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The nature of medicinal materials varies in rigidity and flexibility: rigidity corresponds to Yang, and flexibility corresponds to Yin. Therefore, rigid medicines are active, while flexible medicines are tranquil. Rigid and active medicines move swiftly; being swift, they act rapidly without leaving residue, their onset is quick, but their potential to cause death is also violent. Flexible and tranquil medicines move slowly; being slow, they nourish subtly and continuously, their onset is delayed, but their potential to cause death is also gradual. Those without insight tend to favor one extreme, causing harm that is beyond measure. Those advocating a middle ground propose the theory of mutual restraint. Yet, how can one not know that flexible medicines are naturally slow and cannot be forced to be fast; rigid medicines are naturally fast and cannot be forced to be slow. If both slow and fast-acting medicines are used together, the slow ones must yield to the fast ones to act first. After swallowing, one only sees the action of Yang medicines in the Yang aspects, but does not see the action of Yin medicines in the Yin aspects. If the condition suits Yang medicines, the Yin medicines initially show no effect, and instead, disaster may brew after the Yang medicines have passed their peak; if the condition suits Yin medicines, the Yin medicines have not yet taken effect, but damage from the Yang medicines has already occurred in an instant. Thus, this theory of balance is also erroneous. In summary, prescribing medicine according to the disease is the way to act in accordance with nature, achieving results through non-action.

Any medicine that expels pathogenic factors inevitably damages healthy Qi; any medicine that tonifies deficiency inevitably retains pathogenic factors; any medicine that draws pathogens out of a specific meridian inevitably guides pathogens into that meridian. Therefore, Ephedrae Herba (Mahuang) and Cinnamomi Ramulus (Guizhi) release the exterior, but can also lead to the collapse of Yang; Poria (Fuling) and Alismatis Rhizoma (Zexie) promote urination and leach out dampness, but can also scorch body fluids. From this, we understand that no medicine is unbiased. Precisely because each medicine has its bias, it can treat diseases characterized by bias. If the Creator had endowed all medicines with a balanced nature, how would diseases be cured? It all lies in the management of these medicines. The ability to manage them depends entirely on the physician's firm judgment of symptoms. When using expelling medicines, distinguish whether healthy Qi is deficient; once pathogens are expelled, healthy Qi recovers naturally. When using tonifying medicines, know whether pathogens are completely cleared; when healthy Qi prevails, pathogens find it difficult to interfere. By weighing degrees of severity and distinguishing the sequence of administration, and by applying divine flexibility to the principle of "prescribing according to symptoms," the art of treatment is complete. Why must one advocate Ginseng Radix (Renshen) and Astragali Radix (Huangqi), while regarding Rheae Fructus (Dahuang) and Natrii Sulfas (Xiaosha) as enemies!

Tangye (decoctions) are also beverages. The *Suwen* chapter "Jingmai Bielun" states: When food and drink enter the stomach, their essence floats up and is transported to the spleen; the spleen disperses essence upward to the lungs; the lungs govern all vessels, distributing essence to the skin and hair, where skin and vessel essences merge; they regulate water passages, transporting fluids downward to the bladder; water essence spreads throughout the body, and the essence of the five zang organs circulates together. This describes the order of distribution above and below after ingestion, which parallels the principle of how medicine interacts with disease after entering the stomach. Because the effect manifests first in the upper part, light and clear medicines act first, corresponding to diseases in the upper body. However, the effect of those acting first subsides first; by the time water essence spreads throughout the body, the light and clear medicines have lost their potency. Their inability to treat lower diseases, and their insufficient capacity to hinder lower treatments, is due to natural momentum. Heavy and turbid medicines act later; when they reach the spleen and lungs, they have not fully exerted their effect. Only after water essence spreads throughout the body does their power finally reach the lower parts, corresponding to lower diseases. This explains why light and clear medicines treat the upper body, while heavy and turbid medicines treat the lower body. The classics state: "For nearby conditions with odd/even combinations, prepare small doses; for distant conditions with odd/even combinations, prepare large doses." This takes the speed of drug action and the height/depth of the location as the rationale. For drugs entering the zang organs, it is merely stated that the five flavors enter the stomach and return to their preferred organs, such as sour flavor entering the liver first, etc., not implying they do not enter other organs. Later generations, unaware of the ancients' intent in formulating prescriptions, claimed that certain drugs enter specific meridians, or certain drugs enter multiple meridians. One might ask: Is stomach Qi driven by drug Qi, or is drug Qi driven by stomach Qi? This is self-evident. Yet some erroneously follow this view. For example, in Guizhi Tang (Cinnamon Twig Decoction), since it primarily treats Taiyang disease, people assume Cinnamomi Ramulus (Guizhi) belongs solely to the Foot-Taiyang meridian. They fail to consider that Taoin disease also uses Cinnamomi Ramulus (Guizhi), and that Zhenwu Tang (True Warrior Decoction), Lizhong Tang (Regulate the Middle Decoction), and Sini Tang (Reverse Cold Decoction) all have examples of adding Cinnamomi Ramulus (Guizhi). Alas! How strange. In summary, treating diseases with decoctions involves distinguishing flavors and odors, not meridians, which differs greatly from acupuncture methods.