I’ve been reading up on the Great Depression (never say those of us at TPC don’t know how to have fun) and am struck by one overriding thought: Even with the benefit of 80 years of hindsight, economists still can’t agree on either what went wrong or how the economy got back on track.

There is an important lesson here. Famously impatient, Americans not only expect government to fix today’s economic crisis, they demand it. Barack Obama, I suspect, will have about a year to turn things around before the public turns on him. Yet, if we still don’t know what happened in 1929, how can we expect policymakers to truly understand—and correctly address—events happening in real time today?

Here is the dirty little secret: Obama doesn’t know how to fix the recession. Nobody does. It is easy (though wrong) to say Herbert Hoover failed to grasp the enormity of the Depression. But in truth, FDR had little better idea of how to respond to the crumbling economy. In fact, Roosevelt was famous for trying one solution after another until something seemed to work.

There is broad agreement on what government should not do, such as run a tight monetary policy or restrict trade. But there is much less consensus on what it should do. How much will pulling fiscal levers boost demand? Is massive new government spending really such a good idea? Will tax cuts get people and businesses spending again?

When credit markets collapsed earlier this year, George Bush turned to his group of wise men for advice. They included Fed Chairman Ben Bernanke, a highly-regarded student of the Depression. President-elect Obama has now chosen his team, including Christy Romer, another expert on that era. They are very smart, and have access to far better data than their counterparts had in the 30s.

But today’s data are not that good. And there is no magic bullet. For instance, Treasury Secretary Hank Paulson has been roundly criticized for his on-again-off-again use of the TARP. First, he insisted on using the $700 billion to acquire bad assets, then he decided to pump liquidity into financial institutions, and now he is giving $13 billion to the auto companies. This seems incoherent because it is. But trial-and-error is as inevitable in economics as in medicine—another field based more on art than science.

Obama’s stimulus plan, which by some accounts will cost $850 billion or more, will include its share of bad ideas. Some initiatives may at first seem to make perfect sense but will inevitably have dreadful unintended consequences. And there will be good ideas that should be in the plan but fail to make the cut.

Those of us who kibbitz from the sidelines will identify mistakes and demand better alternatives. But we should have no illusions. Getting a huge, complex, and still poorly understood economy back on track is not going to be simple and it will take time. Obama, despite the stratospheric expectations he’s raised, is going to make errors. We will, and should, call him on those poor choices. But we’d be fools to expect perfect diagnosis and treatment from the incoming Administration.