Better to Judge Between Two Enemies Than Two Friends1
The Sage Bias Tells Why
(from Diogenes Laertius's Lives of Eminent Philosophers)

THE SAGE BIAS stated that he would rather decide a case between two of his enemies than between two of his friends. In the latter case, he would make one of his friends his enemy. In the former case, he would make one of his enemies his friend.

Endnotes
1 Diogenes Laertius, Lives of Eminent Philosophers I.87. ⇑