vad menar men med The impossible trinity?

A country that wishes to fix the value of its currency and also have an interest-rate policy that is free from outside influence cannot allow capital to flow freely across its borders.  If the exchange rate is fixed but the country is open to cross-border capital flows, it cannot have an independent monetary policy.  And if a country chooses free capital mobility and wants monetary autonomy, it has to allow its currency to float.

Källa: https://www.economist.com/blogs/economist-explains/2016/09/economist-explains-economics-4