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The *Neijing* (Inner Canon) states: "When pathogenic factors are exuberant, it is excess (Shi); when healthy qi is depleted, it is deficiency (Xu)." It also states: "For what is in excess, use draining (Xie); for what is deficient, use tonifying (Bu)." I consider that the terms "excess" and "deficiency" are the key to grasping the mechanism of disease; "tonifying" and "draining" are the guiding principles for treatment methods. Generally, human illness does not go beyond the two categories of deficiency and excess, and a physician's treatment does not go beyond the two methods of tonification and drainage. If there is any doubt regarding deficiency or excess, then tonification and drainage cannot be applied. Examining symptoms and palpating the pulse to ascertain deficiency and excess is indeed the primary concern in clinical practice. However, there are cases where symptoms appear like excess but the pulse indicates deficiency, or the pulse appears like excess but the symptoms indicate deficiency. If one discards the pulse and follows the symptoms, it is difficult to trust them as true; if one discards the symptoms and follows the pulse, one fears they may be false. What then should be done? One must understand that the principle of all things is: in excess conditions, the form is firm and contracted, and the color is dark and aged; in deficiency conditions, the form is floating and puffy, and the color is delicate and tender. The manifestation of disease on the tongue follows this same rule regarding form and color. (Therefore, for diseases belonging to excess, the tongue must be firm and contracted with a dark, aged appearance; for diseases belonging to deficiency, the tongue must be puffy and floating with a delicate, tender appearance. By distinguishing in this manner, whether it is deficiency or excess, true or false, can be clearly understood at a glance, even without examining symptoms or palpating the pulse.)

Once deficiency and excess are distinguished, the decision to tonify or drain naturally becomes clear. However, both deficiency and excess have their own yin and yang aspects, and yin and yang interplay to create variations in deficiency and excess. Therefore, when distinguishing yin and yang within the context of deficiency and excess, clinicians must not confuse them. If the distinction is not made correctly, one might mistake yin-excess for yang-excess, or yang-deficiency for yin-deficiency, leading to errors. Furthermore, some conditions are fundamentally yang-deficiency, yet classical texts describe them as yin-deficiency, causing misunderstanding and significant harm. For instance, the text says: "Yin-deficiency leads to night sweats." Here, "yin" refers to the Hand-Taiyin Lung Meridian, and "deficiency" refers to Lung Qi deficiency. It also says: "Yin-deficiency causes night fever." Here, "yin" refers to the Foot-Taiyin Spleen Meridian, and "deficiency" refers to Spleen Qi deficiency. Although both are termed "yin-deficiency," there is a distinction between the Hand-Taiyin and Foot-Taiyin meridians. Named "yin-deficiency," these are actually symptoms of Spleen and Lung Qi deficiency. Regrettably, physicians through the ages have never clarified this meaning, mistakenly identifying Spleen and Lung Qi deficiency as Kidney Water insufficiency. Consequently, they use formulas for nourishing yin and descending fire. Day after day, heavy yin forces downward, leading to Earth confinement and Metal failure (Spleen/Stomach impairment and Lung Qi exhaustion), resulting in loose stools and hoarseness, placing patients in a fatal state without realization. This error stems solely from the two characters "yin," adhering rigidly to textual meanings and spreading misinformation. From ancient times to the present, across the vast world, countless lives have been lost due to this, which is truly heartbreaking. Moreover, such cases are numerous in the classics and difficult to enumerate entirely. The root cause lies in the mixing of yin and yang, and the blurring of deficiency and excess, making it difficult to clarify analysis based solely on pulses and symptoms. (Who would know that in yin-deficiency with yang-excess, the tongue must be dry; in yang-deficiency with yin-excess, the tongue must be slippery; in yin-deficiency with yang-excess and vigorous fire, the tongue must be dry and parched; in yang-deficiency with yin-excess and declining fire, the tongue must be slippery and wet.) By distinguishing in this way, whether it is yin or yang, who is excess and who is deficiency, becomes clearly visible. How could there be doubts resembling yin or yang, leading to errors of excessive yin or excessive yang, thereby bringing premature death upon people?