The number 7 in playing cards stands for darkness. Yin-Yang and Five Elements are simple materialistic dialectical methods in ancient China. Yin and Yang are two sides of the same opposite, and they complement each other. There is a famous saying in Zhouyi: "One yin and one yang is the way." Yin He explains everything in the world. The Five Elements is a further elaboration of Yin and Yang, which extracts the essential attributes of Jin Mu's five substances in nature, namely water, fire and earth, and explains nature.