. Females belong to Yin, and Yin corresponds to the Moon; thus, blood is full and menstruation occurs. Therefore, when men and women unite at the appropriate age, their congenital True Qi and acquired Essence and Blood harmonize in Yin and Yang, enabling conception. During this union of Yin and Yang, an abundance of Yang naturally results in a male, which is the Dao of Heaven creating males. An abundance of Yin naturally results in a female, which is the Dao of Earth creating females.

One should wait until thirty to marry after achieving seminal emission, and wait until twenty to marry after the onset of menstruation, all to ensure that Yin and Yang are fully matured, so that the offspring born will be robust, strong, and long-lived.

[Note] A male achieves seminal emission at sixteen but must wait until thirty to marry; a female experiences the onset of menstruation at fourteen but must wait until twenty to marry. This is done to allow Yin and Yang to become fully complete and solid. Only then can intercourse lead to pregnancy, pregnancy to gestation, and the child born will be sturdy and long-lived. Nowadays, girls who have not yet undergone the capping ceremony (Ji Li) may approach men as soon as Tian Gui begins. This causes Yin Qi to leak prematurely, damaging the body before it is complete and stirring movement before it is solid. Consequently, although intercourse may occur, conception fails; if conception occurs, gestation fails; and if birth occurs, the child is inevitably fragile and short-lived.

For males, accumulating essence requires reducing desires. Intercourse must be timed correctly and not missed. One must wait for the fertile window to arrive; the unbearable intensity of sexual desire marks the true mechanism for conception.

[Note] The method of accumulating essence lies solely in reducing desires. When engaging in intercourse with a woman, one must seize the opportune time and not be too early or too late. Generally, after a woman's menstrual flow ends once per month, there is a day of "Yun" (gestation/ovulation). During this time, internal Qi steams and generates heat; she appears intoxicated and infatuated, experiencing an unbearable urge to engage in intercourse. This is a natural seasonal rhythm and the true mechanism for conception and biological generation.