TWENTY-ONE YEARS AGO this fall I sat in my first economics class and listened to the conventional wisdom of the time, as set forth by the eighth edition of Paul Samuelson’s classic introductory text Economics. Today that book retains a valued place on the shelf of my office, in part a reminder of what is enduring in economic thought, but also a testament to the ability of the economic mainstream, if not individual economists, to change long-cherished beliefs.