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The *I Ching* (Book of Changes) states: "Taiji generates Liangyi (Two Modes), Liangyi generates Sixiang (Four Images), Sixiang generates Bagua (Eight Trigrams). When the Eight Trigrams interlock and interact, all things are born." Taiji contains Yin within Yang; the images of Liangyi show Yang dividing Yin. Yang cannot stand alone; it must rely on Yin to establish itself. Therefore, Yang takes Yin as its foundation, and Yin is the mother of Yang. Yin cannot manifest independently; it must await Yang to become visible. Therefore, Yin takes Yang as its commander, and Yang is the father of Yin. Rooted in both Yin and Yang, the principles of Heaven and Humanity are one. In terms of fixed positions, Yang is above and Yin is below, establishing the structure of opposing forms. In terms of Qi transformation, Yin rises and Yang descends, magnifying the function of dynamic circulation.

Let us discuss this using trigrams: Qian represents Heaven. To the left of Qian is Kan Water, and to the right is Dui Water. This indicates water moving in the sky, but it is not actual water; rather, it is the Yin Qi of water rising into the sky. If Yin rises to the sky without sufficient Qi transformation, dark mists will rise everywhere, altering the celestial phenomena. Kun represents Earth. To the left of Kun are Zhen Thunder/Fire, Xun Wind/Fire, and Li True Fire. This indicates fire emerging from underground, but it is not actual fire; rather, it is the Yang Qi of fire descending to the earth. If Yang descends to the earth without complete Qi circulation, the land becomes saline and barren, altering the terrestrial phenomena. However, discussing trigram images remains abstract; let us verify this through seasonal observations. Consider a year: after the Summer Solstice, intense heat prevails. If Yin Qi were not secretly generated and heavy rains did not fall periodically, the earth would turn to ashes. This is clear evidence of Yin Qi rising. Moreover, when Yin Qi rises to the sky and receives Heaven's distribution, it permeates boundlessly. After the Winter Solstice, cold condenses and freezes. If Yang Qi were not secretly stored and spring waters did not flow, people would freeze to death. This is clear evidence of Yang Qi descending. Furthermore, when Yang Qi descends to the earth and receives the earth's incubation, it develops infinitely. Specifically, Yin Qi does not rise by itself; it requires Yang Qi to ascend. The Yang Qi on earth is the Yang Qi that descended from heaven; assisting Yin's ascent with Yang, we speak of "Yin ascending" rather than "Yang ascending." Yang Qi does not descend vacuously; it must contain Yin Qi. The Yin Qi in heaven is the Yin Qi that rose from earth; transforming with Yang following Yin, we speak of "Yang descending" rather than "Yin descending." Thus, Yin and Yang are mutually rooted, originally one Qi, divided only by the movements of ascent and descent!

Speaking of humans: Human Yin Qi ascending refers to the essence and Qi derived from food and drink processed by the Spleen and Stomach, rising to the Lung. As the classics state, "Fluids enter the Stomach, overflow with essence and Qi, are transported upward to the Spleen, where the Spleen scatters the essence, transporting it upward to the Lung." This is like water moving in the sky. Since Qi contains Water, it is called Yin Ascending, yet Water never separates from Qi. If it is not the steaming of Qi and Water together, but rather pathogenic water flooding upward, water will overflow from high sources, leading to conditions such as Lung distension and wheezing cough. Since Qi and Water originate in the Stomach, the Stomach must be full of food and drink for Yin Qi to flourish. Hence, the classics say: "Essence Qi is born from Grain Qi." If the Stomach Qi itself becomes diseased, the source of transformation and generation is cut off. How can one expect the Kidney's grasping function to store Yang Qi securely? Human Yang Qi descending refers to the Yang Qi of the Lung descending to the Kidney, just as Heavenly Yang Qi hides in the Earth. This is like fire emerging from underground. Water is transformed by Qi, hence called Yang Descending, yet Qi never separates from Water. If it is not the moistening interaction of Qi and Water, but rather dry Yang descending, the Metal (Lung) will wither and Water (Kidney) will exhaust, leading to conditions such as consumptive cough and bone-steaming fever. Thus, Yang Qi cannot descend vacuously; it must contain Yin Qi. The True Yin of the Lung is the Yin Qi rising from the Spleen, Stomach, and Kidney. Only when the Yin Qi of the Spleen, Stomach, and Kidney rises to the Lung and receives the Lung's distribution can Yin Qi fill the entire body. The classics state: "The Kidney connects upward to the Lung." And again: "Without Yang, Yin cannot be generated; without Yin, Yang cannot transform."

However, the ascent and descent of Yin and Yang are invisible. Let us use a pot and steamer as an analogy. The water and grain in the pot represent Water; the fire under the pot represents Fire. The steam rising from the pot is Yang Qi; the moisture within the steam is Yin Qi. But only when the pot is full of water and grain, heated by the fire below, and covered by the lid above, can the moisture in the steam warm and nourish, steaming upward and spreading downward. Qi containing Water is Yin Ascending; Water transformed by Qi is Yang Descending. If the pot lacks water and grain, cooking without rice results in the fire beneath holding only deficient Yang, and the lid becoming a useless vessel. Or, if there is water and grain but no fire underneath, not only can essence and Qi not be steamed and transported, but even dregs cannot be melted; without a lid, not only is control lost, but spillover becomes uncontrollable. Are not Yin and Yang two Qi mutually dependent, inseparable even for a moment?

Weighing these two Qi, Yin receives Yang, and Yang commands Yin. Disease occurs if Yang Qi fails to reach even a fraction; death ensues if Yang Qi is exhausted even by a fraction. Therefore, humans should prioritize Yang Qi. Yet, while Yang Qi is important, Yin Qi is also crucial. Why? Human affairs and diseases often injure Yin. The classics say: "Stillness allows the Spirit to reside; movement leads to consumption." Daily toil involves movement; movement produces less than it consumes. Disease is also a form of movement; movement causes the Six Evils to transform into Fire. Transforming into Fire inevitably damages Yin, so in such cases, Yin Qi should be prioritized. For example, in sailing, the driving force is Qi, and the medium is Water; only with sufficient Water can Qi be vigorous. Similarly, consider a lamp: the flame is Fire, and the oil is Water; only with sufficient Oil can the Fire shine brightly. Qi is the commander of Blood, and Blood is the vessel for Qi. This is the principle that Yang commands Yin based on Yin. Without Yin, Yang has no basis and perishes. (Death occurs from the depletion of Yin fluids or the emaciation of large muscles.) Therefore, the term "Yin-Yang" is read as "Yin-Yang," not "Yang-Yin," which brings sudden enlightenment!