The examination to become a London cabbie is possibly the most difficult test in the world — demanding years of study to memorize the labyrinthine city’s 25,000 streets and any business or landmark on them. As GPS and Uber imperil this tradition, is there an argument for learning as an end in itself? At 10 past 6 on a January morning a couple of winters ago, a 35-year-old man named Matt McCabe stepped out of his house in the town of Kenley, England, got on his Piaggio X8 motor scooter, and started driving north. McCabe’s destination was Stour Road, a...