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Cistanche Herba (Roucongrong) has a mild temperature and a sweet, sour, and salty taste. Its elemental nature belongs to Earth, yet it incorporates the characteristics of Water and Fire. It vigorously tonifies essence and blood; however, sudden or heavy use may instead induce loose stools, making it unsuitable for patients with Spleen deficiency diarrhea. It should be used after being washed with wine. For impotence (Yang shi bu ju), this herb is indispensable and must not be omitted.

The difficulty in prescribing medication lies not in treating straightforward cases (Shun yong), but in treating complex or paradoxical cases (Ni yong); and the difficulty in treating paradoxical cases lies not merely in using them oppositely, but in ensuring that such opposite application aligns precisely with the patient's condition. Physicians typically know only to treat heat with cold, cold with heat, obstruction with unblocking, and unblocking with obstructing—applying cold remedies thoroughly to heat patterns and hot remedies thoroughly to cold patterns. However, the *Huangdi Neijing* (Yellow Emperor's Inner Canon) proposes principles such as "treating obstruction with obstruction" (Sai yin sai yong), "treating discharge with discharge" (Tong yin tong yong), "treating cold with heat" (Han yin re yong), "treating heat with cold" (Re yin han yong), and "avoiding heat when using hot medicines, avoiding cold when using cold medicines" (Yong re yuan re, yong han yuan han). How are these to be explained? "Treating obstruction with obstruction" refers to cases like abdominal distension caused by Spleen deficiency, which is treated with Ginseng Radix et Rhizoma (Renshen) and Atractylodis Macrocephalae Rhizoma (Baizhu); once the Spleen Qi is tonified, the distension naturally resolves. "Treating discharge with discharge" refers to dysentery accompanied by internal heat due to seasonal pathogens, or constipation due to dry feces retained in the intestines; treatment involves administering Tiaowei Chengqi Decoction (Stomach-Regulating Orderly Flow Decoction) to purge the bowels, thereby achieving cure through unblocking. "Treating cold with heat" means that although the primary drug is cold in nature, it is counter-assisted (Fan zuo) by hot herbs. "Treating heat with cold" means that although the primary drug is hot in nature, it is counter-assisted by cold herbs, thereby preventing rejection (Ge ju) reactions where the body expels the medicine. This is what is meant by: one must first address the primary cause and subdue the underlying etiology. "Avoiding heat when using hot medicines, avoiding cold when using cold medicines" implies that while cold diseases should be treated with hot medicines and heat diseases with cold medicines, one should aim only for therapeutic efficacy without overuse; excessive use will lead to injury caused by the medicine itself. Therefore, it is said: Diseases have no fixed form, physicians have no fixed prescriptions, and medicines have no fixed properties; success depends entirely on the practitioner's ability to learn and apply wisely.

My family possesses a copy of the book *Yao Jian* (Mirror of Medicines), compiled by Du Wenxie, a physician from Anhui during the Ming Dynasty. I appreciate its compilation of essential statements on herbal usage from past masters, integrated with personal clinical experience, making it convenient for study and inspiring for enlightenment. Thus, I have written these few words to share it with the world.
Zhang Yaoqing, August 1973, Shanghai