There are people who owe more to bitter enemies than to apparently pleasant friends
 Marcus Tullius Cicero, On Friendship (44 BC). copy citation

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Author Marcus Tullius Cicero
Source On Friendship
Topic enemies
Date 44 BC
Language English
Reference
Note Translated by E. S. Shuckburgh
Weblink http://www.gutenberg.org/files/2808/2808-h/2808-h.htm

Context

“But if a man's ears are so closed to plain speaking that he cannot bear to hear the truth from a friend, we may give him up in despair. This remark of Cato's, as so many of his did, shews great acuteness: "There are people who owe more to bitter enemies than to apparently pleasant friends: the former often speak the truth, the latter never." Besides, it is a strange paradox that the recipients of advice should feel no annoyance where they ought to feel it, and yet feel so much where they ought not.” source