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All things differ in form from humans, yet humans utilize them to nourish life and eliminate disease. Why is this? It is because Heaven and Earth are also among the myriad things; only because their forms are extremely vast can they possess life.

In generating humans, [Heaven and Earth] imparted pure Qi; in generating animals, the Qi is mixed; in generating plants, the Qi is biased. However, what humans consider "pure" refers merely to the state of the body at birth. Once affected by pathogenic factors such as Wind, Cold, Summer-heat, and Dampness, or disturbed by emotional fluctuations like Joy, Anger, Worry, and Overthinking, this purity becomes diluted. When diluted, Qi is injured; when Qi is injured, the physical form deteriorates. Conversely, those things with mixed or biased natures can instead use their acquired characteristics to tonify and rescue [the human body].

The Sages understood this principle. To save people, one must first know the medicinal substances. Qi cannot violate natural principles, and form cannot depart from Qi. Observing color to distinguish taste, examining sound to identify texture, weighing weight and measuring length to assess shape—these are methods for understanding the physical form of substances. Predicting seasonal changes, detailing habits, distinguishing flourishing and declining states, and identifying suitable soils are methods for seeking the Qi of substances. Once form and Qi are grasped, the Nature (Xing) of the substance is obtained. Nature is the underlying principle of a substance's generation. Based on this, Materia Medica was established and decoctions formulated to treat human diseases. Excess is drained, deficiency is tonified, cold is treated with heat, heat is treated with cold, warmth is cleared, coolness is warmed, following the disease with contrary treatment, and opposing the disease with orthodox treatment.

Sometimes substances are grouped by similarity, sometimes fears and taboos are used to correct imbalances and restore equilibrium. Initially different, ultimately the same. Heaven and Earth generate these substances, and the Sages preserve and utilize them. The power of creation is shared equally between Heaven and Earth and the Sages; Heaven and Earth do not monopolize it.

During the late Han Dynasty, Zhang Zhongjing’s *Jin Gui Yao Lue* (Essential Prescriptions of the Golden Cabinet) and *Shanghan Lun* (Treatise on Cold Damage) contained many prescriptions that were largely remnants of pre-Shang-Zhou methods. The rationale behind their medication usage aligned seamlessly with the *Shennong Bencao Jing* (Divine Farmer's Materia Medica). Careful diagnosis guided prescription selection, yielding results as immediate as an echo. Since the Tang Dynasty, however, understanding of drug properties has become unclear. Prescriptions were often self-composed, such as in *Qianjin Fang* (Prescriptions Worth a Thousand Gold) and *Waitai Miyao* (Secret Essentials from the Imperial Library). While the drug properties generally did not contradict the conditions being treated, the subtle variations of ancient methods were rarely followed. During the Song and Yuan dynasties, the number of medicinal materials increased daily, but their properties were not thoroughly researched. Physicians relied on subjective judgment, leading to persistent errors. Even when using substances recorded in the *Bencao Jing*, their profound and subtle meanings were often overlooked. Consequently, prescriptions failed to be effective formulas, and drugs lost their true efficacy. Occasional successes were mere coincidences rather than reliable methods. This was undoubtedly due to the neglect of the *Bencao Jing*.

I privately lament this! I wish to elaborate in detail, but my observations are limited. Ancient and modern names differ, geographical origins vary, and flavors and odors diverge. Moreover, medicines rarely used by contemporary physicians lack identification and collection. Some possess unique abilities and natures, with principles hidden and obscure, difficult to deduce at present. If I were to attempt explaining the entire classic, I would risk acting against my conscience and misrepresenting the Sages. Therefore, I have selected only those hundred substances that are familiar through observation and whose principles can be reasoned out. For these, I trace the origin and explore the fundamental reasons, elucidating the rationale behind their use. Thus, the intent of the ancient Sages in creating prescriptions to treat diseases becomes clearly visible, while other aspects remain unexplained. Future scholars may perhaps comprehend the whole through this, and although my humble and crude work may invite ridicule, my reverence and caution deserve respect.

Written by Xu Dachun of Songling, aboard a boat on the Yangtze River, during the first half-moon of the fourth month of the year Bingchen (1736), the first year of the Qianlong reign.