But I must at the very beginning lay down this principle — friendship can only exist between good men.

[Sed hoc primum sentio, nisi in bonis amicitiam esse non posse]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
“Laelius De Amicitia [Laelius on Friendship],” ch. 5 / sec. 18 (44 BC) [tr. Shuckburgh (1909)]
    (Source)

Original Latin. Alternate translations:

• "But first of all, I am of opinion, that except among the virtuous, friendship cannot exist." [tr. Edmonds (1871)]
• "But I consider this as a first principle, -- that friendship can exist only between good men." [tr. Peabody (1887)]
• "This, however, I do feel first of all -- that friendship cannot exist except among good men." [tr. Falconer (1923)]
• "But first of all, I think this: except among good people, friendship cannot exist." [Source]