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Stephen R. Covey, A. Roger Merrill, Rebecca R. Merrill (1997). “First Things First Every Day: Daily Reflections- Because Where You're Headed Is More Important Than How Fast You Get There”, p.115, Simon and Schuster
Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.19, St. Martin's Press
Tony Robbins (2007). “Awaken the Giant Within: How to Take Immediate Control of Your Mental, Emotional, Physical and Financial”, p.451, Simon and Schuster