Energy is ever-changing and ever-moving. No movement is possible without an opposing force. The Chinese have divided everything into Yin or Yang. The principle of Yin and Yang has to do with equality and balance. Yin and yang are mutually dependant they can not exist without one and other and it is a continous interaction.
They exist in complementary, non-antagonistic pairs where one does not exclude the other. This shows clearly in our twelve important organs. Every Yin organ is dependent and reciprocal to its Yang counterpart.