John C. Maxwell (2011). “The Maxwell Daily Reader: 365 Days of Insight to Develop the Leader Within You and Influence Those Around You”, p.36, Thomas Nelson Inc
Socrates, Plato, Aristotle (1967). “Wit and Wisdom of Socrates, Plato, Aristotle: Being a Treasury of Thousands of Glorious, Inspiring and Imperishable Thoughts, Views and Observations of the Three Great Greek Philosophers, Classified Under about Four Hundred Subjects for Comparative Study”