For 150 years (from approximately 1785 to 1935), the vast majority of economists took the smooth operation of this market-clearing mechanism as inevitable and inviolable, based largely on belief in Say's law. But the Great Depression of the 1930s caused many economists, including John Maynard Keynes, to doubt their classical faith. If markets were supposed to clear, how could ruinously high rates of unemployment persist for so many painful years? Was the market mechanism not supposed to eliminate such surpluses? In one interpretation, Keynes identified imperfections in the adjustment mechanism that, if present, could introduce rigidities and make prices sticky. In another interpretation, price adjustment could make matters worse, causing what Irving Fisher called "debt deflation". Not all economists accept these theories. They attribute what appears to be imperfect clearing to factors like labor unions or government policy, thereby exonerating the clearing mechanism.