Failure

From the book The Best of Success, compiled by Wynn Davis:

Failure -  Successful people are not afraid to fail. They have the ability to accept their failures and continue on, knowing that failure is a natural consequence of trying. The law of failure is one of the most powerful of all the success laws because you only really fail when you quit trying.

"There can be no failure to a man who has not lost his courage, his character, his self-respect, or his self-confidence. He is still a king."
   -  Orison Swett Marden

"The credit belongs to the man who is actually in the arena; whose face is marred by dust and sweat and blood; who strives valiantly; who errs and comes short again and again; who knows the great enthusiasms, the great devotions, and spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement; and who at the worst, if he fails, at least fails while daring greatly..."
   -  Theodore Roosevelt

"The freedom to fail is vital if you're going to succeed. Most successful men fail time and time again, and it is a measure of their strength that failure merely propels them into some new attempt at success."
   -  Michael Korda