That city [is best to live in,] in which those who are not wronged, no less than those who are wronged, exert themselves to punish the wrongdoers.

Solon (c. 638 BC - 558 BC) Athenian statesman, lawmaker, poet

Quoted in Plutarch, Parallel Lives, “The Life of Solon,” sec. 18.5 [tr. Perrin (1914)]
    (Source)

Alt. trans.:

• "That [city is best managed] in which those who are not wronged espouse the cause of those who are, and punish their oppressors." [tr. Stewart, Long (1894)]
• "That [city is best modeled] where those that are not injured try and punish the unjust as much as those that are." [Source]
• "That [city is best modeled] where those who are not injured, are as ready to prosecute and punish offenders, as those who are." [tr. Langhorne, Langhorne (1819)]
• "The city [is best governed of all] where those who have not been wronged show themselves just as ready to punish the offender as thouse who have been." [tr. Scott-Kilvert (1960)]
• Paraphrased as "Justice can be secured in Athens if those who are not injured feel as indignant as those who are," in Earl Warren, "The Law and the Future," Fortune (Nov 1955).