Reducción al absurdo

 A corollary to the law of one price is the theory that exchange rate (between the currencies of two nations) movements are bounded by the concept of purchasing power parity (PPP). Essentially the PPP concept extends the arbitrage-free ideal to the realm of international trade. One US Dollar should command the same purchasing power (measured in a basket of goods) as one Pound, Euro or Yen. If, to the contrary, one Dollar can buy more goods in the US than the equivalent quantity of Yen in Japan, then the value of the Yen to the Dollar is out of line. In this case, we say the Yen is overvalued to the Dollar.

Si nosotros podemos comprar más bienes en euros en nuestro barrio que su equivalente en rupias en la India es que una de las dos esta mal valorada, algo falla y entonces los economistas dicen HOUSTON , WE HAVE A PROBLEM pero previamente los mercados dan dinero a unos y palos a otros hasta que  se resuelva.