Men of sense often learn from their enemies. It is from their foes, not their friends, that cities learn the lesson of building high walls and ships of war . . .
Source: In Wisdom of the Ages at Your Fingertips, MCR software, 1995.
-- Aristophanes, ( 448BC-385BC), Greek dramatist; He was the greatest comic playwright of ancient world; wrote 55 plays of which 11 survive today.