There are three things which one must have in order to be credible; for this much is the number of those by which we believe in, save demonstrations. And these are wisdom, virtue and favour. For people are deceived in what they say or advise either by all these or by one of these; for either through lack of wisdom their judgement is at fault, or they do make a good judgement but through viciousness they do not say what they think, or they are wise and virtuous, but not friendly. This is the reason why one may not give the best advice, although he knows what it is. And these are the only possible ways. It follows, then, that he who is thought to possess these three qualities is necessarily credible to his audience.
Bibliography: Aristotle, Art of Rhetoric (1378a.6 to 1378a.15)
Translation: George Kotsalis