Read pp. 128-132, part of chapter 4 "Entrepreneurial Profit" of The Theory of Economic Development: An Inquiry Into Profits, Capital, Credit ... By Joseph Alois Schumpeter

http://books.google.com/books?id=-OZwWcOGeOwC&pg=PA128&source=gbs_toc_r&cad=4#v=onepage&q&f=false