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The syndrome of external injury by cold pathogen, along with diseases caused by irregular diet and physical overexertion, as well as internal injury due to diet, all present with fever and aversion to cold. The world generally mistakes the disease of internal injury due to irregular diet and insufficient labor recovery for an external injury by cold pathogen, a syndrome of excess in the exterior. Consequently, they wrongly apply purgative methods to the exterior, leading to countless unjust deaths! This is entirely due to the failure to distinguish between these fevers (internal vs. external). Now, I will analyze them in detail.

External injury by cold pathogen presents with fever and aversion to cold occurring simultaneously. The fever manifests as "xi xi" fever (a mild, fluttering heat felt on the surface) or "fu fu" fever (a gentle, brushing heat), arising from the skin and hair follicles, like feathers brushing against the body. This clearly indicates that the heat is in the exterior; this is when the cold pathogen invades the highest level of the body (the skin and hair). The skin and hair follicles belong to the Yang aspect and are nourished by Wei Qi (defensive Qi). When the cold pathogen attacks, it stagnates the Yang aspect, preventing Yang from extending, thus causing fever. The patient's face becomes red, nasal passages feel congested, there is mental restlessness, and one feels slightly like exposing the skin, yet still cannot tolerate the cold. This superficial deficiency heat is limited to these symptoms. Regarding aversion to cold, even with thick clothing, lowered curtains, or proximity to fire, one cannot ward off the cold. It worsens day by day until the pathogen transmits inward and forms interior excess syndromes, at which point it ceases. The cold and heat occur together without interruption.

In cases of internal injury due to irregular diet or physical overexertion, there may also be headache, stiff neck, and lower back pain, which bear a slight resemblance to Taiyang syndrome (syndrome of the Foot-Taiyang Bladder channel), but the rest are different, as discussed in the treatises. In diseases of internal deficiency, the exterior lacks Yang Qi and cannot withstand wind and cold; this occurs frequently. The irritant heat arising from the Kidney region occurs occasionally and is quite unlike external cold injury. As for aversion to wind and cold, it is because Spleen-Stomach deficiency causes Ying Qi (nutritive Qi flowing within the vessels) to flow downward and invade the Liver and Kidney, marking the beginning of flaccidity and reversal syndromes (Wei Jue). If Stomach Qi is normal, food entering the stomach allows Ying Qi to ascend, benefiting the Heart and Lungs, and nourishing the Yuan Qi in the skin and collaterals of the Upper Jiao. However, if Ying Qi flows downward, the Heart and Lungs receive no nourishment, the skin lacks Yang, and the protective barrier of Ying and Wei Qi is lost. Thus, the Yang aspect in the skin and hair becomes weak; merely seeing wind or cold, or staying in shady, cold places without sunlight, triggers aversion. This happens constantly without interruption. Simply avoiding wind and cold, staying in warm places, or adding clothing to warm the skin will make the aversion disappear. This heat is not caused by external cold invading the skin, nor is it heat in the hair follicles. Rather, it is dampness from the downward-flowing Spleen-Stomach affecting the Kidney region, blocking the lower part, causing Yin Fire (hyperactive ministerial fire from the lower jiao) to rush upward. This produces a steaming, irritable heat that penetrates to the top of the head and through the skin of the whole body. Relief comes only by removing clothes, staying exposed, moving to cool places, or sweating after extreme heat. Do external cold injuries with aversion to cold and fever involve sweating? If sweating occurs, the illness resolves. By this distinction, is it not as easy to see as black and white?

When internal deficiency leads to injury, irritant heat arises. Or, inhaling wind-cold qi stagnates Yin Fire, causing obstruction in the throat and diaphragm. The inhaled air tries to enter but is rejected by the fire in the Chong Mai (Penetrating Vessel) above the diaphragm, preventing Yin Qi from entering. The Qi in the chest is suppressed by external wind-cold and cannot extend, causing the mouth to open and eyes to stare. In severe cases, sound emits externally, Qi cannot move up or down, gets stuck in the throat, and Qi seems about to fail. Alternatively, hiccups, vomiting, or regurgitation may occur, and the onset of irritant heat must have a cause for this syndrome to appear, at which point the exterior deficiency syndrome of aversion to wind and cold reappears. The weakness of the exterior is exploited by Yin Fire; irritant heat arises briefly and passes, leaving the exterior deficient in Yang, unable to withstand wind and cold, which then reappears. Thus, the lack of Yang in the exterior is constant, while irritant heat is intermittent. These two do not occur together; when irritant heat arises, the cold subsides, and when cold arises, the irritant heat subsides. This differs from external injury where cold and heat occur simultaneously without interruption.

All diseases may present with body heat, also called muscle heat or intercutaneous heat, which is known only upon palpation. This is tangible heat of the body tissues. One must wait for the harmonization of Yin and Yang and resolution through sweating. One should not rely solely on this for differentiation, as both deficiency/excess and internal/external diseases can present with it, making it difficult to distinguish. Instead, use the patient's subjective sensation of fever, aversion to cold, and irritant heat as the criterion for differentiation.