Those who so act and so live as to give proof of loyalty and uprightness, of fairness and generosity; who are free from all passion, caprice, and insolence, and have great strength of character — men like those just mentioned — such men let us consider good, as they were accounted good in life, and also entitled to be called by that term because, in as far as that is possible for man, they follow Nature, who is the best guide to good living.

[Qui ita se gerunt, ita vivunt, ut eorum probetur fides integritas aequitas1 liberalitas, nec sit in eis ulla cupiditas libido audacia, sintque magna constantia, ut ei fuerunt, modo quos nominavi, hos viros bonos, ut habiti sunt, sic etiam appellandos putemus, quia sequantur, quantum homines possunt, naturam optimam bene vivendi ducem.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
“Laelius De Amicitia [Laelius on Friendship],” ch. 5, part 3 / sec. 19 (44 BC) [tr. Falconer (1923)]
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Original Latin. Alternate translations:

Those who so conduct themselves, and so live that their honor, their integrity, their justice, and liberality are approved; so that there is not in them any covetousness, or licentiousness, or boldness; and that they are of great consistency, as those men whom I have mentioned above; -- let us consider these worthy of the appellation of good men, as they have been accounted such because they follow (as far as men are able) nature, which is the best guide of a good life.
[tr. Edmonds (1871)]

Those who so conduct themselves, so live, that their good faith, integrity, equity, and kindness win approval, who are entirely free from avarice, lust, and the infirmities of a hasty temper, and in whom there is perfect consistency of character; in fine, men like those whom I have named, while they are regarded as good, ought to be so called, because to the utmost of human capacity they follow Nature, who is the best guide in living well.
[tr. Peabody (1887)]

We mean then by the "good" those whose actions and lives leave no question as to their honour, purity, equity, and liberality; who are free from greed, lust, and violence; and who have the courage of their convictions. The men I have just named may serve as examples. Such men as these being generally accounted “good,” let us agree to call them so, on the ground that to the best of human ability they follow nature as the most perfect guide to a good life.
[tr. Shuckburgh (1909)]

Those who comport themselves in such a way, who live in such a way that their loyalty, integrity, fairness and generosity are proven, such that there is no desire, lust, and insolence in them, and such that they have great steadfastness of character (like those whom I named just before), we consider ought indeed to be called good men (as is customary), because they follow (as much as humans can) nature -- the best leader in proper living.
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