This conclusion comes from the famous Mundell-Fleming model in international macroeconomics. For example, if a country implements an expansionary monetary policy (increasing the money supply), arbitrage opportunities will arise by selling local currency and buying dollars. If the government wants to maintain a fixed exchange rate of the local currency against the US dollar, it must limit the free flow of capital (free arbitrage); Otherwise, the result of arbitrage is that the exchange rate of the local currency against the US dollar falls. If the government doesn't want to restrict free arbitrage and the local currency depreciates, it can only use foreign exchange reserves to intervene in the market, sell dollars and buy local currency, but the increased money supply will be recovered and the monetary policy will be invalid. This rule, called "impossible trinity" by economist Krugman, restricts the basic behavior mode of international finance, just like three people playing with hammers, scissors and cloth together. Only when the three parties have two ways to play can the game have a result.