Smith said nothing about Petty in "The Wealth of Nations". In his published writings, there is nothing apart for a reference in a letter to Lord Shelburne, one of Petty's aristocratic descendants. Petty continued to exercise influence. Karl Marx thought, as did Petty, that the total effort put in by the aggregate of ordinary workers represented a far greater contribution to the economy than contemporary ideas recognised. This belief led Petty to conclude in his estimates that labour ranked as the greatest source of wealth in the kingdom. By contrast Marx's conclusions were that surplus labour was the source of all profit, and that the labourer was alienated from his surplus and thus from society. Marx's high esteem of Adam Smith is mirrored in his consideration of Petty's analysis, as witnessed by countless quotations in his major work "Das Kapital". John Maynard Keynes also wrote at a time of mass discord, as unemployment was rampant and economies stagnant during the 1930s. He showed how governments could manage aggregate demand to stimulate output and employment, much as Petty had done with simpler examples in the 17th century. Petty's simple £100-through-100-hands multiplier was refined by Keynes and incorporated into his model.