二者，阴也，后天之形；一者，阳也，先天之气。神由气化，气本乎天，故生发吾身者，即真阳之
气也；形以精成，精生于气，成立吾身者，即真阴之精也。经云∶女子二七天癸至，男子二八天癸至。又
云∶人年四十而阴气自半。所谓阴者，即吾之精，造吾之形也。人生全盛之数，惟二八后至四旬外，前后
止二十余年，则形体渐衰。故丹溪引日月之盈亏，以为“阳常有余，阴常不足”，立补阴丸为神丹。不知
天癸未至，本由乎气，而阴气自半，亦由乎气，是形虽属阴，而气则从阳也。故人身通体之温者，阳气
也。及既死，则形存气去，此阳脱在前，阴留在后。可见生由乎阳，死亦由乎阳。非阳能死物也，阳来则
生，阳去则死。故经云∶阳气者，若天与日，失其所，则折寿而不彰。可见人之生，只此一息真阳为营运。孰
谓阳常有余，而以苦寒之味伐此阳气乎？
人身捍卫冲和不息之谓气，扰乱变动妄行之谓火。火与气，二而一，一而二者也。顾人身之气，有
正气，亦有邪气；人身之火，有少火，亦有壮火。少火生人之元气，是火即为气。此气为正气。壮火食
人之元气，是气即为火。此气是邪气。邪气有余即为火，若正气有余，便是人身之元气。人身元气生于命
门。命门者，精神之所舍，而为阳气之根也。故命门之火旺，则蒸糟粕而化精微，所谓人非此火不能有生
者是也。是火即是气，不可误认有余之邪。气为生人少火，立命之本也。若正气有余，不可便指为火。丹
溪之言殊欠明白。
Yin refers to two substances, which constitute the postnatal form; Yang refers to one energy, which is the prenatal Qi (vital energy). Shen (spirit) arises from Qi transformation. Since Qi originates from Heaven, the true Yang Qi (true yang vital energy) is what generates our body. Form is composed of Jing (essence), and Jing is born from Qi; thus, the true Yin Jing (true yin essence) is what establishes our body. The Classics state: "In girls, at age fourteen, Tian Gui (heavenly water/reproductive essence) arrives; in boys, at age sixteen, Tian Gui arrives." It also states: "When a person reaches forty, Yin Qi naturally halves." What is referred to as Yin is our Jing, which constructs our form. The period of full physical prime in human life is only from after age sixteen to beyond age forty, lasting merely over twenty years, after which the form gradually declines. Therefore, Zhu Danxi drew an analogy with the waxing and waning of the moon to propose that "Yang is often in excess, while Yin is often deficient," and established Buyin Wan (Nourishing Yin Pill) as a miraculous medicine. He did not realize that before Tian Gui arrives, it depends on Qi; and the halving of Yin Qi also depends on Qi. Although form belongs to Yin, Qi follows Yang. Thus, the warmth throughout the human body is due to Yang Qi. After death, the form remains but Qi departs; this shows that Yang departure occurs first, while Yin substance remains later. This demonstrates that life depends on Yang, and death also depends on Yang. It is not that Yang kills things, but rather that when Yang comes, there is life; when Yang goes, there is death. Hence, the Classics say: "Yang Qi is like the sun in heaven; if it loses its proper place, lifespan is shortened and vitality is obscured." This shows that human life relies solely on this breath of true Yang for operation. Who claims that Yang is often in excess and uses bitter-cold flavors to attack this Yang Qi?
The state of guarding, harmony, and ceaseless movement in the human body is called Qi; the state of disturbance, change, and erratic movement is called Huo (fire). Huo and Qi are two yet one, and one yet two. Regarding Qi in the human body, there is Zheng Qi (upright qi) and Xie Qi (pathogenic qi); regarding Huo in the human body, there is Shao Huo (mild fire/physiological fire) and Zhuang Huo (excessive fire/pathological fire). Shao Huo generates Yuan Qi (original qi) in humans, meaning Huo is Qi. This Qi is Zheng Qi. Zhuang Huo consumes Yuan Qi in humans, meaning Qi becomes Huo. This Qi is Xie Qi. Excess Xie Qi constitutes Huo; if Zheng Qi is in excess, it is simply the body's own Yuan Qi. Human Yuan Qi is born from Ming Men (Gate of Life). Ming Men is where Shen and Jing reside and serves as the root of Yang Qi. Therefore, when the fire of Ming Men is vigorous, it steams and transforms dregs into fine nutrients, which is what is meant by "humans cannot have life without this fire." This fire is Qi, and it should not be mistakenly identified as excess pathogenic factors. Qi is the Shao Huo that gives life and is the foundation of existence. If Zheng Qi is in excess, it should not be immediately labeled as Huo. Zhu Danxi's statement is particularly unclear.