I think Anais Nin might've "borrowed" that one from Immanuel Kant - I do think it's very true, though :-)

I'm a bit emotional today and listening to old songs...I really like these lyrics from Simon & Garfukel's, 'The Boxer':

In the clearing stands a boxer
And a fighter by his trade
And he carries the reminders
Of ev'ry glove that layed him down
Or cut him till he cried out
In his anger and his shame
"I am leaving, I am leaving"
But the fighter still remains