This is the use that may be lawfully made of censuring and judging our enemies; that we may be sure we are not culpable for the same misdemeanors which we condemn in them. On the contrary, we may reap no less advantage from our being judged and censured by our enemies. In this case Antisthenes spake incomparably well, that if a man would lead a secure and blameless life, it was necessary that he should have either very ingenuous and honest friends, or very furious enemies, because the first would keep him from sinning by their kind admonitions, the latter by their evil words and vehement invectives.
Source: Plutarch’s Morals. Translated from the Greek by Several Hands. Corrected and Revised by William W. Goodwin, with an Introduction by Ralph Waldo Emerson. 5 Volumes. (Boston: Little, Brown, and Co., 1878).