What is Yin and Yang?

The theory of Yin and Yang holds that everything in the universe has two opposing yet unified aspects of Yin and Yang. Yin and Yang are in constant movement and interaction, and such movement and interaction are the root cause of the movement and changes of all things. The emergence, development, and demise of all things in the universe are the results of the continuous interaction between the Yin and Yang aspects of things. The theory of Yin and Yang is an extremely important thought for understanding and mastering the laws of nature, and it plays a vital role in the I Ching divination.