Louis O. Kelso, Patricia Hetter Kelso (1967). “Two-factor Theory: the Economics of Reality; how to Turn Eighty Million Workers Into Capitalists on Borrowed Money, and Other Proposals”, New York : Vintage Books
Joseph A. Schumpeter (2011). “The Theory of Economic Development: An Inquiry into Profits, Capital, Credit, Interest, and the Business Cycle”, p.74, Transaction Publishers
Francis Quarles (1844). “Enchiridion Institutions, Essays and Maxims, political, moral & divine. Divided into four centuries. By Francis Quarles”, p.31