To appear the friend of a man when in reality we are no longer so, is to reserve to ourselves the means of doing him an injury by surprising honest men into an error.
 Jean-Jacques Rousseau, Confessions (1782). copy citation

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Author Jean-Jacques Rousseau
Source Confessions
Topic reality error
Date 1782
Language English
Reference
Note Translated by Samuel William Orson
Weblink https://en.wikisource.org/wiki/Confessions_(Rousseau)

Context

“for I had perceived secret ruptures turned to my prejudice, because they left the mask of friendship in possession of my most cruel enemies. The rules of good breeding, established in the world on this head, seem to have been dictated by a spirit of treachery and falsehood. To appear the friend of a man when in reality we are no longer so, is to reserve to ourselves the means of doing him an injury by surprising honest men into an error. I recollected that when the illustrious Montesquieu broke with Father de Tournemine, he immediately said to everybody: "Listen neither to Father Tournemine nor myself, when we speak of each other, for we are no longer friends."” source