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Cold belongs to Yin, and heat belongs to Yang. Fever is a manifestation of Yang, arising from the disharmony between Yin Qi and Yang Qi. The disharmony of Yin Qi with Yang Qi constitutes Yin Deficiency (deficiency of Yin fluids). However, Yin resides within and serves as the anchor for Yang; if Yin is deficient, Yang has no anchor, and if Yang lacks an anchor, Yang Qi also becomes deficient. Thus, fever may be attributed to either Yin Deficiency or Yang Deficiency. For fever due to Yin Deficiency, nourishing blood and enriching Yin is appropriate; for fever due to Yang Deficiency, tonifying Qi and assisting Yang is indicated. When Yin and Yang are in harmony within the body, there is no heat; when they are disharmonious, fever occurs. According to general principles, one should merely harmonize Yin and Yang and regulate Qi and Blood. However, the onset of heat varies in intensity. Mild heat refers to a gentle and gradual fever, caused by internal deficiency of Yin Blood; sudden or violent heat refers to intense burning fever, caused by hyperactive and harmful Yang Qi. Fever resulting from internal deficiency of Yin Blood should be treated with nourishing herbs; fever resulting from hyperactive Yang Qi should be treated with warm-natured herbs. Generally, mild heat is "rooted" heat, whereas sudden/violent heat is "unrooted" heat. Unrooted heat is termed "Tuore" (fever due to collapse/escape of Yang); Tuore is heat generated by the external escape of Yang Qi. For such heat, treatment should seek its underlying nature to subdue it, hence the saying: "Sweet-warm [herbs] eliminate great heat." In nine out of ten cases of disease involving fever, although the body presents with fever, there must be an underlying fundamental syndrome. Physicians must observe the complexion, palpate the pulse, examine the exterior-interior status, assess deficiency-excess conditions, and analyze thoroughly in conjunction with the fundamental syndrome to grasp the essence.

Heat is Yang Qi; Cold is Yin Qi. Aversion to cold (fear of cold) occurs because the pores throughout the body fail to receive the protective function of Yang Qi on the exterior, resulting in shivering and chilliness. The eighty-four thousand pores throughout the human body are governed by the defensive Qi of the Taiyang (Greater Yang) channel. If the pathology affects the Qi aspect of Taiyang, the entire body will experience aversion to cold. From the head and neck down to the back and spine lies the pathway of the Taiyang channel; if the pathology affects the Taiyang meridian itself, aversion to cold will be localized to the back. Beyond aversion to cold, there is also "shen han" (generalized sensation of coldness). Shenhhan is characterized by feeling constantly cold despite wearing multiple layers of clothing, caused by the inability of the fire-heat Qi of the Sanjiao (Triple Burner) to warm the muscles externally. The *Shanghan Lun* states: "When the form is cold and there is aversion to cold, this indicates injury to the Sanjiao." Form being cold with aversion to cold is synonymous with shenhhan. In the initial stage of Shanghan (cold damage), the body invariably feels cold, because cold is the intrinsic nature (Ben Qi) of the Taiyang channel. The classics state: "Above Taiyang, Cold Qi governs." This indicates that Taiyang takes Cold as its root and Heat as its branch. Besides aversion to cold, there is also aversion to wind. The classics state: "Above Jueyin (Terminal Yin), Wind Qi governs." The skin and hair of the whole body are governed by Taiyang Qi. Within the skin and hair, in the interstices of the muscles and fascia, lie the blood that nourishes the flesh, which are governed by Jueyin Qi. Pathology affecting the root Qi of Taiyang leads to dysfunction of the skin and hair, resulting in aversion to cold; pathology affecting the root Qi of Jueyin leads to dysfunction of the muscle interstices, resulting in aversion to wind. Wind and cold are difficult to distinguish; differentiation lies in whether the pathology involves the skin/hair or the muscle interstices. When the pathology is in the skin and hair, the pores close and resist, leading to anhidrosis (absence of sweating); when the pathology is in the muscle interstices, the pores open, leading to sweating. Generally, contraction and closure characterize Cold, while stirring and opening characterize Wind. Both Wind and Cold arise because the healthy Qi is internally deficient, allowing pathogenic Qi to follow. In some cases, although the patient has fever, aversion to cold persists during the fever; the treatment method should employ acrid-warm herbs to address the heat. The classics state: "If the disease has not returned to its root, treatment should follow the root." This is what is meant.