From http://philebersole.wordpress.com/2010/11/17/inflation-through-the-ages/
"Ideally, according to monetarists, there should be a money supply expanding at exactly the same rate that the economy as a whole expands.  This would be impossible in practice, because nobody can know how fast the economy is expanding until after the fact.  So monetarists advocate increasing the money supply at a constant rate – say 2 percent a year – which is consistent with the long-term growth rate of the economy."