The Impossible Trinity or 'trilemma' suggests that it is impossible for a country to maintain a fixed exchange rate, free capital movement and an independent monetary policy at one and the same time.

Transcript -- The Principle of Comparative Advantage

Ever shaken an invisible hand? Been flattened by a falling market? Or wondered what took the bend out of Phillips' curve? David Mitchell helps reveal some of the great dilemmas faced by governments trying to run an economy - whether to save or spend, control inflation, regulate trade, fix exchange rates, or just leave everyone to get on with it and not intervene. You'll learn why Adam Smith put such a high price on free markets, how Keynes found a bold new way to reduce unemployment, and what economists went on to discover about the impact of policy on people's and businesses' behaviour - which may not always be entirely rational...