The law of Yin and Yang is the natural order of the universe, the foundation of all things, mother of all changes, the root of life and death.
The Yin Yang theory holds 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, etc. The movements and changes of yin and yang give impetus to the development of everything or in the words of 《The Yellow Emperor's Classic of Medicine》, "The law of Yin and Yang is the natural order of the universe, the foundation of all things, mother of all changes, the root of life and death."
Yin and yang represent two opposite aspects of every object and its implicit conflict and interdependence. Generally, anything that is moving, ascending, bright, progressing, hyperactive, including functional disease of the body, pertains to yang. The characteristics of stillness, descending, darkness, degeneration, hypoactivity, including organic disease, pertain 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 ad infinitum. 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.