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The substance of Yin and Yang is referred to as Qian (Heaven) and Kun (Earth); their function is referred to as Water and Fire; their transformation is referred to as Form and Qi (Vital Energy). Speaking from the perspective of generation and destruction, Yang generates while Yin destroys; speaking from the perspective of cold and heat, Yang is hot while Yin is cold. Regarding the mechanism of their generation and transformation, Yang acts first and Yin follows later; Yang exerts influence while Yin receives it. In the pre-heaven stage, form is transformed from Qi, which is Yang generating Yin; in the post-heaven stage, Qi is transformed from Form, which is Yin generating Yang. Form is Jing (Essence), and Jing is Water; Shen (Spirit) is Qi, and Qi is Fire. The two Qi of Yin and Yang must not be biased; when balanced, Qi is harmonious and generates living things; when biased, Qi becomes discordant and destroys living things. The Classics state: "When Yin is peaceful and Yang is secure, Spirit and Mind are regulated; when Yin and Yang separate and break apart, Essence and Qi are exhausted." This is how the ancient kings taught people to observe Yin and Yang and preserve vital Qi.

Yang is one (odd number), and Yin is two (even number); therefore, even numbers belong to Yin, representing post-heaven form; odd numbers belong to Yang, representing pre-heaven Qi. Shen arises from the transformation of Qi, and Qi originates from Heaven; thus, what generates my body is the True Yang Qi. Form is composed of Jing, and Jing is generated by Qi; thus, what establishes my body is the True Yin Qi. *Shanggu Tianzhen Lun* (Treatise on the Heavenly Truth of Antiquity) states: "At fourteen years of age, the Tian Gui (menstrual essence) arrives for women; at sixteen years of age, the Tian Gui arrives for men." Does this not indicate that Yin is difficult to complete in post-heaven development? *Yinyang Yingxiang Dalun* (Great Treatise on Yin-Yang Correspondences) states: "When a person reaches forty years of age, their Yin Qi naturally halves." Does this not indicate that Yin declines easily in early stages? Therefore, the period of full vitality lasts only from after sixteen years of age until beyond forty years of age, spanning no more than twenty-plus years, after which the physical form gradually declines.

The path of Yang is often solid/excess, while the path of Yin is often deficient/empty. Thus, Zhu Danxi cited the waxing and waning of the sun and moon to propose the theory that "Yang is often in excess and Yin is often deficient," establishing formulas such as *Buyin Wan* (Nourishing Yin Pill) and *Dabu Wan* (Great Tonifying Pill). It is only regrettable that he regarded Huangbai (Phellodendri Cortex) and Zhimu (Anemarrhenae Rhizoma) as miraculous elixirs, leading to their widespread family transmission and household use. Little did they know that the non-arrival of Tian Gui originally depends on Qi, and the natural halving of Yin also depends on Qi. Although form belongs to Yin, Qi still adheres to Yang. This is the critical juncture of life and death, which must not fail to be distinguished.

The significance of Yang is vast. Yin is governed by Yang; what concerns the origin of creation and the root of life and nature is solely this. Let us distinguish the most important aspects: first, Form and Qi; second, Cold and Heat; third, Water and Fire.

Regarding Form and Qi: Yang transforms into Qi, and Yin forms into structure; thus, form inherently belongs to Yin. However, the warmth throughout the entire body is Yang Qi; the aliveness and activity of life are Yang Qi; the unfathomable spiritual clarity of the five sense organs and five Zang organs is also Yang Qi. If spiritual awareness is completely extinguished and the body becomes cold as ice, although the form remains, the Qi has departed. This occurs because Yang collapses first, leaving Yin behind; is this not Yin exceeding Yang? This is the distinction between Form, Qi, Yin, and Yang.

Regarding Cold and Heat: The warmth of spring and summer is Yang; the cold of autumn and winter is Yin. During the intense heat of late summer, grasses, trees, insects, and animals suffer from being roasted, yet the more they are roasted, the more abundant they become; without heat, there is no flourishing. But upon a single night of wind and frost, they stiffen and wither across the fields. This shows that heat can generate life, while cold lacks vitality; heat causes no harm, but cold is to be feared. Is this not Cold overpowering Heat? This is the distinction between Cold, Heat, Yin, and Yang.

Regarding Water and Fire: These are the powers of creation, manifesting in four images: the Sun is Taiyang (Greater Yang), Fire is Shaoyang (Lesser Yang), Water is Taiyin (Greater Yin), and the Moon is Shaoyin (Lesser Yin). These are the true forms of the Four Images. There is no Taiyin within Yang, and no Taiyang within Yin; this represents the exclusive dominion of Yin and Yang. The Sun shining in the sky is Yang within Yang, is it not Taiyang? The Moon shining in the sky is Yin within Yang, is it not Shaoyin? Water flowing on earth is Yin within Yin, is it not Taiyin? Fire existing on earth is Yang within Yin, is it not Shaoyang? These great principles were unknown to Zhu Danxi, who therefore cited the waxing and waning of the sun and moon to prove the deficiency and excess of Yin and Yang. Furthermore, regarding the nature of Yin and Yang, the greater ones have rigid Qi; thus, the Sun cannot be extinguished, and Water cannot be dried up. The Sun is the root of Fire, and Water is the root of the Moon. The lesser ones have soft Qi; thus, Fire sometimes rests, and the Moon sometimes wanes. Fire is the remnant of the Sun, and the Moon is the remnant of Water. Only that which does not extinguish is True Fire; that which comes and goes intermittently, how could it be Yuan Yang (Primordial Yang)? Therefore, only the fire of True Yang can generate living things, whereas the ordinary fire of prairie fires can only scorch objects. Heaven generates Water with One ("Tian Yi Sheng Shui"). "One" refers to the first number of Heaven; One is Yang. Without One, it would stop at Six. Thus, Water's ability to generate life relies on this One; Water's ability to transform into Qi relies on this One. Therefore, in spring and summer, when Earth obtains Water, things grow; in autumn and winter, when Earth obtains Water, things do not grow or develop, and Water itself dies. Who is the master of Water's generation and movement? Is it not Yang? This indicates that Yang exists within Water, but Water itself is not Yang.

If Yang Qi is not充盈 (abundant), then the scope of vitality is narrow, let alone if there is no Yang at all. Therefore, Yang fears only decline, and Yin fears only excess; it is not that Yin can become excessive on its own, but rather that Yin becomes excessive due to Yang's decline. The birth of all things depends on Yang, and the death of all things also depends on Yang. It is not that Yang kills things; when Yang arrives, there is life; when Yang departs, there is death. Consider the example of the Sun: When the Sun travels through the southern land, corresponding to winter, it is not that there is no Sun at that time, but merely that it is slightly farther away, making severe winter hard to withstand and causing all things to wither. Thus, the harmony of heaven and earth relies solely on this Sun; the survival of all things relies solely on this Sun. If there were no Sun, even if heaven and earth were vast, they would merely be a mass of cold matter. Humans are a small universe; gaining Yang brings life, losing Yang brings death. Decline of Yang is the gradual onset of Wang Yang (Collapse of Yang).

When the Sages created the *I Ching* (Book of Changes), they first established a single line, setting up the Ancestor of Yuan Yang, clarifying the primal prosperity of Yang virtue in the initial six of Kun (Earth). Mentioning "treading on frost, firm ice approaches," they worried about the gradual growth of Yin Qi, guarding against its interference with generative processes.

The *Neijing* (Inner Canon) states: "The key to Yin and Yang is that when Yang is dense, Yin becomes secure." This says that what Yin relies upon is Yang as the master. It also states: "Human Yang Qi is like the sky and the sun; if it loses its proper position, lifespan is shortened and brilliance is obscured. Therefore, the movement of heaven should rely on the brightness of the sun." It is evident that humanity's greatest treasure is but this breath of True Yang. Who says Yang is often in excess, and wishes to use bitter-cold substances to attack this Yang Qi, thinking that preserving life can be achieved this way?

A guest asked: "To consolidate this Yang, what plan should be adopted?" I replied: "Simply knowing the root is the key; this is the Mingmen (Gate of Life)." What is called Mingmen is where we receive our pre-heaven existence; it is where our post-heaven self is nurtured. The gate of life is also the door of death; human flourishing, decline, safety, and danger are all tied to this. Because it is the source of vital Qi, strong Qi leads to strength, and weak Qi leads to illness. Although this is a place of extreme Yin, it is actually the residence of Yuan Yang. As for the Spleen and Stomach, they are the foundation of post-heaven food and water, yet they are merely children of Yuan Yang.