Fair enough. Keynes didn't publish the General Theory until 1936.
In a Financial Times article* quoted extensively by DeLong, John Kay makes this remark about the first generation of Keynesians, who engaged in ad hoc-ery in trying to determine the consumption function (not this), "which related aggregate spending in a period to current national income," and thus get a handle on the multiplier effect:
But you would not nowadays be able to publish similar work in a good economics journal. You would be told that your model was theoretically inadequate – it lacked rigour, failed to demonstrate consistency. To be “ad hoc” is a cardinal sin. Rigour and consistency are the two most powerful words in economics today.
This is a damning indictment of macroeconomics, as practiced by the neoclassical school. I level this criticism specifically at them, since the opposing Keynesians seem to have rather useful models, and - more importantly - are not bound by giving more credence to the model than they give to the real world. And perhaps most importantly of all, Keynesians are either roundly criticized as idiots, or simply ignored, and have no influence on current policy. Neo-classicists and their near equivalents, whether of the Chicago, Austrian or Libertarian persuasions, are the opposite: if the model conflicts with reality (as it sooner or later must, models always and everywhere being simplified and therefore inaccurate approximations) then it is reality that is somehow wrong. And these are the Very Serious People who determine and influence policy.
A criticism of Keynesian economics is that it was unable to anticipate the stagflation of the 70's. That's true, it wasn't. It's funny, though, that neo-classicists, who were also equally clueless about stagflation (though I guess Friedman and Phelps did have a clue) had - and still do not have - any explanation of the high employment of the 30's, and of now, come to think of it, other than the "Great Vacation" theory.
In 1970, Keynes had already been dead for 24 years. A more vital Keynes perhaps would have made an attempt to analyze, understand and explain stagflation. That was his approach to difficulties 40 years prior, anyway. The thing to remember about Keynes is that he did not overthrow the edifice of economics as it existed in the 30's. He expanded it to include realms that were otherwise incomprehensible. It's reasonable to assume that he would have done the same in the 70's, had the rather inconvenient passing-away not long since intervened.
The problem with conservative economists of all stripes is not that they are always wrong. As hard as I am on them and their models, they get some things right, some times. The problem is that they give full credence and dependence to contrived "comprehensive and universal descriptions of the world" and little or none to contrary facts and data that actually occur in the world their models so inadequately describe.** Hence, they do not recognize that the universe of economics contains different realms, and that when in these different realms, different policy solutions are needed.
Austerity is right in certain times and places, but not at a time and place when unemployment is high and interest rates are bumping against the zero interest boundary. That they refuse to acknowledge the need for fiscal stimulus, or even that the zero interest bound is of any relevance is quite telling, ispo facto.
The Keynesian solutions were tried in the 30's, and they worked. It is quite likely they would work today, if there were the political will to employ them, rather than Very Serious (and Very Painfully Destructive) Austerity. And, since they are tried and true methods, there would be no need for an ad hoc approach.
* You have to register with FT to read the article - but go ahead: it's free, and worth it!
** The 4th pillar of the conservative mental process manifests itself as some sort of denial of reality.