UPDATE (June 13, 2013): the earliest prototype of the "lump-of-labor fallacy" that I have now been able to document occurs in Dorning Rasbotham's 1780 pamphlet, Thoughts on the Use of Machines in the Cotton Manufacture.
Dear Professor Krugman,
I am writing to you because three times over the last 14 months your authority has been invoked to me on behalf of the assertion that people who advocate shorter working time as a remedy for unemployment are guilty of a "lump-of-labor fallacy" assumption that there is only a fixed quantity of work in the world. As did John Maynard Keynes, I believe that working less is one of "three ingredients of a cure" for unemployment. I find it odd to learn that I (and presumably Keynes) am thereby assuming a palpable absurdity: that the amount of work to be done is invariant.
