Keynes wrote a lot of things about economics. What is usually referred to as “Keynesian economics” is stimulus, generally assumed to be deficit, spending by the government during downturns.
It is very important to remember that this spending is supposed to be, according to Keynes, offset by reducing spending during boom times. A lot of folks forget this part when bashing “Keynesian economics”.
The problem with “Keynesian economics” is that, while potentially reasonable in theory, it fails (and is never tried) in practice. Economic downturns are too short for government intervention. There aren’t enough “shovel ready” projects (especially once you get the EPA, et al, involved), so the spending actually ends up occurring too late. Also, when did government ever back off? [During the Reagan years, we ran up the debt to break the Soviet Union, which may have saved us money over the long run, but after that we never hit the brakes].
Please note that what I have written above are thoughts I would use when discussing Keynes with a liberal or other idiot. I think a lot less of Keynes’ theories than one might assume reading the above.
True Keynesian theory (which would call for a balanced budget over the business cycle would, at best, have the effect of leveling out the upward and downward swings in an economy. Even this is not without a cost and over many years a society would not be as wealthy under Keynesian theory as it would under a free market approach.