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(Starting from the original meaning rooted in body regions, and extensively citing to clarify it)

The Three Yin and Three Yang of the twelve meridians cannot be rigidly or forcibly matched with the Zang-Fu organs, as detailed in previous discussions. The names of the Three Yin and Three Yang of the twelve meridians originate from the division of body regions (Fenye). Why are there divisions into Three Yin and Three Yang? The answer is: this takes imagery from Heaven and Earth. When standing facing south, the Yangming meridian is in front, representing the peak of Yang Qi, not implying that Dryness Qi is in front; the Taiyang meridian is at the back, representing being distant and external, not implying that Cold Qi is at the back; the Shaoyang meridian is on the side, between front and back, not implying that Fire Qi is on the side. The same logic applies to the Three Yin meridians. Names are determined solely by region, orientation, and interior-exterior relationships, not by the Six Qi (Wind, Cold, Dryness, Heat, Dampness, Summer-Heat) to establish their meanings. Therefore, the Three Yin and Three Yang in the human body are empty positional concepts. One might ask: If the Three Yin and Three Yang are empty positions, why does the *Neijing* (Classic of Internal Medicine) often say that Dryness diseases are called Yangming, Cold diseases are called Taiyang, Fire diseases are called Shaoyang, Earth diseases are called Taiyin, Heat diseases are called Shaoyin, and Wind diseases are called Jueyin? The answer is: this is borrowing the names. Yangming borrows the name of Dryness Metal, but it does not necessarily have to be in front of the body; Metal Qi connects with the Lung, not exclusively with the Stomach and Large Intestine meridians. Jueyin borrows the name of Wind Wood, but it does not necessarily have to be on the side of the body; Wind Qi connects with the Liver, not extending to the Pericardium meridian. Taiyang, Shaoyang, Taiyin, and Shaoyin all follow this same principle. This is borrowing names due to Qi pathologies, and there are also cases where names are borrowed due to meridian pathologies. When the Stomach meridian is diseased, it is called Foot-Yangming; when the Large Intestine meridian is diseased, it is called Hand-Yangming; this does not necessarily mean Dryness Qi is causing the disease. When the Kidney meridian is diseased, it is called Foot-Shaoyin; when the Heart meridian is diseased, it is called Hand-Shaoyin; this does not necessarily mean Fire Qi is causing the disease. Generally, when a person is attacked by pathogens, if the attack is on the face, it falls under Yangming; if on the nape, it falls under Taiyang; if on the cheek, it falls under Shaoyang. Here, the terms Yangming, Taiyang, and Shaoyang refer to body regions, not to specific meridians, nor to the Six Qi. Pathogens attacking humans first invade the areas between skin and hair follicles; at this stage, the meridians and vessels have not yet immediately become diseased. If the Qi and Blood within the vessels are not robust, pathogens will seep into the vessels. Some pathogens seep into the Yang meridians, some into the Yin meridians, and some reach the regions of the Three Yin without yet entering the vessels. The Qi of the meridians connects with the Zang-Fu organs, and its transmission mechanism is extremely rapid. Once pathogens enter the meridians, their entry into the Zang-Fu organs becomes unstoppable. Therefore, Yang meridians can also present Interior syndromes. If pathogens reach the regions of the Three Yin but have not entered the vessels, this constitutes an Exterior syndrome of the Three Yin, which can still be cured by sweating. Ancients suspected that the *Shanghan Lun* (Treatise on Cold Damage) only discussed Foot meridians and ignored Hand meridians. In fact, the Three Yin and Three Yang mentioned in the treatise refer merely to body regions. The regions of the Foot meridians are large, hence they present more symptoms; the regions of the Hand meridians are small, hence they present fewer symptoms. If pathogens enter the vessels, Qi flows along fixed pathways, and meridians influence each other; thus, Hand meridians naturally have their own diseases. Therefore, when the *Shanghan Lun* mentions Hand meridian syndromes, these are always Interior syndromes. Tao Jian'an said: The six meridians of the foot represent the geographical boundaries of injury. Zhang Jingyue said: The Foot meridians are long and distant, extending from top to bottom and connecting all four limbs, so one can examine whole-body diseases by palpating them; the Hand meridians are short and proximal, intersecting among the Foot meridians, which is why Shanghan discusses only Foot meridians and ignores Hand meridians. Shanghan involves Exterior pathogens; to seek Exterior syndromes, one should examine the whole body, and among the meridians throughout the body, only the six Foot meridians encompass everything! ("Whole body" refers to the trunk/shell, contrasted with Zang-Fu organs.) Zhang Shiweng said: Transmission occurring only in Foot meridians means the pathogen is in the body surface and has not entered the Zang-Fu organs; if it enters the Zang-Fu organs, it cannot remain solely in the Foot meridians. Alas! Observing the arguments of various experts, can one not suddenly realize this? However, when pathogens are in the body regions, it generally refers to the skin and muscle layers, yet the syndromes clearly manifest in specific meridians with distinct boundaries. What is the reason for this? (*Zhang Lun* states: Each of the five Zang and six Fu organs has its boundaries, and their diseases have distinct shapes.) The answer is: The arrival of pathogens must follow a specific path. For example, if attacked on the nape, it falls under Taiyang. When the Taiyang region is occupied by pathogens, the healthy Qi in that region becomes constrained. When healthy Qi is constrained, it cannot coordinate with the rising, falling, slowing, and accelerating of Qi in the vessels. Although pathogens have not yet entered the vessels, the healthy Qi in the vessels is already affected. Thus, the entire body uniformly manifests Taiyang syndromes. After prolonged constraint, the Interior Qi also becomes deficient, allowing pathogens to invade internally due to deficiency. In summary, when pathogens are in the body regions, syndromes appear only outside the trunk; when pathogens enter the meridians, syndromes involve the Zang-Fu organs. Cases where Interior syndromes appear suddenly without pathogens entering the meridians must be due to pathogens directly striking the Sanjiao (Triple Burner); direct strike to the Sanjiao makes entry into Zang-Fu organs easier. The Sanjiao is the internal region; the Three Yin and Three Yang are the external regions. Regions are the departments of Wei Qi (Defensive Qi); meridians are the pathways of Ying Qi (Nutritive Qi).