Only when the stock of wealth can be related to human energy by purchasing living energy as "labor power", offered for sale by people who have no other means of using their labor to ensure their livelihood: and only when it can relate that labor power to purchased machines - embodiments of past transformation of nature by human energy expended in the past - only then does "wealth" become "capital".

Chapter 10, Crises and Differentiation in Capitalism, p. 298. - Europe and the People Without History