精血先后分男女，或以奇偶少多分，或以子宫左右定，是皆不晓个中因，欲识此中真消息，干道阳男
坤女阴。
【注】分男女之说，先贤有以血先至裹精则成男、精先至裹血则成女，精血散分并裹
则为骈胎、品胎之原者；有以月水尽后一、三、五日成男，二、四、六日成女，与夫经水断
后一、二日成男，四、五日成女者；有以受气于左子宫成男，受气于右子官成女者，皆
各执一见，殊为不晓此中因也。盖独男独女之胎，可以日数论，骈胎、品胎，或男或女，
亦可日数论乎？稽之史载，一产三子、四子，有半男半女，或男多女少，男少女多者，则
一、三、五日为男，二、四、六日为女之说，不可凭矣！抑岂有一日受男，而二日复受女
之理乎？丹田，命门也。在男子曰精室，在女子曰子宫。形如合钵，并无两歧可分。曰
左右，则是有两子宫矣。此说尤属不经。然则何以定之？亦惟以会合天人，阳盛干道成
男，阴盛坤道成女，斯足为确论耳。
古以双胎精气盛，不成男女或兼形，阴阳变常驳气盛，事之所有理难明。
【注】古以双胎，乃精气有余，歧而分之，血因分而摄之故也。若男同孕者，刚日阳时
也；女同孕者，柔日阴时也；男女同孕者，刚日阴时，或柔日阳时也，其他或有不成男女，
男不可为父，女不可为母，与男女之兼形者，又皆阴阳变常，驳气所感，事之所有，理之
所无，莫可稽考者也。
The sequence of arrival of Jing (Essence) and Xue (Blood) determines the sex of the fetus, or it is distinguished by odd/even days or quantity, or by the left/right position of the uterus. These are all views that fail to understand the true cause within. To know the real truth here, one must recognize: following the Heavenly Way (Yang) results in a male, while following the Earthly Way (Yin) results in a female.
[Note] Regarding theories on fetal sex, ancient sages held various views. Some believed that if Blood arrives first and envelops Jing, a male is formed; if Jing arrives first and envelops Blood, a female is formed; and if Jing and Blood disperse and envelop each other, this is the origin of twin or multiple fetuses. Others believed that conception on the 1st, 3rd, or 5th day after menstruation ends results in a male, while the 2nd, 4th, or 6th day results in a female; or that conception 1-2 days after menstruation ends yields a male, and 4-5 days yields a female. Still others believed that conception in the left uterus produces a male, and in the right uterus produces a female. All these views are subjective and fundamentally misunderstand the underlying principles. Furthermore, can the sex of a single fetus be determined by days? Can the sex of twins or multiples also be determined by days? Historical records show cases of triplets or quadruplets, with mixed sexes or varying ratios of males to females. Thus, the theory that "odd days produce males, even days produce females" cannot be relied upon! Is there any logic where a male is conceived on one day and a female on the next? The Dantian (Lower Abdomen) is the Mingmen (Gate of Life). In men, it is called the Jing Shi (House of Essence); in women, it is the Zigong (Uterus). Its shape is like a joined bowl, with no division into two. To speak of "left and right" implies two separate uteri, which is an absurd notion. So how is sex determined? It is solely through the harmony of Heaven and Humanity: when Yang Qi is predominant, following the Heavenly Way produces a male; when Yin Qi is predominant, following the Earthly Way produces a female. This is the only reliable conclusion.
Ancient texts state that twins arise from abundant Jing and Qi, resulting in forms that may not distinguish between male/female or possess combined characteristics. This is due to abnormal changes in Yin and Yang and the prevalence of mixed/impure Qi. Such phenomena exist but their underlying principles are difficult to clarify.
[Note] Ancients believed that twins occur because Jing and Qi are excessive, splitting into two, with Blood dividing to nourish them. If two males are conceived together, it occurs during Yang times of Yang days (Gang Ri); if two females are conceived together, it occurs during Yin times of Yin days (Rou Ri); if a male and female are conceived together, it occurs during Yin times of Yang days or Yang times of Yin days. Other cases, such as individuals lacking distinct sexual characteristics, males unable to father children, females unable to bear children, or those with combined male/female traits, are caused by abnormal Yin-Yang changes and influence from mixed Qi. While these phenomena exist, they contradict normal principles and cannot be historically verified.