Dr. Robert C. Worstell, Dorothea Brande, Eark Nightingale, Claude M. Bristol, Napoleon Hill (2017). “Mindset StackingTM Inspirational Journal VolumeSS02”, p.112, Lulu.com
Napoleon Hill (1960). “The Law of Success in Sixteen Lessons: Teaching, for the First Time in the History of the World, the True Philosophy Upon which All Personal Success is Built”
Dr. Robert C. Worstell, James Breckenridge Jones, Napoleon Hill (2015). “Think and Grow Rich, Updated and Complete - With If You Can Count to Four...”, p.12, Lulu.com