Eighteenth-century thinkers, such as David Hume and Adam Smith, understood the relationship between the amount of money in circulation and the prices at which goods and services were bought and sold: 'if we consider any kingdom by itself, it is evident, that the greater or less plenty of money is of no consequence, since the prices of commodities are always proportioned to the plenty of money'. In the long run, more money means higher prices. The quantity theory of money, later refined and popularised by the American economists Irving Fisher and Milton Friedman, had been born.