Aristophanes Quote

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.
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.

Birds (414 BC) [tr. Hickie 1853, vol. 1, p. 322; l. 375]