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When pathogenic factors invade the exterior, it results in Yang deficiency. The use of warm-natured drugs is intended to assist Yang. Once Yang is assisted, Yin pathogens will naturally dissipate. This is why releasing-the-exterior drugs are often warm in nature. As stated in the classics: "In cases of Yang deficiency and Yin excess, sweating (diaphoresis) leads to recovery."

When pathogens transmit inward, they transform into heat and damage Yin. The use of cold-natured drugs is intended to suppress excessive Yang. Once Yang is suppressed, True Yin can grow and recover on its own. This is why purgative drugs for clearing interior excesses are often cold in nature. Therefore, the classics state: "In cases of Yin deficiency and Yang excess, purgation (downward draining) leads to recovery."

If the Yin meridians directly contract cold pathogens, Yang becomes insufficient while Yin becomes excessive. Thus, acrid-hot drugs are used to assist Yang and suppress Yin. This explains why warming-the-interior drugs are often hot in nature.

On the Theory that Taiyang Cold Damage Transmits Only to the Foot Meridians and Not the Hand Meridians:
It is acceptable to say that Cold Damage injures the Foot meridians but not the Hand meridians; however, it is incorrect to say that Cold Damage transmits only within the Foot meridians and does not transmit to the Hand meridians. The six Hand meridians all belong to Metal and Fire, so Metal and Fire do not fear cold. The six Foot meridians belong to Earth, Wood, and Water. Water freezes when encountering cold; Wood sheds its leaves when encountering cold; Earth cracks and loses firmness when encountering cold. These cannot withstand the cold, hence they are susceptible to injury. From this perspective, is it not true that the Foot meridians are injured while the Hand meridians are spared? The character "transmit" (chuan) here must be a scribal error; I speak out of reflection on this matter.

Zhang Zhongjing defined sixteen primary types of Cold Damage. Other symptoms such as mania (fa kuang), delirious speech (chan yu), monotonous repetition of words (zheng sheng), chest binding with pain (jie xiong), and epigastric fullness/stuffiness (pi man) are all caused by the transmission and transformation of these sixteen primary diseases. How can they be generally categorized simply as Cold Damage?

Cold Damage refers to illness that manifests immediately after exposure to cold pathogens during winter. Its symptoms include headache and stiffness of the neck and occiput, stiffness of the lower back and spine, fever with aversion to cold, absence of irritability, anhidrosis (lack of sweating), and a pulse that is floating, tight, and choppy. If contracted between the Frost Descent (Shuang Jiang) solar term in winter and the Spring Equinox (Chun Fen), Mahuang Tang (Ephedra Decoction) is appropriate.

Wind Injury refers to contracting wind pathogens. Its symptoms include headache, body heat, aversion to wind, and spontaneous sweating, with a pulse that is floating and moderate (slow/loose). Guizhi Tang (Cinnamon Twig Decoction) is appropriate. If the pulse is tight and choppy, accompanied by anhidrosis and aversion to cold, it is Cold Damage. A tight pulse indicates aversion to cold; a choppy pulse indicates anhidrosis. Since cold injuring the Ying (Nutrient) level belongs to Yin, and Yin governs storage/closure, anhidrosis occurs. Therefore, the light and ascending properties of Ephedrae Herba (Mahuang, 麻黄) are used to release the exterior. If the pulse is floating and moderate with aversion to wind, it is Wind Injury. A floating pulse indicates Wind Injury; a moderate pulse indicates spontaneous sweating. Therefore, the sweet and warm properties of Cinnamomi Ramulus (Guizhi, 桂枝) are used to resolve the muscle layer.