 It occurs when the consumable good is labour itself.To understand this phenomenon, compare the change in the cost of performing the Molière play "Tartuffe" in 1664 and in 2007 with the change in cost of calculating a large number of sums from an accounting ledger.In 1664, you needed two hours and twelve actors to perform Molière's play, and it would take, say, twelve accountants working for two hours to add up all the sums in an accounting ledger.