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Yu Chang said: The principles of Ying (Nutritive Qi) and Wei (Defensive Qi) are what the sages value most. The *Ling Shu* states that Zong (Pectoral) Qi accumulates in the Upper Jiao, Ying Qi originates from the Middle Jiao, and Wei Qi originates from the Lower Jiao; this refers to their root sources.

Wei Qi is rooted in the Lower Jiao; it is a faint Yang within Yin. It travels to the Middle Jiao, passes through the state where Yin and Yang coexist there, ascends to the Upper Jiao, and generates Yang Qi independently. Thus, Wei Qi is essentially Clear Yang Qi, but because it arises from the Turbid Yin of the Lower Jiao, it is referred to as "Turbid" being Wei. When the human body reaches dawn (Ping Dan), Yin is exhausted and Yang governs alone. Upon opening the eyes, its Qi ascends to the head and exits at the Jingming (BL1) acupoint of the Foot-Taiyang Bladder Meridian. Therefore, Wei Qi travels externally on the Hand and Foot Taiyang meridians during the day. It is said that Yang Qi primarily governs the exterior throughout the day. Following the acupoints of the Taiyang meridian, it emerges upward as the sequence of movement. It is also said that the Taiyang meridian governs the exterior. Wei Qi is fierce and rapid; it does not travel together with the Zong Qi of the Upper Jiao along the meridians, but moves independently between the skin and muscle layers of various meridians. Thus, Wei Qi travels outside the vessels, warms the muscles, fills the skin, thickens the interstices (Cou Li), and controls the opening and closing of pores.

Ying Qi is rooted in the Middle Jiao; it is Yin within Yang. It travels to the Upper Jiao, follows the Zong Qi of the Upper Jiao, descends to the Lower Jiao, and generates Yin Qi. Thus, Ying Qi is essentially Turbid Yin Qi, but because it arises from the Clear Yang of the Upper Jiao, it is referred to as "Clear" being Ying. Ying Qi is quiet and focused; it must travel together with the Zong Qi of the Upper Jiao along the meridians. It begins at the Taiyuan (LU9) acupoint of the Hand-Taiyin Lung Meridian, then proceeds through the Hand-Yangming Large Intestine Meridian, Foot-Taiyang Bladder Meridian, Foot-Shaoyin Kidney Meridian, Hand-Jueyin Pericardium Meridian, Hand-Shaoyang Sanjiao Meridian, Foot-Shaoyang Gallbladder Meridian, Foot-Jueyin Liver Meridian, and returns to the Hand-Taiyin Lung Meridian. Hence, it is said that the Taiyin meridian governs the interior, and Ying Qi travels within the vessels.

Wei Qi travels in the Yang aspect twenty-five circuits during the day. When it is dominant, it enters from the exterior to intersect with Ying. Ying Qi travels in the Yin aspect twenty-five circuits during the night. When it is dominant, it exits from the interior to intersect with Wei. Their coming and going, penetrating and connecting, proceed side by side without contradiction, never ceasing for a moment. Ying contains Wei, and Wei contains Ying. If they were divided into two separate entities, how could one speak of the subtlety of sharing the same origin? Once Ying and Qi have an excess or deficiency, the resulting harm is incalculable. If Wei Qi is excessive, the body becomes hot; heat causes the interstices to close, leading to panting and labored breathing requiring flexion and extension of the torso, absence of sweat, dry teeth, and mental restlessness. If Ying Qi is excessive, the body becomes cold; cold causes sweating, constant chilliness, frequent shivering, and cold extremities. If Wei Qi is deficient, the body becomes cold; if Ying Qi is deficient, the body becomes hot. Although these symptoms may appear similar, there are distinctions. As for the non-movement of Ying and Wei Qi, food and drink cannot enter, and the form becomes insensible. If Ying and Wei Qi become stagnant and cease, essence and Qi will slacken and deteriorate, the Spirit will depart and cannot be recovered. Therefore, the sages stated that when Yin and Yang, sinews and vessels are harmonized, and marrow is solid, Qi and Blood follow suit. In such cases, the interior and exterior are harmonious, pathogens cannot harm, hearing and vision are acute, and Qi stands firm as usual. This shows that regulating Ying and Wei is the primary task for human life. Deeply contemplating its mechanism, one feels that Wei Qi should be prioritized. The Classics say: "When Yang Qi is broken and scattered, Yin Qi perishes." This indicates that Wei Qi is the impregnable fortress protecting Ying Qi. To examine Wei Qi is to identify the mother of all diseases; this indicates that Wei Qi is the throat and tongue for the entry and exit of pathogens. The *I Ching* says: "One Yin and one Yang is called the Dao." Supporting Yang and suppressing Yin is done to the utmost extent, and the path of immortality is likewise so. Alas! Nasal breath connects with Heaven, while oral breath connects with Earth. People only know to nourish Ying Qi using the breath from the mouth, but those who understand the Dao nourish Wei Qi using the breath from the nose. Nourishing Ying Qi inevitably indulges the mouth and harms life; nourishing Wei Qi involves consuming Heavenly Qi to connect with the Divine Spirit. The difference between the two changes month by month and year by year; is it necessary to ask?

Appendix: Five Questions on Ying and Wei

Q: Wei Qi travels in Yang for twenty-five circuits during the day; does it hide at night? Ying Qi travels in Yin for twenty-five circuits during the night; does it hide during the day?
A: The circulation of Qi in the human body does not cease day or night; it is merely one Qi. Speaking of two Qi from the perspective of Yin and Yang: Day belongs to Yang, governed by Wei Qi; Night belongs to Yin, governed by Ying Qi. Wei Qi also travels in Yin at night, and Ying Qi also travels in Yang during the day, but when not in their dominant phase, they do not govern that period. It is like the movement of the sun and moon, which originally has no distinction between day and night, but their degrees traversing the sky are distinct.

Q: Ying Qi travels inside the vessels, and Wei Qi travels outside the vessels. Who defines the boundary?
A: This meaning was already clarified in the previous discussion. Further deducing the theory: Heaven wraps Earth, Yang wraps Yin, and Qi wraps Blood; this is the natural principle. Ying and Wei travel together within the meridians. Yin naturally resides inside as the guard of Yang, and Yang naturally resides outside as the protector of Yin. This is what is meant by proceeding side by side without contradiction. It is like an army setting up camp: the general naturally stays in the center, and the soldiers naturally guard the exterior. In marital relations, the man naturally occupies the outer position, and the woman the inner position. The sages simply describe the normal state.

Q: The *Nan Jing* (Difficulty 22) states that the *Jing* (Classics) mentions pulses having "Shi Dong" (Movement-causing) and "Suo Sheng Bing" (Disease-producing) conditions, where one pulse transforms into two diseases. The meaning remains unclear today.
A: This precisely discusses the sequence of diseases caused by Ying and Wei. "One pulse transforming into two diseases" means that for the same meridian, disease manifests as two types due to differences in depth. When pathogens enter superficially, Qi stagnates and fails to move, so Wei Qi is diseased first. When pathogens enter deeper, blood becomes congested and fails to nourish, so Ying Qi is diseased. Thus, Ying disease follows Wei disease. If Wei did not first present "Shi Dong" symptoms, how would Ying subsequently produce "Suo Sheng" diseases? Later, Zhang Zhongjing's *Shang Han Lun* (Treatise on Cold Damage) stated that the Taiyang meridian governs the exterior on the first day, distinguishing Wind injuring Wei, Cold injuring Ying, and Wind-Cold injuring both Ying and Wei, presenting corresponding pulse and symptom patterns. And treating hundreds of variations, its profound essence enters the divine, surpassing the merits of Xuanyuan (Yellow Emperor) and Qibo.

Q: What is the method for routinely regulating Wei Qi?
A: Whenever sunset approaches, the gate of Yang Qi in the body closes. One must pay careful attention to protect it and not open it again. The Classics say: "At dusk, gather and repel; do not disturb sinews and bones, do not expose oneself to fog and dew." This comprehensively covers the meaning of regulating Wei Qi. "Gathering" means storing Shen (Spirit) internally; "Repelling" means defending against pathogens externally. Like a gatekeeper who closes at dusk and opens at dawn, what fear of violent intruders would there be? Even if encountering years of deficiency or months of emptiness, if the Qi within the body is naturally solid, how can empty pathogens invade?

Q: Do diseases of the Extraordinary Vessels also relate to Ying and Wei?
A: Although the diseases governed by the Extraordinary Vessels differ from those of the Regular Meridians, their relation to Ying and Wei is the same. If the Yin Mai (Yin Linking Vessel) cannot maintain Yin, leading to a sense of loss and depression, it is due to weak Ying Qi. If the Yang Mai (Yang Linking Vessel) cannot maintain Yang, leading to laxity and inability to self-control, it is due to declining Wei Qi. Disease of the Yang Mai causing aversion to cold and fever indicates pathogens entering Wei Qi, which governs Qi. Disease of the Yin Mai causing heart pain indicates pathogens entering Ying Qi, which governs Blood. This corresponds to the Classics' mention of Lung-Wei and Heart-Ying. Disease of the Yin Qiao Mai presents with flaccidity in Yang and tension in Yin, indicating Yang disease and Yin health. Disease of the Yang Qiao Mai presents with flaccidity in Yin and tension in Yang, indicating Yin disease and Yang health. Such diseases often appear alongside regular meridian diseases. Recognizing them as ailments received by Ying and Wei removes all doubt. Generally, one Qi circulates throughout the body, penetrating everywhere. The Twelve Regular Meridians have Ying and Wei, and the Eight Extraordinary Vessels also have Ying and Wei. The Extraordinary Vessels, attached within the boundaries of the Regular Meridians, receive simultaneous infusion. Deduced from Yang Mai, Yin Mai, Yang Qiao, and Yin Qiao: the longitudinal course of the Chong Mai, the horizontal course of the Dai Mai, the anterior course of the Ren Mai, and the posterior course of the Du Mai—are any of these not the flow of one Qi? With the flow of one Qi, Yin and Yang are distinguished. How could the meaning of Ying and Wei fail to penetrate anywhere?

[Laws Two Articles]

First Law: Treating Wei disease by addressing Ying, or Ying disease by addressing Wei, failing to distinguish true from false pathogens, treating mild diseases heavily, or treating severe diseases lightly, thereby reversing the order and harming patients, constitutes the fault of the physician.

Second Law: Physicians who cannot discern the variations in depth, deficiency/excess, cold/heat, and sequence of Ying and Wei diseases remain ignorant even in old age, comparable to children, and are unworthy of mention.