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Yin and Yang are differentiated from a single Qi (vital energy). They should be balanced and harmonious, avoiding bias and separation. Sometimes they exist in opposition, sometimes in flow; there are points of convergence and divergence; they mutually generate and restrain each other. Nothing in heaven and earth can exist without them. The subtlety and profundity of their principles are truly not easily expressed.

Examining medical texts, some argue that Yin is prone to deficiency while Yang is prone to hyperactivity, emphasizing the supplementation of Yin as the priority. Others state that Yin governs destruction/salting down while Yang governs growth/life, insisting on supporting Yang as the primary task. Such views, while having profound logic within their own contexts, are not absolute conclusions. Why? Because they fail to distinguish between "Chang" (constancy/normalcy) and "Bian" (change/pathology).

Let us verify this with the four seasons and day and night: Spring and Summer belong to Yang, Autumn and Winter to Yin; these two are distinct without any discrepancy. Night belongs to Yin, Day to Yang; in total, there is no excess or deficit. This represents the constancy of Yin and Yang. Considering changes in the earth: when extreme drought occurs, it is a sign of Yang excess and Yin deficiency, necessitating sweet rain; when prolonged cloudy/rainy weather persists, it is a sign of Yang decline and Yin excess, relying on the bright sun. Those who adhere to specific theories also have valid reasoning.

Regarding human diseases: when Water (Kidney Yin) is deficient and Fire (Heart Yang) is flourishing, only cooling methods can save the raging fire; thus, one cannot exclusively emphasize Yang. When Qi escapes and Spirit is obscured, only warming methods can dissipate the Yin shadow; thus, one cannot exclusively prioritize Yin. Are these not biased views, and therefore not definitive conclusions?

Examining the teachings of ancient Confucian scholars regarding the general outline: movement and stillness serve as roots for each other; this is called "Liu Xing" (flowing circulation). Dividing into Yin and Yang establishes the Two Modes (Liang Yi); this is called "Ding Wei" (fixed positioning). Regarding substance and function: Heaven uses Yang to generate all things and Yin to complete them. Only through the combination of both can development proceed. When Yang is functional, Yin is honored; when Yin is functional, Yang is honored. They change with time and alternate in use. Yang cannot stand alone; it must rely on Yin to establish itself, so Yang takes Yin as its foundation. Yin cannot manifest by itself; it awaits Yang to appear, so Yin follows Yang as its leader. Yin and Yang generate each other; their nature depends on mutual necessity. Therefore, if Yang departs, Yin exhausts; if Yin ends, Yang perishes.

However, the way of Yin favors following Yang and opposes resisting Yang; the nature of Yang favors submergence and opposes emission. Yang governs advancement and Yin retreat; Yang governs growth and Yin wanes. Advancing and growing implies strong Qi; retreating and waning implies weak Qi. Yang is firm, warm, and dense, residing in the southeast, governing Spring and Summer, engaged in production and growth. Yin is soft, strict, and congealed, residing in the northwest, governing Autumn and Winter, engaged in gathering and storing. Production and growth constitute life; gathering and storing constitute death. It seems Yang should be prioritized. Yet, observing that heaven does not produce without earth, nor husband without wife, and that "Yuan" (origin) is born from "Zhen" (steadfastness/endurance), we see that the transformation of heaven and earth requires condensation before dispersion. Without Zhen, there is no Yuan, which would not be the endless path of generation. Thus, Yin should not be considered light. The theories of the ancients did not lean towards bias or weight.

Therefore, when Yin and Yang attain righteousness, they are balanced like a scale; when they lose harmony, they become contrary like ice and charcoal. Viewing from the aspect of change: Yang governs heat, Yin governs cold; they cannot be confused as one. Viewing from the aspect of unchangeability: the flow of Yin Qi is Yang, the condensation of Yang Qi is Yin; how can they be separated into two? Moreover, Yin and Yang hide within each other's dwellings. Thus, injuring Yang affects Yin, and injuring Yin affects Yang. The waxing and waning of Yin and Yang are infinite; the retreat of Yang is the birth of Yin, and the retreat of Yin is the birth of Yang. As stated in the *Nei Jing* (Inner Canon): The Way of Yin and Yang is like a ring without beginning or end. If one says Yang can generate Yin but Yin cannot generate Yang, is this reasonable? Grain, beans, and trees may be born in Spring and mature in Autumn, or born in Autumn and mature in Spring. Only isolated Yang cannot generate, and isolated Yin cannot grow. In summary, discussing constancy, neither should be favored over the other; discussing change, there is a great disparity between rigid and flexible. Extreme cold leads to freezing death; extreme heat leads to burning demise; excess leads to destruction. Clear skies make flora flourish; rain and dew make vegetation lush; harmony leads to life. However, modern people's constitutions often exhibit bias. The key lies in clinical practice: do not dismiss the shortcomings of traditional biased theories, but rather utilize their strengths effectively. The principles of Yin and Yang cannot be fully exhausted in a single statement.