Institutio Oratoria

From this it follows that there is no truth in yet another argument which contends that arts know when they have attained their end, whereas rhetoric does not. For every speaker is aware when he is speaking well. These critics also charge rhetoric with doing what no art does, namely making use of vices to serve its ends, since it speaks the thing that is not and excites the passions.

But there is no disgrace in doing either of these things, as long as the motive be good: consequently there is nothing vicious in such action. Even a philosopher is at times permitted to tell a lie, while the orator must needs excite the passions, if that be the only way by which he can lead the judge to do justice.