Socrates: The fact is, as we said at the beginning of our discussion, that the aspiring speaker needs no knowledge of the truth about what is right or good... In courts of justice no attention is paid whatever to the truth about such topics; all that matters is plausibility... There are even some occasions when both prosecution and defence should positively suppress the facts in favor of probability, if the facts are improbable. Never mind the truth -- pursue probability through thick and thin in every kind of speech; the whole secret of the art of speaking lies in consistent adherence to this principle.
--Plato, Phaedrus 272
Note: There are a few links below to Perseus. To see the figures in question, you'll often need to examine the Greek versions of these texts.