This emphasis on mental character lasted for a time, but it has abated. There’s less talk of sin and frailty these days. Capitalism has also undermined this ethos. In the media competition for eyeballs, everyone is rewarded for producing enjoyable and affirming content. Output is measured by ratings and page views, so much of the media, and even the academy, is more geared toward pleasuring consumers, not putting them on some arduous character-building regime.

In this atmosphere, we’re all less conscious of our severe mental shortcomings and less inclined to be skeptical of our own opinions. Occasionally you surf around the Web and find someone who takes mental limitations seriously. For example, Charlie Munger of Berkshire Hathaway once gave a speech called “The Psychology of Human Misjudgment.” He and others list our natural weaknesses: We have confirmation bias; we pick out evidence that supports our views. We are cognitive misers; we try to think as little as possible. We are herd thinkers and conform our perceptions to fit in with the group.

But, in general, the culture places less emphasis on the need to struggle against one’s own mental feebleness. Today’s culture is better in most ways, but in this way it is worse.

The ensuing mental flabbiness is most evident in politics. Many conservatives declare that Barack Obama is a Muslim because it feels so good to say so. Many liberals would never ask themselves why they were so wrong about the surge in Iraq while George Bush was so right. The question is too uncomfortable.

There’s a seller’s market in ideologies that gives people a chance to feel victimized. There’s a rigidity to political debate. Issues like tax cuts and the size of government, which should be shaped by circumstances (often it’s good to cut taxes; sometimes it’s necessary to raise them), are now treated as inflexible tests of tribal purity.

To use a fancy word, there’s a metacognition deficit. Very few in public life habitually step back and think about the weakness in their own thinking and what they should do to compensate. A few people I interview do this regularly (in fact, Larry Summers is one). But it is rare. The rigors of combat discourage it.

Of the problems that afflict the country, this is the underlying one.