Conservatives would have us believe that they hold a monopoly on common sense. Rush Limbaugh, Sean Hannity, Bill O’Reilly, and many other right-wing rabble-rousers regularly portray themselves as defenders of the good, old-fashioned common sense of average Americans against an out-of-touch liberal elite. A growing cadre of ambitious politicians likewise aims to lead a crusade in the name of “commonsense conservatism.” Glenn Beck has even gone so far as to publish a runaway bestseller that explicitly piggybacks on Thomas Paine’s Common Sense to argue against the danger of “out-of-control government” and the forces of organized foolishness that would foist it on the American people.

The unanimity is impressive. But it is also ridiculous. The fact...