Institutio Oratoria

A similar impression of grace and charm is produced by rhetorical figures, whether they be figures of thought or figures of speech. For they involve a certain departure from the straight line and have the merit of variation from the ordinary usage.

In a picture the full face is most attractive. But Apelles painted Antigonus in profile, to conceal the blemish caused by the loss of one eye. So, too, in speaking, there are certain things which have to be concealed, either because they ought not to be disclosed or because they cannot be expressed as they deserve.