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The principle of Yin and Yang is the foundation of health preservation and disease treatment, yet it is difficult for people to comprehend due to its unfathomable subtleties. The function of Yin and Yang lies in mutual assistance (Xiangji), not in mutual harm (Xiangze). Mutual assistance means harmony: there must be Yang within Yin, and Yin within Yang. Mutual harm means injury: if Yang harms Yin, it leads to scorching and withering; if Yin harms Yang, it leads to stillness and extinction. All diseases are essentially caused by the mutual harm of Yin and Yang, resulting in a loss of harmony.

Generally, the nature of Yin favors stillness and dislikes movement, while the nature of Yang favors warmth and fears cold. When they harm each other, Yin fears the hyperactivity of Yang; thus, when Yin encounters Yang pathogenic factors, it becomes dry or withered. Yang fears the toxicity of Yin; thus, when Yang encounters Yin pathogens, it floats away or scatters. This mutual jealousy between Yin and Yang is indeed hard to predict. In severe cases where false and true symptoms appear similar (Jia Zhen Si Yi), this phenomenon is evident, particularly in Shanghan (Cold Damage) and Li Ji (Dysentery).

In severe cases of dysentery today, patients often present with heat signs in both the upper and lower parts, but these are not true heat. How can we know this? For instance, irritability may resemble heat but is not true heat; restlessness may resemble mania but is not true mania. There is inner vexation and discomfort that defies description. This is not a true Yang heat syndrome. It occurs because essence and blood are damaged, fire lacks water support, and Yin loses its stillness, causing such irritability. Furthermore, "flying" refers to heat floating upward, and "walking" refers to heat sinking downward. Heat floating upward causes thirst, throat sores, red face, and body heat. Heat sinking downward causes burning pain in the orifices, yellow urine, or bloody stools. These are not excess heat syndromes. They occur because Water and Fire clash, Yang is driven away by Yin, and Fire leaves its proper position, leading to this floating and sinking behavior.

People today, seeing such symptoms, claim that diagnosing disease relies on external signs. Since the heat signs are so obvious, how can one call it cold? This is absurd! They only see external heat but ignore internal cold; they see heat above and below but ignore cold in the Middle Jiao (Middle Burner). They do not realize that irritability stems from Yin deficiency, and floating/sinking stems from Yang deficiency. Do listeners truly believe me, or remain skeptical? Between doubt and belief, differentiation is essential.

For example, if the body surface shows heat signs, one should dislike heat, yet refuses to remove clothing or blankets. Or if the umbilical and abdominal areas prefer warmth and benefit from warming compresses and massage, this indicates external heat but internal cold. Similarly, if the nine orifices show heat signs, one might desire cold drinks. However, some may want their mouth cool but fear the abdomen being chilled. If cold liquids are swallowed, it may increase nausea/vomiting, worsen abdominal pain, cause blockage and distension, or even though there are mouth/tongue sores, the patient desires hot drinks. This indicates heat above and below but cold in the Middle Jiao. Additionally, consider conditions like inherent weak Yang or pulse/color lacking spirit as discussed earlier. If one assumes it is merely fire and treats with cold/cooling herbs, ignoring the underlying cold, the cold medicine enters the stomach and directly attacks the Middle Jiao. The external heat is irrelevant, but the internal cold worsens. Consequently, the "flying" Qi flies more, and the "walking" Qi sinks more. This is adding frost to snow. To keep the solitary Yang from extinguishing is impossible. Therefore, treating such cases requires guiding the fire back to its source (Yin Huo Gui Yuan). Warming the Dantian allows fire to seek dryness (stability); stabilizing the lower origin allows Qi to return to Essence. This is the divine principle of Yin-Yang inversion. Many fail to understand this, remaining lost in the sea of suffering without finding the shore. I elaborate on this to pass down the mantle of universal salvation.