Money appears with the constant change of commodity exchange value. At the end of primitive society, people's products were surplus, so they had to exchange between different tribes. For example, one tribe has extra sheep and must exchange them with other tribes. By chance, a man took a rabbit to exchange, and two people met together, so a sheep was exchanged for five rabbits. It can be seen here that the value of 1 sheep is represented by five rabbits, and this sheep is now represented by five rabbits, which is an initial form of value expression. With the progress of society and the emergence of a large number of surplus products, this kind of transaction has become more and more frequent. Finally, people realized that gold and silver are materials to measure the value of all commodities, so money came into being.

It can be seen that money itself is a commodity, but this commodity has special functions. It can be exchanged with any other commodity as a universal equivalent.