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Lei Zhenjun said: The principles of heaven and earth are none other than Yin and Yang; how can diseases of the human body depart from Yin and Yang? The *Neijing* (Classic of Internal Medicine) has exhaustively discussed Yin and Yang.
However, it only discusses the subtle details, failing to address the general outline. Among the Yin and Yang of the human body, the most significant are Qi and Blood. Although the *Neijing* briefly mentions them, it does not fully expound on their supreme importance. Generally, the most critical aspect of Qi and Blood lies in the excess of Qi and the deficiency of Blood. When Qi is in excess, Yang becomes vigorous while Yin declines; when Blood is in excess, Yin becomes vigorous while Yang declines. In cases where Yang is vigorous and Yin declines, one should tonify Blood; in cases where Yin is vigorous and Yang declines, one should tonify Qi. For conditions characterized by Yang excess and Yin decline, it is appropriate to drain Qi; for those with Yin excess and Yang decline, it is appropriate to drain Blood. To determine the appropriateness of tonifying or draining Yin and Yang, one need only observe whether Qi and Blood are in excess or deficient.

Lei Zhenjun said: Day and night are crucial for distinguishing the Yin and Yang nature of diseases, yet they are also the most difficult to distinguish. Yang-pattern diseases tend to worsen during the day and improve at night because Yang Qi and pathogenic Qi mutually intensify each other. However, there are Yang-pattern diseases that do not worsen during the day, due to the deficiency of Yang Qi. Yin-pattern diseases tend to improve during the day and worsen at night because Yin Qi and pathogenic Qi mutually intensify each other. However, there are Yin-pattern diseases that improve at night, due to the deficiency of Yin Qi. When Yang Qi and pathogenic Qi mutually intensify, this indicates a condition where Yang Qi is not yet deficient; thus, Yuan Yang (Primordial Yang) dares to contend with pathogenic Qi without ceasing. Although the symptoms appear severe, the disease is actually mild. One should assist Yang Qi to dispel pathogens rather than merely dispelling pathogens without tonifying Yang Qi. When Yin Qi and pathogenic Qi mutually intensify, this indicates a condition where Yin Qi is not yet depleted; thus, True Yin contends with pathogenic Qi incessantly. Although the presentation appears aggressive, the disease is not actually severe. One should assist Yin Qi to drive out pathogens rather than merely driving out pathogens without tonifying Yin Qi. When Yang Qi is deficient, symptoms do not worsen during the day; although the presentation seems mild, the pathogenic factor is actually substantial. This is because Yuan Yang is extremely deficient and dares not contend with Yang-type pathogens, showing a tendency to retreat rather than advance, which is not due to Yang being vigorous and refusing to fight. When Yin Qi is deficient, symptoms may improve at night; although the presentation seems superficial, the pathogenic factor is actually deep-seated. This is because True Yin is severely weak and dares not confront Yin-type pathogens, showing a hurried avoidance, which is not due to Yin being vigorous and refusing to engage. These distinctions based on day and night should not be deceived by the appearance of the disease. However, one must not rigidly adhere to these rules. Some diseases worsen both day and night, some improve both day and night, some fluctuate in severity, and some show no clear pattern. This reflects the indeterminacy of Yin and Yang and the difficulty in strictly applying day-night distinctions. One must not blindly adhere to theories of tonifying Yang alone; instead, one should vigorously tonify Yin while slightly assisting with Yang-tonifying herbs. In this way, Yin and Yang will be nourished, and pathogens will flee spontaneously without combat.
Zhang Zhenjun said: Discussions on Yin and Yang cannot transcend the subtleties found in the classics.