Historia et scribitur ad narrandum non ad probandum. - Quintilian

We should not speak so that it is possible for the audience to understand us, but so that it is impossible for them to misunderstand us.

Laudem virtutis necessitati damus.

In either case the orator should bear clearly in mind throughout his whole speech what the fiction is to which he has committed himself, since we are apt to forget our falsehoods, and there is no doubt about the truth of the proverb that a liar should have a good memory.