Birds (414 BC) 
Context: Epops: You're mistaken: men of sense often learn from their enemies. Prudence is the best safeguard. This principle cannot be learned from a friend, but an enemy extorts it immediately. It is from their foes, not their friends, that cities learn the lesson of building high walls and ships of war. And this lesson saves their children, their homes, and their properties.
Chorus [leader]: It appears then that it will be better for us to hear what they have to say first; for one may learn something at times even from one's enemies.
(tr. Anon. 1812 rev. in Ramage 1864, p.  45 http://books.google.com/books?id=AoUCAAAAQAAJ&pg;=PA45)
                                    
“In war you learn your lessons, and they stay learned, but the tuition fees are high.”
Storm of Steel (1920)
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Ernst Jünger 16
German writer 1895–1998Related quotes
Robert Rosen (2013), Essays on Life Itself Chapter 18
“You Been Burned baby lessons learned.”
                                        
                                        In And Out Of Love 
Music, 7800° Fahrenheit (1985)
                                    
said in an interview quoted by Javier C. Hernandez of The New York Times http://www.nytimes.com/2013/10/14/nyregion/from-his-fathers-decline-de-blasio-learned-what-not-to-do.html.