The worker becomes poorer the more wealth he produces and the more his production increases in powerand extent.The worker becomes anevercheaper commodity the more good he creates. The devaluation of the human world increases in direct relation with the increase in value of the world of things. Labour does not only create goods; it also produces itself and the worker as a commodity, and indeed in the same proportion as it produces goods.

-Marx, Karl Heinrich
  Collected in T B Bottomore (trans and ed) Early Writings (1964), p.121.