Conversation, then, in which the Socratics are the best models, should have these qualities. It should be easy and not in the least dogmatic; it should have the spice of wit. And the one who engages in conversation should not debar others from participating in it, as if he were entering upon a private monopoly; but, as in other things, so in a general conversation he should think it not unfair for each to have his turn. He should observe, first and foremost, what the subject of conversation is. If it is grave, he should treat it with seriousness; if humorous, with wit. And above all, he should be on the watch that his conversation shall not betray some defect in his character. This is most likely to occur, when people in jest or in earnest take delight in making malicious and slanderous statements about the absent, on purpose to injure their reputations.
Cicero, De Officiis, XXXVII (134)