Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
Still it is in the nature of things conceivable that just causes may lead two wise men to take different sides, since it is held that wise men may fight among themselves, provided that they do so at the bidding of reason. I will therefore reply to their criticisms in such a way that it will be clear that these arguments have no force even against those who concede the name of orator to persons of bad character. For rhetoric is not self-contradictory.
The conflict is
nor would the pilot's art exist, because in sea-fights pilots may be found on different sides; nor yet could there be an art of generalship, since general is pitted against general. In the same way rhetoric does not undo its own work. For the orator does not refute his own arguments, nor does rhetoric even do so, because those who regard persuasion as its end, or the two good men whom chance has matched against one another seek merely for probabilities: and the fact that one thing is more credible than another, does not involve contradiction between the two.