“The best thing for being sad," replied Merlin, beginning to puff and blow, "is to learn something. That's the only thing that never fails. You may grow old and trembling in your anatomies, you may lie awake at night listening to the disorder of your veins, you may miss your only love, you may see the world about you devastated by evil lunatics, or know your honour trampled in the sewers of baser minds. There is only one thing for it then — to learn. Learn why the world wags and what wags it. That is the only thing which the mind can never exhaust, never alienate, never be tortured by, never fear or distrust, and never dream of regretting. Learning is the only thing for you. Look what a lot of things there are to learn.” 
― T.H. White, The Once and Future King

“The mediocre teacher tells. The good teacher explains. The superior teacher demonstrates. The great teacher inspires.” 
― William Arthur Ward

“Those who know, do. Those that understand, teach.” 
― Aristotle

“You cannot teach a man anything, you can only help him find it within himself.” 
― Galileo Galilei

“True teachers are those who use themselves as bridges over which they invite their students to cross; then, having facilitated their crossing, joyfully collapse, encouraging them to create their
own.” 
― Nikos Kazantzakis

“I never teach my pupils, I only attempt to provide the conditions in which they can learn.” 
― Albert Einstein