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Exploring the principle of heaven and earth's yin and yang transforming and generating all things is indeed the root of human life. To skillfully grasp and remedy its subtle applications is truly our great authority in acting on behalf of heaven (Tian Dao). If my understanding of this principle is not profound, I might apply cold therapies when heat is needed (adding ice) or heat therapies when cold is needed (using soup), thereby causing harm to the body, a disaster that is beyond description.

Since my youth, I have read Zhu Danxi's treatise "Yang is Often Excessive, Yin is Often Insufficient," and I have never failed to admire his profound insights. As I matured into adulthood, I began to be half-skeptical. Only after reaching the age of forty (Bu Huo) did I realize his theory was fundamentally erroneous. Therefore, in my *Classified Classic: Records of Seeking Correction* (*Lei Jing · Qiu Zheng Lu*), I included an essay titled "Discussion on the Great Treasure" (*Da Bao Lun*), precisely to correct his fallacies. However, I often feared that my shallow insight and biased words would bring disaster to future generations, so I harbored doubts and fears for a long time, hoping for correction from eminent scholars. Unexpectedly, years passed since publication without any feedback. Being accepted by wise and virtuous people gave me some comfort.

In the summer of the Bingzi year, I formed a spiritual connection with a friend who transmitted a few words to me. Upon inquiring about his name, I learned he was Mr. Li from the Sanwu region. Reading his *Zhi Nan* (Guide), he stated: "Yang is often excessive, Yin is often insufficient." This is Danxi's definitive argument. Yet this Mr. Zhang [referring to myself] claims the opposite: "Yang is often insufficient, Yin is often excessive." How can they be so contrary? Is this self-righteousness, or forced argumentation? Or does it have other sources? Let us not waste breath debating right and wrong verbally; instead, let us verify it through human affairs, where right and wrong will become immediately apparent. For instance, from birth, males do not achieve seminal transmission until sixteen, and females do not begin menstruation until fourteen. In decline, males exhaust their essence at eighty-eight (64 years old), and females exhaust their blood at seventy-seven (49 years old). After essence and blood are depleted, what keeps a person alive is only Qi. Qi belongs to Yang, while essence and blood belong to Yin. The production of essence and blood lags behind Qi, and their depletion precedes Qi. Thus, when essence is gone but Qi remains, is this not clear evidence that "Yin is often insufficient and Yang is often excessive"? From this, we know the ancients' golden words were not false, and how easily junior scholars err in their rash judgments. Hearing this view increases my sorrow. I mourn because such words easily sway people, who applaud them without hesitation. Purple confusing red is nothing compared to this. If not clarified, people will remain asleep in a long dream, which is tragic given the gravity of life and death. Yet, following sorrow comes joy. I rejoice because the subtleties of the Great Dao only become clear through refutation. Fortunately, through his statement, I have opened a path to resolve this confusion. Now, I shall refute Mr. Li's words.

If he regards essence as Yin and Qi as Yang, this is not technically incorrect. However, his vision is limited to immediate phenomena, leading him to mistake identities, confusing Zhang with Li. He fails to realize that essence is Water, and Water is Yang. If speaking of Fire and Water metaphorically, Water is indeed Yin and Fire is indeed Yang. But if speaking of transformation and generation, all things originate from Water. Both Pre-Heaven and Post-Heaven energies are rooted here, and Water is the manifestation of Yang's transformation. Why? In the Five Elements, Water originates from One Heaven; in the Six Qi, Water belongs to Taiyang (Greater Yang). Can Water be considered Yin? Furthermore, in the human body, abundant essence leads to strong Yang, while deficient essence leads to impotence. Can Essence be considered Yin? Additionally, health cultivators emphasize "Pure Yang." This Pure Yang refers to Essence. If Essence leaks, where is the Yang? Can Essence still be called Yin? Taoist texts say: "As long as a portion of Yang remains, one does not die; as long as a portion of Yin remains, one cannot become an immortal." This implies immortality requires Pure Yang. According to Mr. Li's logic, simply exhausting one's essence would result in Pure Yang, making the path to immortality trivial. This is laughable! Mr. Li sees only one aspect of Yin and Yang, not the whole. The Way of Yin and Yang, viewed broadly, sustains heaven and earth; viewed minutely, it analyzes every detail. It is vast and small, present everywhere in transformation. If discussing clarity and turbidity, Qi is Yang, Essence is Yin—this is one aspect. If discussing life, death, gathering, and scattering, the generation of essence and blood is Yang. Qi gains life from Yang and dies without it. This is the source of life and the major principle of Yin and Yang.

Human life is like plants. Plants sprout, grow leaves and branches, then produce flowers and fruits. In decline, fruits fall, leaves remain, and they gradually wither. This is the cycle of growth, flourishing, transformation, storage, and death, governed by seasons. Human life is similar: first infancy, then essence and blood, then offspring. In decline, essence and blood deplete while the form remains, gradually ending. This is the cycle of growth, strength, aging, and decline, governed by age. From childhood to old age, living beings are dominated by Sheng Qi (Vital Qi). This lifelong Vital Qi is dominated by Yang Qi, differing only in degree. If human essence is Yin, are fruit and flower maturity also Yin? Are un-withered leaves Yang? Moreover, even if everyone lived to a hundred, it would only be their natural lifespan. How many actually reach this age? Those who die young lack Vital Qi. How does this show Yang is excessive? Strong Yang brings longevity; weak Yang brings premature death. How does this show Yang is excessive? Yang Qi is hard to gain and easy to lose; once lost, it is hard to recover. How does this show Yang is excessive? The *Chapter on Natural Lifespan* states: "At a hundred years old, the five zang organs are empty, spirit and qi depart, and only the corpse remains. Form is Yin; Spirit and Qi are Yang. Spirit and Qi departing while form remains is the conclusion of 'Yang is often insufficient.' How can one say Yang is often excessive?"

Regarding essence and Qi, Yin and Yang can sometimes be discussed separately and sometimes together. Separately, as in the discussion of clarity and turbidity. Together, as in alchemical cultivation where Essence, Qi, and Spirit are the Three Treasures. Pre-Heaven Qi transforms from Spirit to Qi to Essence. Post-Heaven Qi transforms from Essence to Qi to Spirit. These three transform into each other, sharing the same root Qi, hence inseparable. Thus, one skilled in treating Essence can generate Qi from Essence; one skilled in treating Qi can generate Essence from Qi. This is the妙用 (wonderful application) of separability and inseparability. However, Cold and Heat Yin/Yang must be distinguished. Cold is like ice; Heat is like charcoal. Ice and charcoal do not mix; how dare one use them recklessly? I say: Essence-Qi Yin/Yang are inseparable; Cold-Heat Yin/Yang are distinct. This is crucial medical advice. Essence-Blood Yin/Yang refer to constitutional Yuan Qi; Cold-Heat Yin/Yang refer to medicinal treatments. Today, mistaking insufficient Yuan Yang as excessive Fire leads to using bitter-cold inferior drugs as tonics to nourish Yin. Alas! Can the limited Vital Qi of Mount Niu withstand endless Yin stripping? For four centuries, patients have suffered in silence like a mute eating bitter herbs.

Even Danxi, a figure of hope, erred thus; how much more others? Ancients said: "Do not read books that are not sage-like." This applies especially here.

The Way of Heaven and Earth's Yin and Yang is naturally peaceful. Imbalance brings disaster. My claim that "Yang is often insufficient"—is it not also a one-sided view? Because Danxi's theory of nourishing Yin is wrong, I must speak oppositely to save the Vital Qi of ten thousand generations. What humans value most is Life. How can one live? Only through Yang. Without Yang, there is no life. Should one not treasure this Yang? Worrying daily about its deficiency is not excessive. My claim that "Yang is often insufficient" is akin to a man from Qi worrying about spring fading. If my understanding is truly low, I await refutation from enlightened sages.