The Secret Behind Think and Grow Rich

I've been studying Think and Grow Rich by Napoleon Hill lately. In it, Hill repeated makes reference to a principle that he purposefully has not named directly because he feels that readers would benefit more by its indirect discovery. Perhaps this video gives a clue as to the principle's identity. Can you figure out what it is?

It's interesting to hear directly, one eye witness removed, from Andrew Carnegie, one of the richest men who ever lived. His ideas about success are difficult to discount.