Why conservatives are wrong on the economy

From Paul Krugman's blog:

From the very beginning of the Lesser Depression, the central principle for understanding macroeconomic policy has been that everything is different when you’re in a liquidity trap. In particular, the whole case for fiscal stimulus and against austerity rests on the proposition that with interest rates up against the zero lower bound, the central bank can neither achieve full employment on its own nor offset the contractionary effect of spending cuts or tax hikes.

This isn’t hard, folks; it’s just Macro 101. Yet a large number of economists — never mind politicians or policy makers — seems to have a very hard time grasping this basic concept.