“I’m trying to think of a limerick,” said Eustace. “Something like this: Some kids who played games about Narnia Got gradually balmier and balmier—” “Well, Narnia and balmier don’t rhyme, to begin with,” said Lucy. “It’s an assonance,” said Eustace. “Don’t ask him what an assy-thingummy is,” said Edmund. “He’s only longing to be asked.” —C.S. Lewis, The Voyage of the Dawn Treader Recently I came across a row of motivational posters on an office wall. One of them exhorted the aspiring business high-flier to “Clear your mind of can’t”. The quote was attributed to Samuel Johnson (1709–84). “Can’t”? Well,…