[Latin] nisi paria non pugnant [Notes] Irascetur aliquis: tu contra beneficiis prouoca; cadit statim simultas ab altera parte deserta; nisi paria non pugnant. (If any one is angry with you, meet his anger by returning benefits for it: a quarrel which is only taken up on one side falls to the ground: it takes two men to fight.) Seneca the Younger, De Ira (On Anger): Book 2, cap. 34, line 5. [Translation] 
it takes two to make a fight