John Stuart Mill (2017). “JOHN STUART MILL - Ultimate Collection: Works on Philosophy, Politics & Economy (Including Memoirs & Essays): Autobiography, Utilitarianism, The Subjection of Women, On Liberty, Principles of Political Economy, A System of Logic, Ratiocinative and Inductive and More”, p.215, Madison & Adams
Stephen R. Covey (2015). “The Stephen R. Covey Interactive Reader - 4 Books in 1: The 7 Habits of Highly Effective People, First Things First, and the Best of the Most Renowned Leadership Teacher of our Time”, p.449, Mango Media Inc.