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The medical classics state, "When pathogenic factors are exuberant, it is excess; when essential qi is depleted, it is deficiency." These two sentences constitute the critical juncture in treating diseases. Their wording is quite explicit, yet their meaning is profound. I dare to analyze them in detail.

"Excess" refers to the exuberance of pathogenic factors, for which purgation (Xie) should be employed; "deficiency" refers to the weakness of upright qi (Zhengqi), for which tonification (Bu) should be employed. In any disease caused by the struggle between pathogenic factors and upright qi, there is always an excess of pathogens and a deficiency of upright qi, so both aspects can be discussed. Therefore, those who advocate purgation say, "When pathogenic factors are exuberant, it is excess," implying that purgation is indicated; those who advocate tonification say, "When essential qi is depleted, it is deficiency," implying that tonification is indicated. Both sides stick to one view without solid evidence, using textual citations to justify themselves, making it difficult to declare either completely wrong. Consequently, this extremely refined teaching has instead brewed immense harm. The principle at stake must have immutable truths, but unfortunately, physicians fail to perceive them. I propose to analyze this into four categories: which is urgent and which is slow, and whether the condition is "present" or "absent."

In cases with no deficiency (pure excess), the urgency lies in the pathogenic factors; they should be eliminated promptly, as retention will lead to complications. In cases with significant deficiency, the urgency lies in the upright qi; if nourishment is not started early, it will be too late to help when the crisis arrives. When slight deficiency and slight excess coexist, one may treat the excess aspect, clearing it away in one sweep. When severe deficiency and severe excess coexist, what is feared is the deficiency; one should only consolidate the root. By first ensuring one's own invincibility, the pathogenic factors will inevitably recede. In cases of two deficiencies and one excess, one should address the excess while opening a pathway; in cases of two excesses and one deficiency, one should address the deficiency to prevent unpredictable changes. In summary, mistakenly tonifying an excess condition will certainly aggravate the pathogen, though it might still be salvageable; mistakenly purging a deficiency condition causes sudden loss of true qi, which is likely irreversible. Thus, the urgency of excess and deficiency must be carefully considered.

The concept of "presence and absence" refers to examining the presence or absence of pathogenic factors. Wind, cold, summer-heat, dampness, dryness, and fire (Liu Yin, Six Exogenous Pathogens) all contribute to pathogenic accumulation. Whether the pathogen resides in the exterior, interior, zang organs, or fu organs, it must have a specific location; one must identify its origin and attack it directly. This is referred to as "presence," indicating the reality of pathogenic excess. If the disease does not arise from the six climatic pathogens but stems from yin-level disturbances, it is due to emotional desires damaging the interior or fatigue and overwork damaging the exterior. Such conditions resemble pathogens but are not true pathogens, resembling excess but not true excess. This is referred to as "absence," indicating that the pathology lies in the Yuan Qi (Primordial Qi). Physicians who do not understand the distinctions of excess, deficiency, presence, and absence will inevitably mistake counter-indications for indications and symptoms for root causes, thereby shortening lives. Can one not be cautious?