If wane and wax between yin and yang exceeds the normal level, relative predominance or relative decline of either yin or yang will arise, consequently damaging the dynamic balance between yin and yang and leading to imbalance of yin and yang.

1.3.4 Mutual transformation between yin and yang

If yin or yang wane or waxes to the extreme point, it will turn to the opposite. That means yin will change into yang and yang into yin. The key element involved in such a mutual transformation is the degree of wane and wax. The degree that leads to transformation is termed “extreme point” or “excess” in TCM. In Huangdi Neijing, it suggests that “extreme cold generates heat”, ”extreme heat generates cold”, “excessive yin turns into yang” and“excessive yang changes into yin”, all reflecting mutual transformation relationship between yin and yang.

The typical mutual transformation process of yin and yang is well signigied by the variations of yin and yang in the four seasons of a year. From spring to summer, yang waxes while yin wanes, however when such a transformation reached the peak-the summer solstice, yin begins to wax while yang starts to wanes. But when such a transformation reaches the peak- the winter solstice, yang begins to wax while yin starts to wane. Such a change exactly explains the idea that “excessive yin turns into yang” and “excessive yang changes into yin”. With the wane and wax of yin and yang, cold and heat in the climate also alternate. Such an alternation vividly demonstrate the theory that “extreme cold generates heat” and “extreme heat generates cold”.