John Maynard Keynes,
A Tract on Monetary Reform
“ When it is not so restricted, the conception of purchasing power parity becomes much more interesting, but is no longer an accurate forecaster of the course of the foreign exchanges. If, therefore, we follow the ordinary practice of fixing purchasing power parity by comparisons of the general purchasing power of a country’s currency at home and abroad, then we must not infer from this that the actual rate of exchange ought to stand at the purchasing power parity, or that it is only a matter of time and adjustment before the two will return to equality. ”
