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Nan Jing (Classic of Difficult Issues), Chapter 75, states: The medical classics say, "The East is excess, the West is deficiency; drain the South, tonify the North." What does this mean? The answer is: The Five Elements (Wood, Fire, Earth, Metal, Water) should mutually balance and restrain each other.

The East corresponds to Wood, and the West to Metal. If Wood Qi is in excess, Metal Qi should restrain it; if Fire Qi is in excess, Water Qi should restrain it; if Earth Qi is in excess, Wood Qi should restrain it; if Metal Qi is in excess, Fire Qi should restrain it; if Water Qi is in excess, Earth Qi should restrain it. The East corresponds to the Liver (Gan), indicating Liver Excess (Shi); the West corresponds to the Lung (Fei), indicating Lung Deficiency (Xu). The treatment principle is to drain Fire in the South and tonify Water in the North. The South belongs to Fire, which is the Son (Zi) of Wood; the North belongs to Water, which is the Mother (Mu) of Wood. Water overcomes Fire. According to the principle that "the Son can make the Mother full, and the Mother can make the Son empty," draining Fire and tonifying Water aims to prevent Metal from restraining Wood. The classics state: "If you cannot treat the deficiency, what else is there to ask about?" This refers to this very concept. Some hastily argue: "If Metal desires fullness, Fire should restrain it; if Water desires fullness, Earth should restrain it." This implies that the Qi of the Five Elements has both hyperactive and restrained aspects. Why limit it only to the East being excess and the South needing drainage? It must be understood that the first two sentences are introductory phrases leading to the following text, while the latter two sentences are continuations of the preceding context. The meaning is: In the hypothetical scenario where the East is excess and the West is deficiency, one should drain the South and tonify the North. Draining the South means draining the excess of the East (Liver Wood), based on the principle "drain the Son when there is excess"; tonifying the North means tonifying the deficiency of the West (Lung Metal), based on the principle "the Son can make the Mother full." The Lung governs respiration, while the Kidney is the root of vital Qi generation. Therefore, the classics state that the Kidney is the Root (Ben) and the Lung is the Branch (Mo). Xunzi said: "There is no case where the son is wealthy while the father is poor." This illustrates the same logic. However, the core meaning at the beginning and end of the classic emphasizes tonifying deficiency (the beginning and end represent the original meaning of the classic, while the middle section represents Bian Que's explanation of the classic). Hence, it says, "If you cannot treat the deficiency, what else is there to ask about?" One should understand that draining the South (Fire) is also a method to tonify the West (Metal). The phrase "what else is there to ask about" implies that the Qi of the Five Elements can all be deduced and discussed. If, conversely, the West were excess and the East deficient, one would then drain the North and tonify the South. Nevertheless, the Southeast governs the command of growth, so its Qi tends to be excess; the Northwest governs the energy of storage and collection, so its Qi tends to be deficiency. Thus, it is said that in the first half of the year, Yang Qi prevailing is the norm. The statements "If Metal desires fullness, Fire should restrain it; if Water desires fullness, Earth should restrain it" discuss the Qi of the dominant seasons, each having excessive tendencies. "East excess, West deficiency, drain South, tonify North" discusses the Qi of the entire year, involving changes of excess and deficiency. (Combining annual Qi with the Five Elements of the human body allows for comprehensive discussion.) Analyzing the meaning of the classics, one can separate or combine them as appropriate, which is considered to grasp the true essence.