The yin-yang theory is used to explain not only the presence of two opposing aspects of an object or a natural phenomenon, but also that the birth, growth, development, evolution of all things in the universe result from the unification of these opposing aspects.

The Yellow Emperor said in Basic Questions (Su Wen) that Yin and Yang are the law of heaven and earth, the outlines of everything, the parents of all changes and transformations, the root and beginning of birth (life) and destruction (death), the source of all mysteries. To treat diseases, their root causes must be sought.

In general, yang designates the ability or property of outwardness, ascension, light, warmth, exterior, etc., while yin designates the ability or property of inwardness, descent, stillness, darkness, cold, interior, etc.

Yin and yang can each be infinitely divided into yin and yang aspects. Yin and yang are also:

     –      opposite, yet they form a unity, which is the result of their opposition (without opposition, there is no unity);

     –      interdependent – they cannot exist in isolation, neither can exist without the other, the existence of one is dependent upon the existence of the other (there is no day without night, life without death, upper without lower, etc.);

     –      in a dynamic equilibrium – they are not fixed, but in continuous motion and change (growth and decline) = quantitative change;

     –       they transform into each other = a process of qualitative change which occurs as a result of (slow or sudden) quantitative changes and under certain circumstances (“extreme” state – extreme yin will give rise to yang, and extreme yang will give rise to yin).