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One must not be ignorant of medicine, for life is of the utmost importance. The sustenance of life depends solely on Yin and Yang; without understanding Yin and Yang, how can one comprehend medical principles? This is why Yin and Yang cannot but be discussed.

In terms of substance, Yin and Yang correspond to Qian (Heaven) and Kun (Earth); in terms of function, they correspond to Water and Fire; in terms of transformation, they correspond to Form and Qi. From the perspective of generation and destruction, Yang governs generation, while Yin governs destruction. From the perspective of cold and heat, heat belongs to Yang, and cold belongs to Yin. Regarding the mechanism of generation and transformation, Yang acts first and Yin receives subsequently. 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 should most preferably not be biased. If unbiased, Qi is harmonious and generates things; if biased, Qi is discordant and destroys 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." The ancient kings, pitying the premature deaths of the people, therefore established and clarified the Way of Medicine to benefit all generations, teaching people to observe Yin and Yang and preserve the vital Qi. Thus, in the *Huangdi Neijing* (Yellow Emperor's Inner Canon), regarding the principles of Yin and Yang, it fears that people may not understand, repeatedly and earnestly admonishing them. Alas, later students still fail to understand. I propose to first speak of the "two" (referring to Yin/Form) and then speak of the "one" (referring to Yang/Qi). The "two" belong to Yin, representing post-heaven form; the "one" belongs to Yang, representing pre-heaven Qi. Shen arises from Qi transformation, and Qi originates from Heaven; thus, what generates our body is the True Yang Qi. Form is constituted by Jing, and Jing is born from Qi; thus, what establishes our body is the True Yin Qi. Observing the chapter *Shanggu Tianzhen Lun* (Discourse on the Heavenly Truth of Antiquity): "At fourteen, the Tian Gui (heavenly water/menstruation) arrives in women; at sixteen, it arrives in men." Does this not indicate that Yin is generated later and is difficult to complete? Furthermore, observing the chapter *Yinyang Yingxiang Dalun* (Great Discourse on the Correspondences between Yin-Yang and Phenomena): "At forty years of age, Yin Qi is halved." Does this not indicate that Yin declines early and is easily withered? The so-called Yin refers to my Jing, which creates my form. Without form, there is no disaster; with form, there must be decay. Therefore, the period of peak vitality in human life is only from sixteen onwards to beyond forty, lasting merely over twenty years before the form gradually declines. This is indeed a sign of Yin deficiency. From this perspective, even stating that "the Path of Yang is solid/excess, and the Path of Yin is deficient/empty" would be acceptable. Therefore, Zhu Danxi, citing the waxing and waning of the moon, proposed the theory that "Yang is often in excess, and Yin is often deficient," establishing formulas such as *Buyin Wan* (Nourish Yin Pill) and *Dabu Wan* (Great Tonify Pill), treating Huangbai (Phellodendri Cortex) and Zhimu (Anemarrhenae Rhizoma) as divine elixirs, used in every household. Is not the harm caused by this severe? Little do they know that the non-arrival of Tian Gui originally depended on Qi; and the halving of Yin Qi also depended on Qi. Although form belongs to Yin, Qi still follows Yang. This is the pivot of life and death, which must not fail to be distinguished. What I mean by speaking of the "two" first is precisely this. What is meant by the "one"? The "one" is Yang. The significance of Yang is great. Yin is governed by Yang; what concerns the origin of creation and serves as the root of life and nature is nothing other than this. How can we see this? Let us briefly mention three most important points: First, the distinction between Form and Qi; Second, the distinction between Cold and Heat; Third, the distinction between Water and Fire. Regarding Form and Qi: Yang transforms into Qi, Yin forms into shape. Thus, shape fundamentally belongs to Yin, while the warmth throughout the entire body is Yang Qi; the vitality of life is Yang Qi; the mysterious and unfathomable spirit of the five Zang organs and five sense organs is Yang Qi. After death, the body becomes cold as ice, and spiritual awareness is completely extinguished. The form remains, but the Qi has departed. This shows that Yang departs first, and Yin remains later. This is the distinction between Form, Qi, Yin, and Yang—is it not true that Yin exceeds Yang? Second, regarding Cold and Heat: Heat is Yang, Cold is Yin; the warmth of Spring and Summer is Yang, the cold of Autumn and Winter is Yin. During the intense heat of late summer, the world is like a furnace. At this time, all grasses, trees, insects, and animals suffer from scorching heat. Yet, the hotter it is, the more flourishing they become; without heat, they do not thrive. But after a night of wind and frost, they stiffen and wither across the fields. This shows that heat can generate life, but excessive heat causes disease; cold has no generative power, but excessive cold destroys everything. Therefore, heat does little harm, while cold is to be feared. This is the distinction between Cold, Heat, Yin, and Yang—is it not true that Cold is stronger than Heat? Third, regarding Water and Fire: Water is Yin, Fire is Yang. The power of creation lies entirely in Water and Fire. The manifestations of Water and Fire have four aspects: 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, which people have not yet comprehended. Before I could finish speaking, a guest obsessed with medicine passed by me. Hearing this, he was surprised and said: "The Moon is inherently Greater Yin; how can Fire be Lesser Yang? There is no such saying in antiquity. On what basis do you say this? Is there some principle involved?" I replied: "Yang governs the exterior, Yin governs the interior; this is the fixed position of Yin and Yang. There is no Greater Yin within Yang, and no Greater Yang within Yin; this is the exclusive dominion of Yin and Yang. The Sun attached to the sky is Yang within Yang, which is indeed Taiyang. The Moon hanging in the sky is Yin within Yang, which is indeed Shaoyin. Water flowing on the earth is Yin within Yin, which is indeed Taiyin. Fire existing within the earth is Yang within Yin, which is indeed Shaoyang. These great principles were indeed unknown to Zhu Danxi, who cited the waxing and waning of the sun and moon to prove the excess and deficiency of Yin and Yang. How could he know that since Water is greater than the Moon, one should not worry about the insufficiency of Yang and the excess of Yin?" The guest said: "The theory of Greater and Lesser Yin and Yang seems reasonable; however, since Water is greater than the Moon, implying an excess of Yin, there is plenty of fire in the world. Is Yang found only in the Sun?" I said: "There is an even more subtle principle here. The nature of the 'Greater' aspect is 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' aspect has 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, is it truly Yuan Yang (Primal Yang)? Therefore, only True Yang Fire can generate life, while ordinary fires that burn wildly can only scorch and damage things. It has never been heard that life is generated by roasting or baking. How can Fire be used to metaphorize the Sun?" The guest said: "If this is so, then Water is indeed Greater Yin. However, why does it say 'Heaven One generates Water'? Is Water not Yang? And why does it say 'Water can generate all things'? Is Water not the Qi of generation?" I said: "This question is even more profound. 'Heaven One' refers to the One of Heaven; One is Yang. Without One, it stops at Six. Therefore, what allows Water to generate things relies on this One; what allows Water to transform into Qi also relies on this One. Do you not observe that in Spring and Summer, when Earth obtains Water, it can grow and flourish? Is it not because of this One? In Autumn and Winter, when Earth obtains Water, it does not grow or flourish. Is it not because this One is absent? Not only does it not grow, but it freezes itself, meaning Water dies. Thus, it is evident that whatever makes Water generate and flow is dominated by Yang Qi. This means Yang exists within Water; it does not mean Water itself is Yang." The guest said: "Then, if the power of generation and transformation all stems from Yang Qi, those who say Yang is in excess are indeed correct. Why do you worry about its insufficiency? Is this not an overconcern?" I said: "I wrote this treatise precisely for this reason. Fearing that people might not realize this, I first spoke of Form and Qi, then of Cold and Heat, and now of Water and Fire, all to clarify that Yang is not in excess and that one must not ignore its fundamental importance. Yang governs generation, Yin governs destruction. Whenever Yang Qi is insufficient, the scope of generative power is narrow, let alone when there is no Yang. Therefore, Yang fears only decline, and Yin fears only excess. Yin cannot become excessive on its own; it becomes excessive because Yang declines. All things live due to Yang, and all things die due to Yang. It is not that Yang kills things; rather, when Yang comes, there is life; when Yang leaves, there is death. Let us use the Sun as proof to visualize this. When the Sun travels through the Southern Land, corresponding to Winter, although the Sun is present, it is slightly farther away, resulting in unbearable cold and the withering of all things. Thus, the thing that harmonizes Heaven and Earth is the Sun; the thing that generates all things is also the Sun. If there were no Sun, despite the vastness of Heaven and Earth, it would merely be a mass of cold. Would not the six directions be full of ice pots, and the universe a hell? Humans are a microcosm of Heaven and Earth; gaining Yang brings life, losing Yang brings death. Decline in Yang is the gradual onset of Yang collapse; relying on strength is the omen of leading to decline. Can one not fear this? Therefore, when Fuxi created the *I Ching* (Book of Changes), he first formulated one line (the Yang line), establishing the ancestor of Yuan Yang. When King Wen expanded the *I Ching*, in all sixty-four hexagrams, he used Yang to symbolize the gentleman and Yin to symbolize the petty person, clarifying the virtue of Yang Qi. The *Tuan Zhuan* (Commentary on the Decision) of Qian states: 'Great is the Primal Power of Qian! By it all things begin, and it commands Heaven.' This says that the Primal penetrates the four virtues, and Yang is the head of development. The *Yao Ci* (Line Statement) of the first Yin line of Kun states: 'Treading on frost, firm ice arrives soon.' This worries about the gradual growth of Yin, preventing it from hindering generation and cultivation. The *Tuan Zhuan* of Dayou states: 'Great Possession, Supreme Success. Fire is in the sky.' This says the prosperity of Yang virtue shines everywhere. The *Xici* (Great Commentary) states: 'The great virtue of Heaven and Earth is called Generation.' This emphasizes the root of continuous generation. The *Neijing* states: 'The essence of all Yin and Yang is that Yang must be dense/sealed for Yin to be secure.' This says that what Yin relies upon is solely Yang as the master. It also states: 'Yang Qi is like Heaven and the Sun; if it loses its proper place, lifespan is shortened and brilliance is obscured. Therefore, the movement of Heaven should be illuminated by the Sun.' This says that the movement of Heaven and the destiny of humans, the primal roots, lie entirely in the uniqueness of the Great Sun. All these classic teachings, passed down from the six sages—Fuxi, Yellow Emperor, King Wen, Qibo, Duke of Zhou, and Confucius—for millennia, as if coming from one mouth, are surely not merely my private concern? From this, we can see that the greatest treasure of Heaven is but this red Sun; the greatest treasure of Man is but this breath of True Yang. Who says Yang is often in excess? Those who wish to preserve life should not use bitter-cold substances to attack this Yang Qi. Can preservation of life be achieved thus?" The guest said: "Excellent! I have heard the source of my being. Since there is this Dao, is there not a method? To consolidate this Yang, where should the strategy come from?" I said: "Knowing the root is the key." He asked: "What is the root?" I answered: "Mingmen (Gate of Life)." He said: "I have heard that Earth generates all things, so Spleen and Stomach are the root of the five Zang and six Fu organs. You speak of Mingmen; I do not understand." I said: "Do you not observe that at birth, life comes via the umbilical cord, which connects to the Dantian, called Qihai (Sea of Qi), which is also Mingmen. So-called Mingmen: what generated me in pre-heaven is received from here; what I generate in post-heaven is cultivated from here. The gate of life is also the door of death. Therefore, the rise and fall, safety and danger of humans are all tied to this, because it is the source of vital Qi. Strong Qi leads to strength; weak Qi leads to illness. Although this is a place of extreme Yin, it is actually the residence of Yuan Yang. As for Spleen and Stomach, they are the root of post-heaven grain and water, belonging merely to the children of Yuan Yang. If you wish to know medicine, and neglect this mother that gave birth to you, it will be problematic. Words cannot fully express the meaning. Shall I write another treatise, *Zhenyin Lun* (Discourse on True Yin), to elaborate further?" The guest joyfully said: "I am willing to hear the principles again."