"For this reason, entrepreneurial activity is just as  
 necessary a factor in the production of goods as  
technical labor services."  
> Menger, Carl. (1981) Principles of Economics.  
> Translated by James Dingwall and Bert Hoselitz. New  
York: New York University Press.    
  
But Menger also wrote:  
" Entrepreneurial activity must definitely be counted  
as a category of labor services."   
(p. 172).  
  
Menger, Carl. 1871 [1976]. Principles of Economics.  
Trans. James Dingwall and Bert Hoselitz. New York: New  
York University Press  
  
Fred Foldvary