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There are numerous medicinal substances, each possessing its unique nature. Their suitability varies greatly, making it difficult to fully comprehend them. If practitioners fail to grasp the essentials, errors are inevitable. Furthermore, the annotations in *Bencao* (Compendium of Materia Medica) generally describe their capabilities; any substance with a particular strength is unlikely to be overlooked. However, determining which effects are primary and which are secondary, which are beneficial for one condition but detrimental for another, and which are suitable for tonification but not for purging, remains challenging. Learners who lack an understanding of the true nature of herbs and merely rely on diagrams to find remedies often find treatments ineffective, further highlighting the difficulty of prescribing. The principle of using medicines lies not in complexity, but in mastering their flavors and aromas and recognizing their Yin-Yang attributes. By doing so, even with a vast array of substances, one can grasp the key points.

Regarding the distinction between flavors and aromas: all aromas belong to Yang, and all flavors belong to Yin. Aromas originate from Heaven and have four types: Cold, Hot, Warm, and Cool. Flavors originate from Earth and have six types: Sour, Bitter, Sweet, Pungent, Salty, and Bland. Warm and Hot correspond to the Yang of Heaven; Cold and Cool correspond to the Yin of Heaven. Pungent, Sweet, and Bland correspond to the Yang of Earth; Sour, Bitter, and Salty correspond to the Yin of Earth. Yang governs ascending and floating movements, while Yin governs sinking and descending movements. Pungent flavor governs dispersion; its action moves horizontally, thus resolving the exterior. Sweet flavor governs moderation; its action moves upward, thus tonifying the Middle Jiao. Bitter flavor governs purging; its action moves downward, thus eliminating excess pathogens. Sour flavor governs astringency; its nature is contracting, thus treating diarrhea. Bland flavor governs permeation; its nature is promoting urination, thus separating clear from turbid. Salty flavor governs softening hardness; its nature is sinking, thus guiding stagnation. When using pure aromatic substances, one utilizes their dynamic ability to circulate; when using pure flavorful substances, one utilizes their static ability to retain. When both aroma and flavor are present, the subtlety of combination lies in mutual reinforcement. In the pairing of Sovereign and Minister herbs, the key to appropriateness lies in avoiding contradiction. Even when combinations are appropriate, contraindications must be known: first avoid harm, then utilize benefits. If one herb is unsuitable, all potential benefits are lost. Therefore, when aiming to disperse the exterior, one must avoid Sour and Cold substances; when aiming to descend, one must not combine with Ascending and Dispersing substances. For those with excessive Yang, Warm substances should be avoided; for those with deficient Yang, Sinking-Cold substances must not be touched. Upper Excess conditions contraindicate Ascending therapies; Lower Excess conditions contraindicate Constipating therapies. Upper Deficiency conditions contraindicate Descending therapies; Lower Deficiency conditions contraindicate Purging therapies. Substances with dynamic properties, if used excessively, will dissipate; substances with static properties, if used excessively, will extinguish. Sweet substances should not be administered to patients with Middle Jiao fullness; Bitter substances should not be administered to patients with false heat; Pungent substances should not be administered to patients with heat and restlessness; Salty substances should not be administered to patients with blood injury. Sour, associated with Wood, can most effectively overcome Earth (Spleen); therefore, it should be used sparingly in cases of Spleen Qi deficiency. Within Yang, there are Yin manifestations; within Yin, there are Yang principles. If one can illuminate these Yin-Yang relationships, although the pharmacology may seem profound, it is certainly not difficult to penetrate.

Regarding the entry of the Five Flavors, the *Neijing* (Inner Canon) states: "The five flavors enter the stomach and each returns to its preferred organ. Thus, Sour enters the Liver first, Bitter enters the Heart first, Sweet enters the Spleen first, Pungent enters the Lung first, and Salty enters the Kidney first." Long-term consumption increases the Qi of the corresponding organ, which is the normal law of transformation. However, if this increase persists for too long, it becomes the cause of premature death.