Russ Roberts had a much-discussed item recently on Keynesian economics, which included an interesting criticism of Paul Krugman: “Krugman is a Keynesian because he wants bigger government. I’m an anti-Keynesian because I want smaller government. Both of us can find evidence for our worldviews.”

I was glad to see Krugman explain just how mistaken this is.

First of all, while conservatives see smaller government as an end in itself, liberals don’t see bigger government the same way. Think about it: while you often see conservatives crow about, say, reducing discretionary spending as a good thing just because the number is down, do you ever see liberals crowing about a rise in spending, never mind what on? Liberals want government to do certain things, like provide essential health care; the size of government per se isn’t the objective.

Krugman added some related points — Keynesianism isn’t about promoting bigger government; conservatives have traditionally supported Keynesian economics; and basing economic views on political prejudices is a bad idea — but it’s this first point that stood out for me. Regular readers probably know we’ve discussed this before, but I continue to believe it’s one of the key observations in American politics, because it’s fundamental to understanding how both sides of the political divide seek to advance their goals — and the nature of the goals themselves.

For the left, political objectives relate to policy ends. We want to expand access to quality health care. We want to lower carbon emissions to combat global warming. We want to reform the lending process for student loans so more young people can afford to go to college. We want to make public investments to create jobs. There are competing ways to get to where progressives want to go, but the focus is on the policy achievement.

What conservatives often find confusing is that the liberal worldview is not about necessarily increasing the size of government or raising taxes; those mechanisms are only valuable insofar as they reach a desired end-point. Whether the government increases or shrinks in the process is largely irrelevant.

For the right, it’s backwards — the ideological goal is the achievement.

Jon Chait had a terrific piece on this several years ago.

We’re accustomed to thinking of liberalism and conservatism as parallel ideologies, with conservatives preferring less government and liberals preferring more. The equivalency breaks down, though, when you consider that liberals never claim that increasing the size of government is an end in itself. Liberals only support larger government if they have some reason to believe that it will lead to material improvement in people’s lives. Conservatives also want material improvement in people’s lives, of course, but proving that their policies can produce such an outcome is a luxury, not a necessity.

The contrast between economic liberalism and economic conservatism, then, ultimately lies not only in different values or preferences but in different epistemologies. Liberalism is a more deeply pragmatic governing philosophy — more open to change, more receptive to empiricism, and ultimately better at producing policies that improve the human condition — than conservatism.

Now, liberalism’s pragmatic superiority wouldn’t matter to a true ideological conservative any more than news about the medical benefits of pork (to pick an imaginary example) would cause a strictly observant Jew to begin eating ham sandwiches. But, if you have no particular a priori preference about the size of government and care only about tangible outcomes, then liberalism’s aversion to dogma makes it superior as a practical governing philosophy.

Conservatives tend to prefer a different approach that decreases the role of government, not to achieve specific ends, but because decreasing the role of government is the specific end.

This, of course, affects nearly every debate in Washington. When it comes to job creation, for example, the task for Democrats is pretty straightforward: let’s do more of what’s been the most effective, and less of what’s been the least effective. Again, it’s about pragmatism and results based on evidence.

For Republicans, it doesn’t work quite that way — they have ideological ideals that outweigh evidence. GOP leaders could be shown incontrovertible proof that the most effective methods of creating jobs and improving the economy are aid to states, infrastructure investment, unemployment insurance, and food stamps, and they’d still refuse. Why? Because their ideology dictates the response.

The left starts with a policy goal (more people with access to medical care, more students with access to college, less pollution, more jobs, less financial market instability) and crafts proposals to try to complete the task. The right starts with an ideological goal (smaller government, more privatization, more deregulation) and works backwards.

These are not parallel ideologies.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.