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The essence of all zang-fu organs ascends and gathers in the eyes. However, the ability to see clearly at both near and far distances relies on the function of Yang (Yang Qi). The force that sustains this and prevents the light from exhausting is the power of Yin (Yin Qi). When Yin and Yang harmonize, they form "Jingming" (sharp vision), which cannot be achieved by Qi and Blood alone. Generally, Qi and Blood are merely the manifestations (biao) of Yin and Yang and cannot be directly termed as "essence." In contrast, Yin and Yang are the root of Qi and Blood, thus generating the essence.

If Yin is deficient, heat arises, leading to eye diseases. Symptoms include redness, acute swelling, and severe pain, with stabbing pain in the eyeballs worsening at night. Although the swelling and pain are extreme, if the patient can still see objects when looking, it indicates that Yin deficiency generates internal heat, allowing pathogenic fire to invade and further consume the spirit. This depletion prevents the assistance of clear Yang, resulting in redness, swelling, and stabbing pain. Even though opening the eyes is difficult, once opened, vision remains possible. This demonstrates a disease of Yin without disease of Yang, characterizing a condition with "fire" (excess heat).

If Yang is deficient, cold arises, also leading to eye diseases. Symptoms include white nebulae obscuring the pupil, with little or no pain in the eyeball. The vision resembles "Qingmang" (blindness with normal appearance), where the patient can open their eyes but cannot see objects. This indicates that Yang deficiency generates internal cold, allowing turbid Yin to invade, thereby losing the brilliance akin to the sky and sun. This represents a self-disease of Yang, characterizing a condition without "fire" (deficiency cold).

For heat conditions, tonify True Yin using Liuwei Dihuang Wan (Six-Ingredient Rehmannia Pill); for cold conditions, tonify True Yang using Bawei Dihuang Wan (Eight-Ingredient Rehmannia Pill). Through this approach, Yin and Yang naturally restore themselves and resume their harmonious function of sharp vision.

During his youth, Zhang [Feng Chuzhan] suffered greatly from eye diseases. Upon mastering medicine in adulthood, he sought its principles day and night, eventually understanding the underlying logic. After achieving efficacy in self-treatment, he applied this method to help others. Regardless of age, gender, or whether the condition followed childbirth or smallpox, his prescriptions were invariably effective. As stated in the *Classic* (*Huangdi Neijing*): "He who knows the essentials needs only one word to conclude." This refers to the True Yin and True Yang of various diseases.

Some may harbor doubts; let us use a simple analogy: Just as a lamp shines due to fire, the substance that fuels this brightness is oil/wax. With oil/wax but no fire, how can there be light? With fire but no oil/wax, the flame burns fiercely until exhausted. If either element is neglected, the phenomenon of light cannot be formed. Therefore, seeking water within fire ensures the source does not dry up; seeking fire within water ensures the light does not extinguish. One belongs to the tangible (form), the other to the intangible (Qi/Spirit). Because it is intangible, it can generate the tangible. Indeed, the principle of creation stems from the "non-being" (Wu).