At the outset of his last year as chairman of the Federal Reserve, Alan Greenspan had a warning. “History cautions,” he told Congress in 2005, “that people experiencing long periods of relative stability are prone to excess.” The idea wasn’t unique to Greenspan, who passed away late last month at the age of 100. These days, the notion that stability breeds instability is associated with the economist Hyman Minsky, a trenchant critic of the modern financial system.