"If all men had full memory of the past, awareness of the present, and foresight into the future, speech would not be as effective as it is. But since in fact men have little ability to remember the past, observe the present, or foretell the future, speech works easily; with the result that most speakers on most subjects offer only opinion as counsellor to the soul. But opinion is delusive and inconstant, and those who rely on it run grave risks."