The Yin Yang theory states that all phenomena consist of two opposite aspects, yin and yang, which are variously defined as:
- Up and down
- Left and right
- Light and dark
- Hot and cold
- Stillness and movement
- Substance and function
The movements and changes of yin and yang give momentum to the development of everything or in the words of the Suwen,
"Yin and yang are the law of Heaven and Earth, the outline of everything, the parents of change, the origin of birth and destruction...."
Yin and yang represent two opposite aspects of every object and its implicit conflict and interdependence. Basically, anything that is moving, ascending, bright, progressing, hyperactive, including functional disease of the body, relates to yang. The characteristics of stillness, descending, darkness, degeneration, hypoactivity, including organic disease, are associated to yin.
The nature of yin and yang is relative. According to Yin-Yang theory, everything in the universe can be divided into the two opposite but complementary aspects of yin and yang and so on for infinity. For example, day is yang and night is yin, but morning is understood as being yang within yang, afternoon is yin within yang, evening before midnight is yin within yin and the time after midnight is yang within yin. As the Suwen states,
"Yin and yang could amount to ten in number, be extended to one hundred, one thousand, to ten thousand and ever to the infinite."