3. Waning and waxing of yin and yang
Yin and yang always coexist in a dynamic equilibrium in which one waxes while the other wanes. In other words, waning of yin will lead to waxing of yang and vice versa. Take the seasonal and climatic variations for example, it gets warm from winter to spring, and hot fron spring to summer. This is the process of "yang waxing and yin waning." Conversely, it gets cool from summer to autunn, and cold from autumn to winter---the process known as "yin waxing and yang waning". Under normal conditions, the waning - waxing relation of yin and yang is in a state of relative balance. If this relation goes beyond normal limits, the relative balance of yin and yang will not be rmaintained, thus resulting in either excess or deficiency of yin or yang and the occurrence of disease, so far as to endanger one's life.
4. Transformation of yin and yang
In given conditions, either yin or yang may transform into its opposite, i. e. yin may be transformed into yang and yang into yin. If the waning- waxing of yin and yang is ssid to be a process of quantitative change, then that of their intertransformation pertains to a qualitative change based on the quantitative change.
Such a process is mostly a gradual one from quantitative to qualitative change. Su Wen states: "Extreme yin gives rise to yang, while extreme yang gives rise to yin." "Extreme cold brings on heat, while extreme heat brings on cold". This is termed: "Things will develop in the opposite direction when they become extrene." Pathologically, the yin syndrones can be transformed into yang syndromes, and vice versa. It must be pointed out that the decisive factor of the mutual transformation is the conditions, including internal and external conditions, without which such transformation will be by no means likely to occur.
The above statement is the basic content of yin-yang theory, which is also illustrated by "Taijitu" (Yin-Yang Diagram) below. In the diagram the white part indicates yang, and the black part yin. The relationships between the two are both opposite and complementary to esch other. The white spot within yin shows the yang within yin, while the black spot does yin within yang. Meanwhile, they contain the potential for Intertransformation and inter-wane-wax between yin and yang.
From the above, we can see thạt the content of yin-yang theory is composed of four aspects, among which the opposition and waxing-waning contain the opposite of contradiction; the interdependence and transformation contain the unity of contradiction; waxing-waning and transformation contain quantitative change and qualitative change. The forner is the precondition for the latter, the latter is the outcotme of the former.
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