“Fas est ab hoste doceri.
One should learn even from one's enemies.”
Source: Metamorphoses
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Ovid 120
Roman poet -43–17 BCRelated quotes

“It is right to learn even from an enemy.”
Fas est et ab hoste doceri.
Book IV, 428
Variant translations:
It is right to learn, even from the enemy.
Right it is to be taught even by the enemy.
It is right to be taught even by an enemy.
We can learn even from our enemies.
Metamorphoses (Transformations)

“One should forgive one's enemies, but not before they are hanged.”
Statement of 1848, as quoted in The Cynic's Lexicon : A Dictionary of Amoral Advice (1984) by Jonathon Green, p. 91
One must forgive one's enemies, but not before they are hanged.
As quoted in A Mania for Sentences (1985) by Dennis Joseph Enright, p. 10
Variant: We should forgive our enemies, but not before they are hanged

“One should attend to one's enemies, for they are the first persons to detect one's errors.”
§ 5
From Lives and Opinions of the Eminent Philosophers by Diogenes Laërtius
“One cannot fight an enemy if one does not even have the courage to identify him.”
Crossing the Rubicon
Focus Fourteen

Source: Giovanni Gentile: Philosopher of Fascism, (2001), p. xii

Interview with William Warren Bartley, cited in [Bartley, William Warren, w:William Warren Bartley, Werner Erhard: the Transformation of a Man: the Founding of est, Clarkson N. Potter, Inc., 1978, New York, 121, 0-517-53502-5]