Institutio Oratoria

He therefore deceives others, but not himself. When Cicero boasted that he had thrown dust in the eyes of the jury in the case of Cluentius, he was far from being blinded himself. And when a painter by his artistic skill makes us believe that certain objects project from the picture, while others are withdrawn into the background, he knows perfectly well that they are really all in the same plane.

My opponents further assert that every art has some definite goal towards which it directs its efforts, but that rhetoric as a rule has no such goal, while at other times it professes to have an aim, but fails to perform its promise. They lie: I have already shown that rhetoric has a definite purpose and have explained what it is.