There is a scene in the 2005 film Thank You For Smoking where the main character, a fast-talking charismatic spin doctor defending big tobacco, explains what he does for a living to his son. It’s his job, he says, never to be wrong. Never being wrong, though, is not the same as always being right.

“So I’ll ask you: you think chocolate is the end-all of ice cream, do you?” Nick, who is championing vanilla, says to his boy, Joey. Joey answers passionately that he does, whereupon Nick turns the conversation into a debate on freedom of choice. “But you didn’t prove that vanilla was the best,” says Joey. “I didn’t have to,” his Dad replies. “I proved that you’re wrong, and if you’re wrong