Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
Similarly Cornelius Celsus defines the end of rhetoric as
v1-3 p.311
to speak persuasively on any doubtful subject within the field of politics. Similar definitions are given by others, such for instance as the following:— rhetoric is the power of judging and holding forth on such political subjects as come before it with a certain persuasiveness, a certain action of the body and delivery of the words.
There are countless other definitions, either identical with this or composed of the same elements, which I shall deal with when I come to the questions concerned with the subject matter of rhetoric. Some regard it as neither a power, a science or an art; Critolaus calls it the practice of speaking (for this is the meaning of τριβή ), Athenaeus styles it the art of deceiving,