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Duanxi stated: Humans are born by receiving the Qi of Heaven and Earth. The Yang Qi of Heaven transforms into Qi, and the Yin Qi of Earth transforms into Blood. Therefore, Qi is often in excess, while Blood is often deficient. Why is this so? Heaven is the parent of all things; the celestial body is vast, belongs to Yang, and moves outside the Earth. The Earth resides within Heaven, belongs to Yin. The great atmospheric Qi of Heaven supports the Sun. The Sun is full, belongs to Yang, and moves outside the Moon. The Moon is waning, belongs to Yin, and derives its brightness from the light of the Sun. Thus, the waxing and waning of Yin and Yang in the human body also correspond to the phases of the Moon.

After birth, males reach seminal emission at sixteen years of age, and females begin menstruation at fourteen years of age. After forming the physical body, one still relies on nourishment from milk, food, and water for the Yin Qi to mature, allowing it to pair with Yang Qi, enabling the person to grow up and become a parent. Ancient people waited until they were around thirty or twenty (likely meaning adulthood) before marrying, which shows the difficulty in forming Yin Qi and the ancient wisdom of preservation.

Qian Zhongyang's principle that "the Kidney has only tonification and no purgation" embodies this very idea. Furthermore, the commentary on the *Book of Rites* states: "Only after fifty does one need to add measures to nourish Yin." The *Neijing* (Inner Canon) says: "When a person reaches forty, their Yin Qi naturally halves, and their daily activities decline." It also states: "Males cease producing essence at sixty-four, and females cease menstruating at forty-nine." Given how difficult it is to form Yin Qi, yet it suffices for only about thirty years of operation before becoming deficient, how can one indulge in desires without limit when facing such hard-to-form but easily-depleted Yin Qi?

The *Neijing* states: "Yang corresponds to Heavenly Qi and governs the exterior; Yin corresponds to Earthly Qi and governs the interior." Therefore, the Path of Yang is solid, and the Path of Yin is hollow. This is not merely my personal opinion.

Someone asked: "Observing Heaven and Earth, the Sun and Moon, they are so different. Yet why do Cold, Heat, Warmth, and Coolness appear equally balanced across the four seasons without significant disparity?" The answer is: "Extreme movement returns to stillness, and extreme stillness returns to movement, just like inhalation and exhalation. Cold is the extreme of inhalation, where Qi sinks; Heat is the extreme of exhalation, where Qi floats; Warmth is slight exhalation, where Qi rises; Coolness is slight inhalation, where Qi descends. The mechanism behind each breath ensures they are equal and balanced. This discusses the dynamic function. The previous discussion on 'size and deficiency/excess' referred to the static structure."

Another asked: "Taking far examples from Heaven, Earth, Sun, and Moon, and near examples from male and female bodies, discussing 'excess' and 'deficiency,' I understand now. Humans exist within the intersection of Qi. How should one follow the principles of Yin and Yang for preservation?" The answer is: "The Zang organ governing storage and closure is the Kidney; the one governing dispersion and flow is the Liver. Both contain Ministerial Fire (Xiang Huo), which connects upward to the Heart. The Heart's Sovereign Fire (Jun Huo) is easily stirred by external stimuli. When the Heart moves, Ministerial Fire follows swiftly. Even without sexual intercourse, Ministerial Fire may leak secretly. Thus, sages only teach people to gather the mind and nurture nature, a profound intent.

Heaven and Earth form the four seasons through the alternating rise and fall of the Five Elements. The human Five Zang and Six Fu organs respond similarly. The fourth month (Si) and fifth month (Wu) belong to Fire, which is in peak prosperity. Fire controls Metal (Lung); when Fire is strong, Metal weakens. The sixth month (Wei) belongs to Earth, which is in peak prosperity. Earth controls Water (Kidney); when Earth is strong, Water weakens. Moreover, Kidney Water often relies on Lung Metal as its mother to supplement deficiencies. Hence, the *Neijing* earnestly advises nourishing the source of transformation. In summer, ancients lived alone and ate bland foods, carefully protecting the Lung and Kidney, precisely because they feared the dominance of Fire and Earth.

The *Neijing* also says: "Those who preserve Essence will not suffer from Warm diseases in Spring." The tenth month (Hai) and eleventh month (Zi) are times when Great Qi lies dormant and closed, nurturing the original truth for the rising and generating energy of the coming spring. If one does not indulge in desires during this time, damaging oneself, then when spring arrives, the root will be strong, and Qi will not float lightly, preventing warm diseases.

Summer Fire and Earth dominance, and Winter Great Qi dormancy, refer to the "deficiency" of a year. The period before the first quarter moon and after the last quarter moon, when the moon is empty, refers to the "deficiency" of a month. Strong winds, heavy fog, rainbows, lightning, hail, sudden cold or heat, solar/lunar eclipses, worry, anger, fear, sorrow, overeating/drinking, fatigue, mental strain, and excessive labor are all "deficiencies" of a day. If a patient has just recovered from fever or has active sores, this exceeds a single day's deficiency.

Many people today suffer from headache, leg weakness, poor appetite, and body heat in late spring and early summer. Zhang Zhongjing discussed conditions worsening in spring/summer and improving in autumn/winter with a wiry-large pulse, known as "Zhuxia disease" (Summer Sickness). If one encounters these four types of "deficiency," illness seems inevitable.

Having the demeanor of an old man while in one's prime, feeling overwhelmed by caring for parents and raising children, is frightening. As the ancients said, "Do not see what arouses desire, and the mind will not be disturbed." Gentle touches on the body, sounds to the ears, colors to the eyes, and fragrances to the nose—how could anyone remain unmoved? Those skilled in health preservation should keep these five senses at bay. During a monthly or daily "deficiency," one should temporarily avoid sexual intimacy, cherish oneself, preserve Heavenly Harmony, and nourish the source of transformation. If Water is nourished and Yin is not depleted, it will balance with Yang. Then Yang will have a foundation and not fly away recklessly, achieving the harmony of Heaven and Earth interaction. What illness would there be? Let us abide by this, honoring the teaching of respecting the body.

Commentary by physicians of later generations: The human body mirrors Heaven and Earth, sharing the same Qi. Yang is in excess, and Yin is deficient; this is the constant state. Thus, those whose deficiency is exacerbated by irregular living habits and emotional desires usually suffer from Yin deficiency. However, common people do not understand this and often tonify Yang. Therefore, Duanxi's treatise is truly a secret wisdom passed down for millennia, opening the eyes of future generations. The *Neijing* states: "When the body's deficiency meets Heaven's deficiency, and two deficiences interact, the Qi penetrates to the bone and injures the Five Zang internally." If treated within two *hou* periods, injury can be avoided. The meaning is clear, and those who practice health preservation should ponder it deeply.