Institutio Oratoria

So too the doctor seeks to heal the sick; but if the violence of the disease or the refusal of the patient to obey his regimen or any other circumstance prevent his achieving his purpose, he will not have fallen short of the ideals of his art, provided he has done everything according to reason. So too the orator's purpose is fulfilled if he has spoken well. For the art of rhetoric, as I shall show later, is realised in action, not in the result obtained.

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.