IT shows how gifted Jon Stewart is that his best moment happened on someone else’s show. He appeared in 2004 on “Crossfire,” a CNN yelling program, and asked the hosts to take seriously their responsibility to public understanding by having useful conversations instead of shouting matches.

It was Mr. Stewart’s finest hour. He made an earnest pitch for civility in a place where there really was none. Which makes it too bad that in his 16 years of hosting “The Daily Show,” he never lived up to his own responsibility. His prodigious talents — he was smart and funny, and even more of both when he was mad — perfectly positioned him to purge a particular smugness from our discourse. Instead, he embodied it. I loved watching him, and hated it too.

Many liberals, but not conservatives, believe there is an important asymmetry in American politics. These liberals believe that people on opposite sides of the ideological spectrum are fundamentally different. Specifically, they believe that liberals are much more open to change than conservatives, more tolerant of differences, more motivated by the public good and, maybe most of all, smarter and better informed.

The evidence for these beliefs is not good. Liberals turn out to be just as prone to their own forms of intolerance, ignorance and bias. But the beliefs are comforting to many. They give their bearers a sense of intellectual and even moral superiority. And they affect behavior. They inform the condescension and self-righteousness with which liberals often treat conservatives. They explain why many liberals have greeted Tea Partiers and other grass-roots conservatives with outsize alarm. They explain why liberals fixate on figures such as Sarah Palin and Todd Akin, who represent the worst that many liberals are prepared to see in conservatives. These liberals often end up sounding like Jon Lovitz, on “Saturday Night Live,” impersonating Michael Dukakis in 1988, gesturing toward the Republican and saying “I can’t believe I’m losing to this guy!” This sense of superiority is hardly the only cause of our polarized public discourse, but it sure doesn’t help.