But an upright man will, never for a friend's sake do anything in violation of his country's interests or his oath or his sacred honour, not even if he sits as judge in a friend's case; for he lays aside the role of friend when he assumes that of judge.
 Marcus Tullius Cicero, On Duties (44 BC). copy citation

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Author Marcus Tullius Cicero
Source On Duties
Topic interest role
Date 44 BC
Language English
Reference
Note Translated by Walter Miller
Weblink http://www.constitution.org/rom/de_officiis.htm

Context

“But for our guidance in all such cases we have a rule that is short and easy to master: apparent advantages — political preferment, riches, sensual pleasures, and the like — should never be preferred to the obligations of friendship. But an upright man will, never for a friend's sake do anything in violation of his country's interests or his oath or his sacred honour, not even if he sits as judge in a friend's case; for he lays aside the role of friend when he assumes that of judge. Only so far will he make concessions to friendship, that he will prefer his friend's side to be the juster one and that he will set the time for presenting his case, as far as the laws will allow, to suit his friend's convenience” source