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The principles of Yin and Yang are infinitely variable and cannot be fully described. When speaking of Yin and Yang, it may refer to Heaven and Earth (Tiandi), Qi and Blood (Qixue), or Qian and Kun (Qiankun); these are relative entities, but in reality, Yang governs Yin, Heaven envelops Earth, and Blood follows Qi. Therefore, when the Sages composed the *I Ching* (Book of Changes), regarding Qian they said, "Great is the Primordial Power of Qian, which leads Heaven," and regarding Kun they said, "Excellent is the Primordial Power of Kun, which follows and receives from Heaven." The ancients were adept at understanding the meaning of the *I Ching*, knowing that treating blood disorders requires first regulating Qi. In cases of blood loss (Xuetuo), one must tonify Qi; hence there is the theory that "tonifying blood does not necessarily use Siwu Tang (Four Substance Decoction)." For example, in cases of fever due to Blood Deficiency (Xue Xu), the formula Bu Xue Tang (Blood Tonifying Decoction) was established, using Huangqi (Astragalus Root) as the sovereign herb (Jun Yao) at 1 liang, and Danggui (Angelica Sinensis Root) as the minister herb (Chen Yao) at 4 qian. There are more Qi-tonifying herbs than Blood-nourishing herbs, allowing Yang to generate so that Yin grows, because Yang governs Yin and Blood follows Qi. Similarly, in cases of massive hemorrhage where life is critical, Dushen Tang (Single Ginseng Decoction) of 1 liang is decocted and taken at once, using purely Qi-tonifying herbs. At this time, tangible Blood cannot be rapidly generated, so the slightest Qi must be urgently secured, allowing the intangible to produce the tangible. The wonder of Yin and Yang originates from non-being; thus it is said "Nameless." The beginning of Heaven and Earth, the cycle of life and death, waxing and waning, are the constant measures of Yin and Yang, which humans cannot add to or subtract from! The Sages shaped the transformations of Heaven and Earth, assisted their appropriateness, and always embedded the subtle balance of supporting Yang and suppressing Yin, preventing what has not yet occurred and treating disease before it arises. However, birth, aging, illness, and death are unavoidable for humans, though there are differences in longevity. This is how the Way of Qibo and Huangdi began. Shennong tasted herbs, classifying them by Yin and Yang into Cold, Hot, Warm, Cool, Pungent, Sweet, Sour, Bitter, and Salty. Pungent and Sweet belong to Yang; Cold and Cool belong to Yin; Warm and Hot belong to Yang; Sour and Bitter belong to Yin. Yang governs growth, Yin governs destruction. Those who manage human life wish for people to avoid destruction and embrace growth, so they use Sweet-Warm and Pungent-Hot herbs to bring people into the realm of spring wind growth. All Bitter-Cold herbs are unused; not only are Bitter-Cold herbs unused, but Cool herbs are also rarely used, because Cool corresponds to Autumn Qi, and all things cease to grow in autumn winds. If during summer, Summer-Heat pathogenic factors invade, or if illness results from overeating roasted, fried, pungent, and hot foods, Bitter-Cold herbs may be used temporarily, stopping as soon as the disease is cured; they are ultimately not standard items for saving lives.

In the human body, Yin and Yang embrace each other without separating, allowing one to live a normal lifespan of a hundred years. Thus, if Yang tends to float upward and escape, Yin draws it down from below, preventing escape; if Yin tends to sink downward and escape, Yang draws it up from above, preventing escape. This is like Heaven above and Earth below, the fixed positions of Yin and Yang. However, Earth's Qi often rises to meet Heaven, and Heaven's Qi often descends to meet Earth; thus Di Tian Tai (Earth over Heaven, Peace) occurs when they interact, while Tian Di Pi (Heaven over Earth, Stagnation) occurs when they do not. The Sages participated in assisting Heaven and Earth, having methods to transform Pi into Tai. For those with sinking Yang Qi, light-flavored and light-aromatic herbs such as Chaihu (Bupleurum Root) and Shengma (Cimicifugae Rhizome) are used to lift and raise, causing Earth's Dao to rotate left and ascend to the highest heavens; for those with descending-deficient Yin Qi, herbs that respond to the肃杀 (severe/clearing) Qi of Autumn, such as Qumai (Dianthus Herba) and Pianxu (Polygoni Avicularis Herba), are used to suppress and descend, causing Heaven's Dao to rotate right and enter the deepest earth. This is the benefit opened by Li Dongyuan's Buzhong Yiqi Tang (Tonify the Middle and Augment the Qi Decoction): there is no need to forcibly descend turbidity; raising the Clear ensures the Turbid descends naturally.

Spring and Autumn, Day and Night, are the gates of Yin and Yang. In a year, Spring and Summer are Yang, Autumn and Winter are Yin; in a month, after the New Moon (Shuo) is Yang, after the Full Moon (Wang) is Yin; in a day, Day is Yang, Night is Yin; further divided by the twelve double-hours to correspond to the Yin and Yang of the Five Zang Organs. Physicians rely on this to understand the source of disease and apply treatment principles. At Winter Solstice (Dongzhi), one Yang is born; at Summer Solstice (Xiazhi), one Yin is born. These two solstices are most critical. "Zhi" means extreme. Extreme Yin generates Yang, finding life in death, from non-being to being; Extreme Yang generates Yin, from being to non-being, differing in Yang transformation and Yin change. Equinoxes merely divide the middle equally. The most significant is Winter Solstice, hence the *I Ching* says: "The Ancient Kings closed the passes on the Solstice days." "Closing the passes" must be understood broadly; observing the *Monthly Ordinances*: "In this month, fast and conceal oneself, waiting for the determination of Yin and Yang," implies it is not just closing market gates. It is most valued because it marks the beginning of the return of one Yang.

The *Classic* states: "Below Xiang Huo (Ministerial Fire), Shui Qi (Water Qi) supports it; below Water position, Tu Qi (Earth Qi) supports it; below Earth position, Feng Qi (Wind Qi) supports it; below Wind position, Jin Qi (Metal Qi) supports it; below Metal position, Huo Qi (Fire Qi) supports it; below Jun Huo (Sovereign Fire), Yin Jing (Yin Essence) supports it. Excess causes harm, support brings control." For example, at Winter Solstice, Yin is extremely abundant, and Yang is born to support/control it; this is "Yin excess leads to harm, Yang support controls it." At Summer Solstice, Yang is extremely abundant, and Yin is born to support/control it; this is "Yang excess leads to harm, Yin support controls it." Yet at Winter Solstice, when one Yang is born, warmth should gradually increase, why is La Yue (the twelfth lunar month) extremely cold with heavy snow? At Summer Solstice, when one Yin is born, coolness should gradually increase, why are the Sanfu (Dog Days) humid and intensely hot? The answer is: what is coming advances, what has succeeded retreats; in the hidden and subtle moments, it is not easy to clarify. Generally, when Yang hides below, it forces Yin upward, steaming with Water Qi, leading to hard ice; when Yin is abundant below, it forces Yang upward, making well water cold and thunderstorms occur. Today, patients with red face, thirst, restlessness, wheezing, and coughing are universally said to have extreme Fire, but who knows it is forced upward by Kidney Yin-Cold? How many have died from administering Cold-Cool medicines? Unjust!

New Moon and Full Moon distinguish Yin and Yang. The first day is "Dead Soul," Yin extreme, Yang born. By the third day, the crescent appears; by the thirteenth, nearly full; by the fifteenth, full. Gradually after the twentieth, the moon's outline empties, seawater flows east, and human Qi and Blood follow suit. Women's menstruation fills monthly; when full, it overflows. Yin extreme, Shaoyang (Lesser Yang) is born, enabling conception; thus, before the Full Moon belongs to Yang.

Yang is One and Solid; Yin is Two and Hollow. Yin's Two derives from Yang's One, hence called "holding the complete form." The moon waxes and wanes. Humans are born Pure Yang without Yin, relying on the mother's Jueyin (Liver Meridian) breast milk for Yin to begin generating. Thus, males reach seminal emission at sixteen (2x8) and essence ceases at sixty-four (8x8); females begin menstruation at fourteen (2x7) and it ceases at forty-nine (7x7). Human Yin serves only thirty years; thus, Yang is often in excess, Yin is often deficient. Moreover, those indulging desires are many, those restraining them are few. Hence, from youth to old age, the work of tonifying Yin must never cease. Here, "Yin" refers to Yin Essence (Jing), not generically to Yin Blood. Using Siwu Tang (Four Substance Decoction) to tonify Yin today is erroneous. Wang Jiezhai said: "Eight or nine out of ten diseases arise from Water Deficiency, one or two from Fire Deficiency," slightly grasping the intent. Chu Shizhong said: "Males' Yin Essence is already depleted, yet they seek sexual release, causing essence to fail internally, urine passage obstructed like strangury. If Yang is already impotent and further exhausted, large and small bowel movements cause pain; the more painful, the more one defecates; the more one defecates, the more painful. Within Yin there is Water and Fire; Water deficiency is common, but Fire decline is also frequent. No one whose essence is depleted can preserve Yuan Qi alone. Since Yang and Yin root each other, discussing Yin tonification must prioritize Yang, for without Yang, Yin cannot generate."

Males carry Yang on the back and Yin on the front; females carry Yin on the back and Yang on the front. In the human body, left and right divide Yin and Yang. Males: Right belongs to Fire and is Qi; Left belongs to Water and is Blood. Females: Right belongs to Water; Left belongs to Fire. Hemiplegia (Ban Zhi Feng) affects males mostly on the left, females mostly on the right; is this not because Water fails to nourish?

There is the subtlety of rooting Yin in Yang and Yang in Yin. Without exploring the root, Yin and Yang would almost cease. Those discussing Yin and Yang say it is Qi and Blood. But do they know Fire is the root of Yang Qi, and Water is the root of Yin Blood? Observing the earth: the Sun is the essence of Fire, so Qi follows it; the Moon is the essence of Water, so tides follow it. However, this Yin-Yang and Water-Fire share one root, receiving execution daily, returning to life nightly, flowing endlessly, paired and inseparable. Because they share one root and do not separate, Yin and Yang mutually root each other: Yang roots in Yin, Yin roots in Yang. Without Yang, Yin cannot generate; without Yin, Yang cannot transform. Draw Yin from Yang, draw Yang from Yin, seek their belonging, and exhaust their roots. People only know Qi and Blood are Yin and Yang, not knowing Water and Fire are the roots of Yin and Yang. Knowing Water and Fire are Yin and Yang, but mistakenly identifying Heart and Kidney as the true Water and Fire, this is why the Dao remains unclear. Observe the sky: the five planets exist, but the Sun and Moon illuminate the world; are they not True Yin and True Yang? In the human body, the Five Elements of Heart, Liver, Spleen, Lung, and Kidney exist, but what operates among the Zang and Fu organs is Formless Xiang Huo (Ministerial Fire), moving through Yang twenty-five degrees, and Formless Kidney Water, moving through Yin twenty-five degrees. Their root originates from the Pre-heaven True Essence. This is what is truly real. Once it becomes tangible, it is Post-heaven, not True, not Root. Calling it "Root" is like a tree's root, from which branches and leaves grow. Thus, the movement, circulation, ascending, descending, floating, and sinking of human Qi all stem from Sheng Qi (Vital Qi) rooted within, with Shen (Spirit) residing there, governing the mechanism of Yin-Yang movement and stillness. If the mechanism moves with clarity and stillness, generation and transformation are regulated; if the mechanism moves with disturbance, severe diseases arise. This depends on whether the root is solid.

Since there is True Yin and True Yang, what are False Yin and False Yang? These are deceptive and often mislead people. For instance, a patient with high fever, agitation, thirst, and dry tongue—surely a Yang syndrome? But if the face is red, this is Dai Yang (Floating Yang). If the pulse at the Chi position is weak and powerless, while Cun and Guan positions are vast and irregular, this indicates Yin abundance below forcing Yang upward—a False Yang syndrome. Using medicines with false-cold properties to follow its nature and subdue it will calm it instantly. Conversely, a patient fearing cold, wrapped in layers, with cold limbs—isn't this a Yin syndrome? But if the complexion is dull, the pulse is choppy, fine, rapid, and forceful upon pressure, this is a False Cold syndrome: Cold in the skin, Heat in the marrow. Using Pungent-Cool agents, administered warmly, will cure it with one sweat. All these occur because True Qi is insecure, allowing the False to disturb the True. False Yang shows excess where there is deficiency; False Yin shows deficiency where there is excess. Liu Hejian said: "Extreme summer heat makes trees weep sap; extreme Fire resembles Water. Extreme winter cold makes water freeze hard; extreme Yin resembles Yang."

Yin pulses include Chen (Deep), Jin (Tight), and Shu (Rapid). Zhang Zhongjing summarizes them as Wei Xi (Fine and Thin). Chen requires heavy pressure to feel; Jin and Shu are seen within Chen and Xi, unlike Yang syndromes which are Fu Da (Floating and Large) with Jin Shu. Xue Ji said: "People know Shu indicates Heat, but do not know Shu within Chen and Xi indicates severe Cold. In True Yin-Cold syndromes, the pulse often beats seven or eight times per breath, but is powerless upon pressure. One must examine deeply."

There are those biased toward Yin or Yang, due to constitutional endowment. Taiyang individuals: even in winter, they need no cotton, drink water constantly, indulge in sex, have bowel movements every few days, and take Huangqin, Huanglian, Zhizi, Huangbai without surprise. Taiyin individuals: even in summer, they need thick clothes; slight cooling of food/drink causes abdominal pain and diarrhea; they constantly consume Renshen, Baizhu, Jiang, Rougui. Upon sexual activity, they groan incessantly. These two types embody a bias of Yin or Yang. Modern physicians create neutral (neither cold nor hot) formulas, revered by the world as the "King's Way" of medicine. But they do not know that disease arises from bias, and medicine must correct bias; thus, treating Heat with Cold, and Cold with Heat, is the standard rule. Yet why does frequent Bitter-Cold intake intensify accumulated Heat, and annual Pungent-Hot intake deepen chronic Cold? This is ignorance of Yin-Yang belonging. The *Classic* says: "For all cases treated with Cold that remain hot, treat the Yin; for all cases treated with Heat that remain cold, treat the Yang." This is seeking the belonging. Thus, "Cold does not resolve Cold" means Water Deficiency; "Heat does not resolve Heat" means Fire Deficiency. For Water Deficiency, strengthen the master of Water to suppress Solar Flare; for Fire Deficiency, augment the source of Fire to dissipate Yin Shadows, reaching truth beyond mere rules.

Yin and Yang are empty names; Water and Fire are physical realities; Cold and Heat are excessive Heavenly Qi; Water and Fire are human True Yuan. Pathogenic excessive Qi causing disease can be attacked with Cold/Hot medicines; True Yuan causing disease is regulated by True Water/Fire. However, without seeking belonging, medicine will not penetrate. Pre-heaven Water and Fire originate in the same palace; Fire takes Water as master, Water takes Fire as source. Thus, seeking Yin means seeking Water within Fire, ensuring essence does not deplete; seeking Yang means seeking Fire within Water, ensuring brightness does not extinguish. Thus, extreme Cold and Heat diseases achieve balance. Scholars like Gao Shili still mistake Water/Fire for Heart/Kidney, no wonder later generations are confused.

Heaven envelops Earth; Earth is within Heaven; Heaven and Earth are one Taiji. Regarding humans: One Yang resides within Two Yins, Yang within Yin. Males: Yin inside, Yang outside; Females: Yang inside, Yin outside. This is the human Taiji. Regarding objects: Grain belongs to Metal, bran is Hot; Wheat belongs to Yang, husk is Cool. Every object possesses a Taiji. The two (Yin) are Post-heaven forms; the One (Yang) is Pre-heaven Qi. Spirit transforms from Qi; Qi originates from Heaven. Thus, what generates our body is True Yang Qi. Form is formed by Essence; Essence generates from Qi. What establishes our body is True Yin Essence. The *Classic* says: "Females at 2x7 Tian Gui arrives; Males at 2x8 Tian Gui arrives." Also: "At age 40, Yin Qi is half gone." The "Yin" here refers to my Essence forming my Form. The peak of human life is only these twenty-plus years. After this, the form declines. Thus, Zhu Danxi cited the waxing/waning of sun and moon, stating Yang is often in excess, Yin often deficient, establishing Buyin Wan (Yin Tonifying Pill) as a divine medicine. But he did not know that before Tian Gui arrives, it depends on Qi; and Yin Qi halving at 40 also depends on Qi. Although Form belongs to Yin, Qi follows Yang. Thus, the warmth of the whole body is Yang Qi. After death, form remains, Qi departs; Yang escapes first, Yin remains. Life depends on Yang, death depends on Yang. Yang comes = life; Yang goes = danger. Yang complete, Yin solid; Yang escaped, Yin ruined. Hence the *Classic* says: "Yang Qi is like Heaven and the Sun; losing its proper place shortens life and obscures vitality." Clearly, human life relies on this point of True Yang for continuous circulation. Who says Yang is often in excess and should be attacked with Bitter-Cold flavors? The *Zhuangzi* says: "The finger is exhausted like firewood; the fire is transmitted, unknown to end." This means fire transmitted to wood is like spirit transmitted to form. Wood ends, but fire is formless and undying. From ancient times to now, only this fire transmits and life continues. Farmers understand this perfectly. Does the world know it regenerates after exhaustion? Understanding the principle of the previous firewood, the fire transmits undying. Receiving nourishment in the heart, life continues uninterrupted. Understanding health preservation is the reason for life.

Youth fear Fire; Old age fears lack of Fire. Lack of Fire makes transformation difficult and decline easy; presence of Fire makes spirit robust and aging slow. Thus, Fire is the root of human life. Adding fuel to the pot raises moisture and Qi, creating the image of Ji Ji (Completion/Intersection of Water and Fire). Yang Qi values hiding/submerging; hidden, it does not flare; hidden, it lasts long. Like adding oil to a lamp, it shines brighter; covering coals in a stove, the fire does not extinguish.

The meaning of Tian Gui is often explained as Jing and Blood, failing to detail the intent of the *Nei Jing*. Studying the *Nei Jing*: "Females at 2x7 Tian Gui arrives, menstruation按时; Males at 2x8 Tian Gui arrives, essence overflows." Tian Gui comes first, then Jing and Blood follow. Tian Gui is not Jing and Blood, clearly. Tian Gui is the True Water generated by Tian Yi. In the body, it is called Yuan Yin (Primary Yin) or Yuan Qi (Primary Qi). Before birth, this Qi roots in parents, called Pre-heaven Primary Qi. After birth, this Qi accumulates in the body, called Post-heaven Primary Qi. Initially, True Yin is very slight; when mature, Jing and Blood flourish. But True Yin must be sufficient before Jing and Blood transform; True Yin precedes Jing and Blood, Jing and Blood follow True Yin. Pre-heaven True Yin is the root of Post-heaven Jing and Blood. If Tian Gui were Jing and Blood, then at 7x7 (females) and 8x8 (males) when Tian Gui ceases, why does周身 (whole-body) Jing and Blood still circulate in Ying and Wei without depletion? Thus, Tian Gui is not Jing and Blood, clearly.