Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
Now one of these points, namely whether the judges have knowledge of what is being said to them, has nothing to do with the art of oratory. The other statement, that art is concerned with things that are known, does however require an answer. Rhetoric is the art of speaking well and the orator knows how to speak well.
But,it is urged,
he does not know whether what he says is true.Neither do they, who assert that all things derive their origin from fire or water or the four elements or indivisible atoms; nor they who calculate the distances of the stars or the size of the earth and sun. And yet all these call the subject which they teach an art. But if reason makes them seem not merely to hold opinions but, thanks to the cogency of the proofs adduced, to have actual knowledge, reason will do the same service to the orator.
But,they say,
he does not know whether the cause which he has undertaken is true.But not even a doctor can tell whether a patient who claims to be suffering from a headache, really is so suffering: but he will treat him on the assumption that his statement is true, and medicine will still be an art. Again what of the fact that rhetoric does not always aim at telling the truth, but always at stating what is probable? The answer is that the orator knows that what he states is no more than probable.