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A patient with an eye disease asked, "Why does Borneol (Pian Nao) have so many applications in treating eye diseases?" I heard this and smiled, saying, "Do you know its benefits, but also do you know its harms?" The patient was startled and said, "People throughout the world, from children to the elderly, even foolish men and women, all know that Borneol is a medicine for treating eyes. No school of ophthalmology fails to use it as a first choice. Now you alone speak of its harm. Why? Is it merely to shock the vulgar?" I replied, "I am not being pedantic or fond of idle talk, nor am I advocating its absolute abandonment. I simply advocate using it appropriately. Since you suffer from an eye disease, have you used it as eye drops?" He answered, "Yes." I said, "Since you have used it, let me ask you this: Have you experienced cases where, after applying Borneol, one initially feels cool and comfortable, but shortly thereafter experiences irritability, heat, chest tightness, and restlessness? Have you encountered cases where vision becomes more blurred after application? Or cases where opacity worsens and the condition deteriorates?" The patient said, "All such cases exist. Moreover, no eye disease is treated without using Borneol. If your eyes were treated with Borneol, why have they not healed, yet you still boast of its efficacy?" The guest rose with a solemn expression and said, "This is indeed something I have never heard. I dare to ask for instruction."

I said, "Borneol possesses both benefits and harms; its merits and faults are equal. However, while the pros and cons lie in the nature and flavor of Borneol itself, the actual outcome depends on whether the physician uses it appropriately. Listen carefully and reason through this. Generally speaking, eye diseases do not flare up unless provoked by heat, nor do they subside unless addressed by cold—this refers to the general principle. Blood flows when exposed to heat and coagulates when exposed to cold; excessive cold damages blood, while excessive heat damages essence. This is the natural order.

Reviewing various theories on Borneol, some classify it as cold, some as hot, some as neutral, and some as a 'plundering' drug (Jie Yao). All these views fail to grasp the core principles of ophthalmology. Borneol possesses both cold and hot properties; it is Yang within Yin. Its flavor is cool, but its nature is hot. It is truly a plundering drug in ophthalmology. Flavor is tangible; nature is intangible. Blood is tangible; Qi is intangible. Borneol has a cool flavor and a hot nature. Its cool flavor cannot dissipate intangible fire, and its hot nature cannot move tangible blood. Thus, although blood desires to flow due to heat, it is restrained by cold; although fire desires to retreat due to cold, it is aided by heat. Consequently, cold damages blood, and heat damages essence. As the ancients said: 'Cold is not purely cold, and heat is not purely hot. When cold and heat attack together, they damage essence and blood.' The eye is a orifice located at the highest point. Fire nature tends to blaze upward and easily attacks the head. Currently, internal fire is blazing and extremely stifled. Furthermore, since the meridians are deep and distant, internal medicines cannot easily reach the eyes. Therefore, external plundering drugs are used to employ a counter-attack method: borrowing its nature to guide pathogenic fire out through the orifice, and borrowing its flavor to moisten the eye, relieving dryness and pain. Fragrance can also unblock orifices. One should only temporarily use its plundering power, not continuously. Conditions such as corneal ulceration (Ning Zhi), redness and swelling, epidemic acute blindness (Tian Xing Bao Feng), crab-eye deformity (Xie Jing), red filaments, wind-induced erosion, dryness, and pain are indications for its use. Other conditions are not suitable.

If the fire has subsided and there are no symptoms of redness, pain, dryness, or erosion, Borneol should be reduced or omitted. The efficacy of Borneol lies only in dispersing redness, clearing fire, moistening dryness, and alleviating pain. Its harm lies in depleting Yang energy, causing blurred vision, and condensing ocular fluids into white opacities that are difficult to remove. This is due to extreme heat generating cold, and fire transforming with water. I have frequently seen patients with corneal ulcers, red filaments, floral nebulae, or crab-eye deformities where Borneol condensed into large white flakes that could not be removed. Only then did I witness the harm of Borneol generating cold and fire transforming with water.

Generally, using Borneol for eye disease is like using a thief to fight thieves; its effect is rapid. Once the 'thief' is defeated, we win. But if we do not strip its authority and allow it to run rampant again, it will cause chaos. Therefore, whenever Borneol is used to plunder pathogens, after the pathogens recede, continued excessive use will cause ocular fluids to condense and diminish the luster of the eyes. It is essential to reduce the dosage of Borneol for optimal results. Internally, one must still take nourishing and regulating herbs, so as not to damage the pupil spirit (Tong Shen)."