Peter Drucker, Harvard Business Review (2016). “The Peter F. Drucker Reader: Selected Articles from the Father of Modern Management Thinking”, p.8, Harvard Business Review Press
Dr. Robert C. Worstell, Dorothea Brande, Eark Nightingale, Claude M. Bristol, Napoleon Hill (2017). “Mindset StackingTM Inspirational Journal VolumeSS02”, p.150, Lulu.com