Can't Can, and often do can argue with results!

Conservatives before the stimulus: It can't work.

Conservatives after the stimulus: Sure, the economy improved, but we can't know it was the stimulus that did it.

Well, in a certain sense, that's true. Correlation does not imply causation, and since we can't do controlled experiments on the whole economy, we'll never know if anything works. Very scientifically minded of you, conservatives; please keep this in mind when you claim that "tax cuts stimulate growth" or "tax cuts increase tax revenue" or whatever.

But for the rest of us, the evidence is as clear as economic evidence gets. To elaborate, I'll play my Summon Krugman card:

Josh Bivens at EPI has a good overview of the evidence that the stimulus is working. As he says,

A serious look at the evidence argues that this debate should be closed: ARRA has played a starring role in pushing the economy into positive growth.

In another piece, Bivens notes that the usual suspects are now moving the goalposts, conceding that the stimulus is producing growth but saying that it’s not “genuine” growth because … it was caused by the stimulus. Ahem...there’s no distinction between genuine and artificial employment...

[W]hile most of the stimulus has yet to be spent, the rate of spending as a percentage of GDP is already fairly high (take that, Richard Posner), close to the maximum it will reach over the whole course of the plan. That means that we’ve already seen much if not most of the impact of the stimulus on growth.

My 9th-level Krugman defeats your 5th-level Posner!

At any rate, this is why I give Obama a "B" for halting our slide into economic depression. I agree with Krugman that more should have been done (and I also think that stimulus should have contained more infrastructure spending and fewer tax cuts). But the correlation is as clear as correlations get.

The bigger question in my mind is: When will conservatives stop taking economics on faith? The economy went bust during Bush I, boomed under Clinton, stagnated and then went bust under Bush II, and then is recovering under Obama. That isn't hard evidence for anything. But conservatives seem to take it as evidence that Republican economic policies are healthy for the economy, and Democratic policies disastrous. If that isn't a leap of faith, I don't know what is.

Weren't Republicans supposed to be the hard-headed realists in this country? Maybe Democrats are so divided because they have to carry the burden of being our utopian idealists and our pragmatists at the same time, while the Republicans spend their time wringing their hands and worrying feverishly that, somewhere out there, a black person has taken some of their money.

Update: Brad DeLong shows how to do stimulus cost/benefit analysis.