For as long as people have studied progress, they’ve also worried it’s about to end. In 1830, historian Thomas Babington Macaulay asked a simple but powerful question: "If everything behind us shows improvement, why do we assume everything ahead will get worse?" A century later, economist John Maynard Keynes made a similar observation. Despite massive gains in living standards, people were convinced the best days were already behind them. In other words, pessimism about the future isn’t new,...