"He who knows others is wise;
he who knows himself is enlightened.
He who conquers others has force;
he who conquers himself is truly strong"
From "Tao Te Ching".
Not exactly a quote, but I have always liked the epitaph on the tomb of Raphael Sanzio, the painter (despite considering it a little "exaggerated"):
"Here lies Raphael, by whom, the great mother of all things (Nature) feared to be defeated while he lived, and when he died, she too feared to die"