Currency serves as a general equivalent and is independent from general commodities due to the nature of currency itself. It is small in size, easy to divide and carry, and its value is high, so currency will inevitably become the expression of the value of all commodities, and the value of any commodity can be expressed by it. Because money is fluid, if money circulates frequently in society, there may not necessarily be a lot of precious metals in circulation to meet society's money demand, and the money supply is equal to M*V. M currency quantity, V is the velocity of circulation. Furthermore, money demand is affected by factors such as income and interest rates. The higher the income, the lower the interest rate, the higher your consumption demand, and the more money you need. It does not mean that there must be as much currency as there are valuable commodities in society. Subsequently, the manifestation of credit in currency became increasingly obvious, and the actual currency needed in society was greatly reduced.

Read Samuelson's "Economics" and you will have a new understanding of economics.