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The *Lingshu* (Spiritual Pivot) chapter "Da Huo Lun" (Treatise on Great Confusion) states: The essence and qi of the five zang organs and six fu organs all ascend to the eyes, forming the brilliance of vision. The eyes value maintaining a state of clarity and purity. Externally, there are the six pathogenic factors (Liuyin, Six Evils); internally, there are emotional injuries caused by the seven emotions (Qiqing). Diseases affecting the eyes are numerous. If treated improperly, some may cause lifelong harm. I have often seen practitioners in this specialty who either steal a few prescriptions or compound eye drops, then hang up diagrams of eye contours, boasting proudly as specialists. Yet when asked about the causes of their patients' conditions, they remain ignorant of where the diseases originate. Alas, these are blind doctors; how can they treat the eyes? Mr. Gu Yangwu from Tongxiang was renowned for ophthalmology in his time. People suffering from eye diseases, near and far, competed to seek his treatment. I frequently heard those with eye ailments say: "My eyes are preserved thanks to Mr. Gu." Or again: "Without Mr. Gu, my eyes would almost be lost." I privately suspected that since Mr. Gu's treatment of the eyes was so miraculous, he must possess secret teachings not passed down and unique, extraordinary methods, enabling him to surpass ordinary practitioners and gain reputation in his era. Now, having observed Mr. Gu's discussions and recorded medical cases, I realize that his scholarship involves thoroughly investigating the source of all external invasions by the six pathogenic factors and internal injuries by the seven emotions before applying tonifying, purging, warming, or cooling agents. Consequently, his prescriptions are invariably effective, and illnesses invariably recover. Thus, regarding medicine, Mr. Gu does not advocate strangeness nor boast of uniqueness; he simply perceives the root of disease with clarity. Which physician's excellence could surpass this? Even if one were to apply Mr. Gu's scholarship to treat other diseases, where would it not be suitable? That he is now known primarily for ophthalmology does not fully exhaust his capabilities. However, being known for ophthalmology also shows that his full effort is dedicated to this field. Based on his heart to save the world, he has published this *Zhinan* (Guide) to instruct others, ensuring that those treating eye diseases do not go astray. Whether using tonification, purgation, or both simultaneously, each method finds its proper way. Thereby restoring the bright nature of the eyes—would this not be a great joy for the people of this world? This serves as the preface.
In the early summer month of Ji Si year, the fourteenth year of the Jiaqing reign, Zhang Qilin of Haichang respectfully composed.