Productivity -- the amount of output delivered per hour of work in the economy -- is often viewed as the engine of progress in modern capitalist economies. Output is everything. Time is money. The quest for increased productivity occupies reams of academic literature and haunts the waking hours of C.E.O.'s and finance ministers. Perhaps forgivably so: our ability to generate more output with fewer people has lifted our lives out of drudgery and delivered us a cornucopia of material wealth.

But the relentless drive for productivity may also have some natural limits. Ever-increasing productivity means that if our economies don't continue to expand, we risk putting people out of work. If more is possible each passing year with each working hour, then either output has to increase or else there is less work to go around. 

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