Why do bad money drive out good money? Do you understand these economic allusions?

"Bad money drives out good money" is an ancient principle in economics. It is said that in the era of coin circulation, when silver and gold are the standard currency, a country should set a value ratio between gold coins and silver coins, and buy and sell gold and silver freely according to this ratio, and gold coins and silver coins can be circulated at the same time. Due to the change in the value of gold and silver, the change in the value of this metal currency itself and the exchange rate between the two remain relatively unchanged, which leads to the phenomenon that "bad money drives out good money" and makes the dual standard system impossible. For example, when the exchange ratio of gold and silver is 1: 15, when the value of silver eventually decreases due to the reduction of the mining cost of silver, people will exchange silver for gold according to the above ratio and store it, so that silver will eventually flood the currency and repel gold. If, on the contrary, the value of silver goes up and the value of gold goes down, people will change gold into silver according to the above ratio, store the silver, and only gold coins will circulate. In other words, the "good money" with higher actual value is gradually stored by people and left the circulation market, which makes the "bad money" with lower actual value flood the market. This phenomenon was first discovered by British Chancellor of the Exchequer Gresham (1533- 1603), so it is called "Gresham's Law". For example, in a completely free foreign exchange market, that is, a market without any legal intervention, there is no legal parity between currencies, and the values of these currencies are different, among which the trend is firm and the gold content is high. On the contrary, weak currency is considered as soft currency, that is, "bad money". In international trade, people are often willing to accept hard currency, that is, "good currency", rather than soft currency, that is, "bad currency". As a result, the survival of the fittest has formed a situation in which "good money drives out bad money". This is a counterexample of Gresham's Law or Anti-Gresham's Law.