Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
For it is a dog's eloquence, as Appius says, to undertake the task of abusing one's opponent, [*](A cognitor is one who represents another. The litigant may abuse his opponent, but that does not justify his advocate in doing so. ) and they who do so should steel themselves in advance to the prospect of being targets for like abuse themselves, since those who adopt this style of pleading are frequently attacked themselves, and there can at any rate be no doubt that the litigant pays dearly for the violence
It is not uncommon for the litigant to demand a base and inhuman gratification of his rancour, such as not a single man among the audience will approve, for it is on revenge rather than on protection that his heart is set. But in this, as in a number of other points, it is the duty of the orator to refuse to comply with his clients' desires. For how can a man with the least degree of gentlemanly feeling consent to make a brutal attack merely because another desires it?