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The statement in the *Neijing* (Inner Canon) that "if less than three days have passed, sweating may suffice; if three days or more have passed, purgation may suffice" refers specifically to heat diseases, which is why the chapter is titled "Discussion on Heat" (*Re Lun*).

Heat disease occurs when cold pathogenic factors are located in the exterior San Yang meridians (Taiyang, Yangming, Shaoyang), and the cold has transformed into heat; thus, it can be resolved by inducing sweating. If it does not resolve through sweating, the heat pathogen has entered the interior Yin organs/meridians, and thus can be treated by purgation. However, if the cold pathogen remains in the exterior without transforming into heat, or in cases of collapse of exterior Yang, the Taiyang meridian presents a cold syndrome treatable with Sini Tang (Four Reversal Decoction). When cold pathogenic factors directly strike the interior Yin, influenced by the thermal transformation of Ministerial Fire (Xiang Huo) and Sovereign Fire (Jun Huo), the Shaoyin meridian presents a fire syndrome requiring urgent purgation, and the Jueyin meridian presents a heat syndrome manifesting as pus and blood in the stool. These variations depend entirely on the Qi transformation within the human body. Therefore, pathogens in the San Yang meridians are called Yang syndromes; those capable of transforming into heat are called heat syndromes, while those unable to transform into heat are called cold syndromes. Pathogens in the San Yin meridians are called Yin syndromes; those characterized by Yin-cold are called cold syndromes, while those undergoing fire transformation are called heat syndromes. One cannot definitively classify a disease as a heat disease simply because it is located in the Yang, nor assume it must be a cold disease because it is located in the Yin.

A condition where Yang excess mimics Yin is described as "the deeper the Jue (cold extremities), the deeper the heat." The *Jueyin Chapter* states: "In cold damage, from one to two days up to four or five days, if Jue occurs, there must be fever; if there was fever previously, Jue will follow later. The deeper the Jue, the deeper the heat; the milder the Jue, the milder the heat." The phrase "one to two days up to four or five days" means that on the first day, the pathology is in the Taiyang meridian, and on the second day, it is in the Yangming meridian, where the cold has transformed into heat. By the fourth or fifth day, it transmits to the interior Yin, hence it is said that fever must occur. This implies that in the first two days of cold damage, fever necessarily precedes, and by the fourth or fifth day, Jue follows. "Deep" indicates severity, and "mild" indicates lightness. This is because the heat pathogen has penetrated deeply, and the interior Qi cannot reach the exterior; thus, the deeper the internal heat, the deeper the Jue. In such cases, one should understand that during the first two days, the pathogen was in the exterior Yang and had previously presented with fever before transmitting to the interior Yin. (On the first day, headache characteristic of the Taiyang meridian must be present.) Conversely, a condition where Yin excess mimics Yang occurs when cold pathogenic factors directly strike the interior, causing Yin-cold to become excessive and repel Yang to the exterior. Consequently, the patient prefers cold and dislikes heat, removes clothing and bedding, and desires to lie in cool places. In such cases, the person is likely irritable, the pulse is deep and fine, or deficient, floating, and chaotic; the tongue coating is slippery; the facial complexion is pale/clear; or redness floats on the surface while the skin feels cool. If there is fever, it typically begins with a sensation of cold followed by heat; upon palpation, intense heat is felt initially, but after a while, it feels cool again. For both these scenarios, if the etiology and symptoms are carefully examined, the false manifestations will naturally be revealed. If one harbors doubts about Yang mimicking Yin or Yin mimicking Yang beforehand, causing confusion in the mind, it will lead to diagnostic errors.