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Zhu Danxi stated that during summer, when intense heat prevails, the human body is hot both internally and externally; this view is quite accurate. However, Zhang Jingyue argued that in summer, Yin-cold lies hidden within the body, resulting in external heat and internal cold; whereas at the Winter Solstice, Yang-heat lies hidden within, resulting in external cold and internal heat. He used as evidence the phenomenon that at the Summer Solstice, Yin begins to generate, making the ground hot but well water cold; and at the Winter Solstice, Yang begins to generate, making the ground cold but well water warm. His argument seems plausible but is actually incorrect, as it acknowledges the changes of celestial timing (Tianshi) while ignoring geographical conditions (Dili).

The Qi of the human body connects with Heaven and Earth; it indeed changes according to celestial timing but also shifts according to geographical terrain. Just as there are differences among East, West, South, and North, how can there be no distinctions between high/low or deep/shallow? The human body resides on the surface of the earth, not underground. If one were situated inside a well during summer, not only would the interior be cold, but the exterior would also be cold. Could one still claim, as he did, that it is externally hot and internally cold? Conversely, being situated on the ground, not only is the exterior hot, but the interior is also hot, which can be deduced by reflection. Consider the practice of floating melons and sinking plums (in water), swallowing cold water, and drinking ice without feeling cold; the principle is evident from this. Can one do the same in winter?

One might ask: In summer, sweating is profuse; profuse sweating leads to the loss of Yang-Qi (Yang Qi), and the loss of Yang leads to the generation of Yin within, which is called "hidden Yin" (Fu Yin), is it not? I reply: Profuse sweating in summer occurs in everyone, yet does it always lead to the loss of Yang? It merely depletes the Body Fluids (Jinye). Depletion of Body Fluids constitutes Yin Deficiency (Yin Xu); Yin deficiency causes Yang to become even more blazing, which can be observed through short and red urine. If one fails to nourish Lung Metal and Kidney Water, but instead tonifies Heart Fire and Spleen Earth, I consider this to be confused.

Another question arises: Did ancients often use hot-natured agents for treating Heatstroke (Shu Zheng), is it not so? I reply: This is done only when the condition transforms into deficiency-cold; one cannot indiscriminately apply such methods.

Further inquiry: In summer, Yang Qi leaks outward; in winter, Yang Qi gathers inward, is this correct? I reply: When Yang Qi leaks outward, sweat is produced and the interior becomes dry/depleted, thus cooling and moistening agents are appropriate; when Qi gathers inward, it transforms into fluids and nourishes Yin, thus warm and hot agents can be employed. Observing that in summer one feels thirsty with short, red urine, while in winter one feels no thirst with clear, long urine, it becomes self-evident that those whose Yang leaks outward do not have a cold interior, and those whose Yang gathers inward do not have a hot interior.