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The physician's possession of the *Shennong Bencao Jing* (Divine Farmer's Materia Medica) is akin to a carpenter's possession of a plumb line. With a plumb line, rules and compasses are established; with rules and compasses, flexibility and adaptation become possible. Flexibility arises from wisdom and skill, yet it must still be grounded in the standard of the plumb line.

Tracing back, Yan Di's (Emperor Yan) *Bencao Jing* represents the creation of the plumb line; the *Daguan Bencao* and *Zhenglei Bencao* represent the formation of rules and compasses; Li Shizhen's *Bencao Gangmu* (Compendium of Materia Medica) is a culmination within these standards but has not reached the level of complete adaptability. It is like a master craftsman who can teach others the rules and compasses but cannot impart wisdom and skill. Those who can impart wisdom and skill to achieve the path of adaptability are found in the texts of Huang Di's (Yellow Emperor) *Ling Shu* (Spiritual Pivot) and *Su Wen* (Basic Questions). Among those who could apply the Dao of Yan and Huang flexibly according to circumstances, unconstrained by rigid standards, there was only one person in the Han Dynasty: Zhang Zhongjing, the Prefect of Changsha. Zhang Zhongjing, endowed with heaven-given ability, inherited the Dao of Yan and Huang in an unbroken lineage; whatever he did naturally conformed to the standards. It has never been heard that there exists another plumb line outside the Dao of Yan and Huang.

I have often pondered the academic traditions in the medical field. Although there have been notable figures in every generation, those who seek the root, clarify the source, and revere the therapeutic principles of the *Bencao Jing*, other than the *Yuhan Jing* (Jade Box Classic) and *Jingui Yaolue* (Essential Prescriptions of the Golden Cabinet), I have not encountered. After Zhang Zhongjing, Sun Simiao, a Tang Dynasty recluse, uniquely grasped the essence in his *Qianjin Fang* (Prescriptions Worth a Thousand Gold). His prescriptions were sometimes more potent than those of Zhang Zhongjing, making them difficult for later generations to emulate or revere. Consequently, compilers of materia medica generally omitted the unconventional uses of reversal and stimulation found in ancient texts, focusing instead on mild and safe methods to suit contemporary needs. The trivial details therein are hardly worth discussing. Even Li Shizhen, who was erudite across past and present, still abandoned the root for the branches, merely listing the indications of the *Bencao Jing* at the beginning of each herb entry as a gesture to preserve the original intent. Only Miao Xichun (courtesy name Zhongchun) excavated and elucidated the classic meanings, surpassing other physicians. However, in places where the meaning was subtle and hard to explain, he extensively cited the *Bielu* (Supplementary Records) and other texts, interpreting them as canonical statements. Could such confusion of red and purple (right and wrong) fail to deviate from the truth?

In the past, Qiao Zizi of Sanyu authored a volume titled *Bencao Jingshu*. Thirty-five years ago, I happened to read it once in Mr. Nian'e's study. Alas, it was never published, so it can no longer be seen. Therefore, presuming upon my own limited abilities, I briefly state my humble views, outlining the major principles of the *Bencao Jing* and appending the therapeutic methods of various schools. This may enable students to find resources everywhere, staying within the norms of the Yan-Huang tradition, thus becoming superior physicians. A superior physician cures six out of ten cases and cannot raise the dead back to life. I wish that all physicians in the world would carefully avoid relying arrogantly on their strengths to seek opportunistic gains. The common people should indeed heed the warning: "If one does not treat illness improperly, one often receives the benefit of average medicine." By stepping out of the circle of contemporary mediocre practitioners, what plumb line could possibly constrain them?

Written by Zhang Lu, styled Wangshiweng, at Junyong Hall in the spring of the Yihai year of the Kangxi reign.
At the age of seventy-nine.