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When Yang pathogenic factors are in the exterior, there is exterior heat; when Yin pathogenic factors are in the exterior, there is exterior cold. When Yang pathogenic factors are in the interior, there is interior heat; when Yin pathogenic factors are in the interior, there is interior cold. When pathogens reside between the exterior and interior without a fixed location, alternating chills and fever occur. When pathogens are in the exterior, the chest and abdomen are not full or distended; when pathogens are in the interior, the chest and abdomen are painful and distended. When pathogens are in the exterior, patients groan in discomfort; when pathogens are in the interior, patients are restless and agitated. When pathogens are in the exterior, patients can eat; when pathogens are in the interior, patients cannot eat. A lack of desire to eat indicates that pathogens are between the exterior and interior, but not yet severe enough to cause complete anorexia. When pathogens are in the exterior, there is no irritability or vomiting; when pathogens are in the interior, there is irritability, fullness, and vomiting. Whenever initial symptoms include irritability, frequent vomiting, and gradually developing epigastric stuffiness, it indicates that pathogens are transmitting from the exterior to the interior; purgative methods should not be used. In cases where the disease originates in the exterior with all external signs present, but the pulse is unexpectedly deep and faint, this is due to insufficiency of Primordial Yang (Yuan Yang), which fails to reach the exterior. The best strategy is to rescue the interior by assisting Yang and dispersing cold. This has been discussed in the previous volume "Zhuanzhong Lu" and should be cross-referenced.

When pathogenic factors in the exterior cause fever, it is exterior heat without interior heat, suitable for treatment with warming dispersion. When pathogenic factors in the interior cause fever, it is intense interior heat radiating outward, suitable for treatment with clearing heat. Insufficiency of Yang allows Yin Qi to enter the Yang aspect, causing aversion to cold; this is cold due to Yin excess, suitable for warming treatment. Insufficiency of Yin allows Yang Qi to sink into the Yin aspect, causing fever; this is heat due to Yang excess, suitable for clearing heat.
Alternating chills and fever result from the struggle between Yin and Yang: Yin excess leads to chills, while Yang excess leads to fever. Generally, heat belongs to Yang, cold belongs to Yin, the exterior belongs to Yang, and the interior belongs to Yin. Pathogens lodging in the exterior manifest as cold; when pathogens struggle with Yang, chills occur. Pathogens transmitting to the interior manifest as heat; when pathogens struggle with Yin, restlessness and heat occur. For pathogens between the exterior and interior, struggling outward with Yang causes cold, while struggling inward with Yin causes heat. Moving between exterior and interior, or out and in, results in alternating chills and fever, constituting a syndrome of the half-exterior half-interior. Therefore, if cold dominates, there will be more cold manifestations; if heat dominates, there will be more heat manifestations. By carefully examining the trend of chills and fever, one can determine the depth of the pathogen.
The Classics state: "When Yang is weak, there is aversion to cold; when Yin is deficient, there is fever."
Zhang Zhongjing stated: "Fever with aversion to cold indicates onset in Yang [channels]; absence of fever with only aversion to cold indicates onset in Yin [channels]."