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.

Aristophanes (c. 450-c. 388 BC) Athenian comedic playwright

The Birds, l. 375ff (414 BC) [tr. Anon. (1812), Ramage (1864)]
    (Source)

Alt. trans. [Hickie (1853)]:
EPOPS: Yet, certainly, the wise learn many things from their enemies; for caution preserves all things. From a friend you could not learn this, but your foe immediately obliges you to learn it. For example, the states have learned from enemies, and not from friends, to build lofty walls, and to possess ships of war. And this lesson preserves children, house, and possessions.
CHORUS [LEADER]: It is useful, as it appears to me, to hear their arguments first; for one might learn some wisdom even from one's foes.

Alt. trans. [O'Neill (1938)]:
EPOPS: The wise can often profit by the lessons of a foe, for caution is the mother of safety. It is just such a thing as one will not learn from a friend and which an enemy compels you to know. To begin with, it's the foe and not the friend that taught cities to build high walls, to equip long vessels of war; and it's this knowledge that protects our children, our slaves and our wealth.
LEADER OF THE CHORUS: Well then, I agree, let us first hear them, for that is best; one can even learn something in an enemy's school.