Aristotle: "Those that know, do. Those that understand, teach."
Chinese proverb: "I hear; I forget. I see; I remember. I do; I understand."
Lao Tzu: "To realize that you do not understand is a virtue. Not to realize that you do not understand is a defect."
Me: "About the only thing I truly understand is that I'm happiest when I try to understand rather than to BE understood."