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The pathogenic factors of cold damage (Shanghan) and febrile diseases that transmit through the meridians differ from those of miscellaneous diseases. It is essential to differentiate their pulse, symptoms, tongue coating, as well as the indications for exterior-releasing (sweating) and purgative methods, so as to avoid diagnostic errors. The pulse is the mansion of blood and belongs to Yin. At the onset of illness, when healthy Qi contends with pathogens, if the True Qi has not yet declined, the pulse will necessarily be slippery, rapid, and forceful. If the disease persists for a long time with intense heat and declining Qi, the pulse will necessarily be faint, thin, and weak. This describes the progression from a frequent to an extreme pulse; at this stage, one should only nourish Yin to subdue Yang. This is the key to mastering pulse diagnosis.

Some patients present with aversion to cold and fever at the very beginning, followed inevitably by symptoms of thirst, dryness, and heat, or even cold extremities with diarrhea; these are pathogenic factors of heat syndromes transmitting through the meridians. If it remains a heat syndrome throughout, characterized solely by fever without aversion to cold, then regarding cold damage as a disease, it initially presents with headache but strictly lacks symptoms of fever, aversion to cold, and thirst for water. If the onset involves cold extremities and diarrhea, or merely aversion to cold without fever, this constitutes a cold syndrome. This is the subtlety of symptom differentiation.

As for the root of the tongue, it is the orifice of the Heart. The Heart belongs to Fire, governs heat, and corresponds to the brightness of the Li trigram. When a person falls ill and the pathogen is initially in the exterior, the tongue naturally turns red without white coating or other colors. When exterior pathogens enter the half-exterior, half-interior region, the tongue color changes, presenting a white coating that appears slippery. One must not fail to clarify exterior syndromes. Therefore, when pathogens have transmitted into the interior but have not yet resolved, the tongue will necessarily show a yellow coating, indicating that the pathogen has entered the Stomach; urgent purgation is required. Once the yellow coating disappears, the illness will resolve. However, if physicians do not follow the sequence of the disease and erroneously use decoctions or pills, delaying purgation, the coating will inevitably turn black, and various complicated symptoms will arise, making it difficult to treat. If the tongue coating appears like glossy pitch-black, ten out of ten cases will result in death. This occurs because Heart Fire flares up independently and attacks the pathogenic heat; extreme heat generates a transformation into Water characteristics, hence the color turns black to correspond to Water. If all Zang-Fu organs are affected and pathogenic toxins deepen daily, the symptoms will manifest as heat syndromes. Although purgation is indicated, it primarily clears heat from the chest; otherwise, the heat will scatter into the collaterals and Zang-Fu organs, and survival is unlikely. This is analogous to fire burning: initially red, but eventually the fuel turns into black charcoal. This illustrates the principle that "excess leads to harm, and control follows." Now, having clearly presented the twelve tongue images above, and fearing they may not cover all symptoms, I have additionally created twenty-four illustrations and listed the corresponding treatments below. Thus, tracing the source and following the flow truly offers the profound ability to determine life and death.

On the morning days of the first lunar month of the first year of Zhizheng [1341 AD]