Full Idea
Socrates was the first Greek to grasp the truth that if someone has done a nasty thing to me, this does not give the slightest moral justification for doing anything nasty to him.
Gist of Idea
Socrates was the first to grasp that a cruelty is not justified by another cruelty
Source
comment on Socrates (reports of career [c.420 BCE]) by Gregory Vlastos - Socrates: Ironist and Moral Philosopher p.190
Book Reference
Vlastos,Gregory: 'Socrates: Ironist and Moral Philosopher' [CUP 1992], p.190