Carnegie: Never Say "You're Wrong"

I made it a rule, to forbear all direct contradiction to the sentiment of others, and all positive assertion of my own, I even forbade myself the use of every word or expression in the language that imported a fix’d opinion, such as ‘certainly,’ ‘undoubtedly,’ etc., and I adopted, instead of them, ‘I conceive,’ ‘I apprehend, ‘ or ‘I imagine’ a thing to be so or so, or ‘it so appears to me at present. » said Benjamin Franklin

« If you can be sure of being right only 55 percent of the time, you can go down to Wall Street and make a million dollars a day. If you can’t be sure of being right even 55 percent of the time, why should you tell other people they are wrong? «  By Dale Carnegie