The Great Man's sincerity is of the kind he cannot speak of, is not conscious of
 Thomas Carlyle, On Heroes, Hero-Worship, and The Heroic in History (1841). copy citation

Context

“I should say sincerity, a deep, great, genuine sincerity, is the first characteristic of all men in any way heroic. Not the sincerity that calls itself sincere; ah no, that is a very poor matter indeed;—a shallow braggart conscious sincerity; oftenest self-conceit mainly. The Great Man's sincerity is of the kind he cannot speak of, is not conscious of: nay, I suppose, he is conscious rather of insincerity; for what man can walk accurately by the law of truth for one day? No, the Great Man does not boast himself sincere, far from that; perhaps does not ask himself if he is so:” source