Friedman and Keynes both agreed that successful macroeconomic management was necessary - that the private economy on its own might well be subject to unbearable instability ­and that strategic, powerful, but limited economic intervention by the government was necessary to maintain stability. But, while for Keynes, the key was to keep the sum of government spending and private investment stable, for Friedman the key was to keep the money supply -- the amount of purchasing power in readily spendable form in the hands of businesses and households-- stable.