Sourced

• "If something cannot go on forever, it will stop."
  • Herb Stein's Unfamiliar Quotations http://www.slate.com/id/2561/

Unsourced

• "I belong to the don't-know school of economics."
• "Economists are very good at saying that something cannot go on forever, but not so good at saying when it will stop."
  • These quotes were a typical reply to those arguing for urgent fiscal policy measures to address the widening U.S. account deficit in the 1980s, and a reminder that knowing if and when to act in macroeconomics is not easy.