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4. Regarding the determination of fetal sex, some argue it is based on the sequence of arrival between essence (Jing) and blood; others distinguish by odd or even numbers or the quantity of days; still others determine it by whether conception occurs in the left or right uterus. All these views fail to understand the underlying cause. To truly grasp this mechanism, one must recognize that when the Yang pathway (Gan Dao) prevails, a male is born; when the Yin pathway (Kun Dao) prevails, a female is born.

[Note] Regarding theories of fetal sex determination, ancient sages held that if blood arrives first and envelops the essence, a male is formed; if essence arrives first and envelops the blood, a female is formed. If essence and blood disperse and are simultaneously enveloped, twins or multiple fetuses result. Concerning the origin of multiple pregnancies, some claim that conception on the 1st, 3rd, or 5th day after menstruation ends yields a male, while the 2nd, 4th, or 6th day yields a female. Others suggest conception on the 1st or 2nd day yields a male, and on the 4th or 5th day yields a female. Still others assert that conception in the left uterus produces a male, and in the right uterus produces a female. These views are all subjective and demonstrate a profound ignorance of the true causes. Generally, single pregnancies can be analyzed by days. However, for twins or multiples, which may be male, female, or mixed, can they really be determined by days? Historical records show cases of triplets, quadruplets, half-male/half-female, or varying ratios of males and females. Thus, the theory that "odd days yield males, even days yield females" cannot be relied upon. Moreover, how could there be a logic where a male is conceived on the first day and a female on the second? The Dantian is the Mingmen (Gate of Life). In men, it is called the Jing Shi (Essence Chamber); in women, it is the Zi Gong (Uterus). Its shape is like a joined bowl, with no division into two parts. To speak of "left and right" implies the existence of two uteri, which is an absurd notion. So, how then is it determined? It lies solely in harmonizing Heaven and Humanity: when Yang Qi is abundant and follows the Heavenly Way, a male is born; when Yin Qi is abundant and follows the Earthly Way, a female is born. This suffices as a definitive conclusion.

5. Ancient texts state that twins arise from abundant essence and Qi, splitting apart, potentially resulting in undefined sexes or combined forms. This is due to abnormal changes in Yin and Yang and the prevalence of mixed/impure Qi. Such events occur in reality, but their principles are difficult to elucidate.

[Note] Ancients believed that twins result from excess essence and Qi, which divide, with blood subsequently nourishing both. If two males are conceived together, it occurs during "Gang Ri" (Yang days/times); if two females are conceived together, it occurs during "Rou Ri" (Yin days/times); if a male and female are conceived together, it occurs during a Yang time on a Yin day or a Yin time on a Yang day. Other cases may involve individuals who cannot function as fathers or mothers, or those with combined sexual characteristics. These are caused by abnormalities in Yin and Yang and influences from mixed Qi. While such phenomena exist in practice, they lack basis in standard theory and cannot be verified through conventional examination.