"If a man is not to believe in himself, in what is he to believe?"

From the chapter titled "The Maniac" in G.K. Chesterton's Orthodoxy:

.... Once I remember walking with a prosperous publisher, who made a remark which I had often heard before; it is, indeed, almost a motto of the modern world. Yet I had heard it once too often, and I saw suddenly that there was nothing in it. The publisher said of somebody, "That man will get on; he believes in himself." .... I said to him, "Shall I tell you where the men are who believe most in themselves? For I can tell you. I know of men who believe in themselves more colossally than Napoleon or Caesar. I know where flames the fixed star of certainty and success. I can guide you to the thrones of the Supermen. The men who really believe in themselves are all in lunatic asylums." He said mildly that there were a good many men after all who believed in themselves and who were not in lunatic asylums. "Yes, there are," I retorted, "and you of all men ought to know them. That drunken poet from whom you would not take a dreary tragedy, he believed in himself. That elderly minister with an epic from whom you were hiding in a back room, he believed in himself. If you consulted your business experience instead of your ugly individualistic philosophy, you would know that believing in himself is one of the commonest signs of a rotter. Actors who can't act believe in themselves; and debtors who won't pay. It would be much truer to say that a man will certainly fail, because he believes in himself. Complete self-confidence is not merely a sin; complete self-confidence is a weakness. Believing utterly in one's self is a hysterical and superstitious belief.... And to all this my friend the publisher made this very deep and effective reply, "Well, if a man is not to believe in himself, in what is he to believe?" After a long pause I replied, "I will go home and write a book in answer to that question." This is the book that I have written in answer to it.

G.K. Chesterton's Orthodoxy