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Dryness is the transformation of Yangming Metal Qi. In Heaven it is Dryness, in Earth it is Metal, and in Man it is the Large Intestine. The Yangming channel governs with Dryness-Metal, and the Stomach Earth follows this command to transform into Dryness.
The Taiyin channel governs with Damp-Earth, and the Lung Metal follows this command to transform into Dampness. The Dryness of Stomach Earth is the Child Qi rather than its Original Qi. Since Child Qi cannot rival the prosperity of Original Qi, in individuals with Yin excess, Stomach Earth is constantly damp.
The Dampness of Lung Metal is the Mother Qi rather than its Original Qi. Since Mother Qi cannot rival the prosperity of Wood Qi, in individuals with Yang excess, Lung Metal is constantly dry. Taiyin nature is Damp, Yangming nature is Dry. The balance between Dryness and Dampness depends on Middle Qi (Spleen-Stomach Qi). When Middle Qi is prosperous, Xin Metal (Lung) transforms Qi within Wet Earth, so the Lung is not injured by Dryness; Wu Earth (Stomach) transforms Qi within Dry Metal, so the Stomach is not injured by Dampness. When Middle Qi declines, Yin and Yang fail to interact, leading to bias in Dryness and Dampness. If Dampness exceeds Dryness, there will be reduced thirst, decreased appetite, astringent urination, and loose stools. If Dryness exceeds Dampness, there will be frequent hunger and easy thirst, profuse urination, and hard stools. Yin pathogens easily advance, while Yang pathogens easily retreat. Cases of Dampness excess are common; cases of Dryness excess are rare. Cases where Xin Metal transforms Dampness account for eight or nine out of ten; cases where Wu Earth transforms Dryness account for less than two or three out of a hundred. Although Yangming is inherently Dry, in disease, Taiyin often prevails while Yangming often loses. Conditions of Earth Dryness and Water Deficiency, except for the Chengqi Syndrome of Yangming in Cold Damage, are extremely rare. Therefore, Zhang Zhongjing established the principle that it is auspicious if the Shaoyin pulse is weaker than the Fuyang pulse, because Fire excess leads to Earth Dryness, and Water excess leads to Earth Dampness. Dryness conquers Water, but Dampness is insulted by Water. When Water is deficient, life persists; when Earth is deficient, death ensues. Thus, Shaoyin should be weak, and Fuyang should be strong, because Earth can conquer Water, ensuring Middle Qi remains unbroken. There is no case where Middle Qi remains unbroken yet the person dies. Dryness is the intermediary between Cold and Heat. Upper Dryness transforms into Fire and becomes Heat; Lower Dryness transforms into Water and becomes Cold. In patients with regurgitation or dysphagia, stools resemble sheep droppings; their Stomach is Damp, while their Intestines are Dry. Dampness is a Yin pathogen; Yin nature tends downward, so its root originates in Spleen Earth, and its manifestation appears at the knees and ankles. Dryness is a Yang pathogen; Yang nature tends upward, so its root originates in the Large Intestine, and its manifestation appears at the elbows and wrists. This is the fixed position: Yang pathogens reside below, Yin pathogens reside above. However, Upper Dryness also arises from Lower Dampness. In patients with wind-stroke, blood depletion, and tendon contraction, the knees and ankles are Damp, while the elbows and wrists are not necessarily Dry. If Ji Earth (Spleen) were not Damp, Wood would flourish, Blood would flow freely, bones would be weak, and tendons flexible, from where would Wind arise? If physicians can recognize the waxing and waning of Dryness and Dampness, they can ascend step by step to the profound mysteries of Zhang Zhongjing's teachings.