“[For people] to refuse a hearing to an opinion, because they are sure that it is false, is to assume that their certainty is the same thing as absolute certainty. All silencing of discussion is an assumption of infallibility.”

“That would be the death of anyone - to recognize false hopes with a certainty. One mustn't know that. If it is offered, refuse!”

“if any opinion is compelled to silence, that opinion may, for aught we can certainly know, be true. To deny this is to assume our own infallibility.”

“Abandon certainty! That's life's deepest command. That's what life's all about. We're a probe into the unknown, into the uncertain. Why can't you hear Muad'Dib? If certainty is knowing absolutely an absolute future, then that's only death disguised! Such a future becomes now!”

“Scientific knowledge is a body of statements of varying degrees of certainty -- some most unsure, some nearly sure, none absolutely certain.”

“No one can know a false thing, one can only believe it with certainty until it is contradicted. (Page 7)”