From Carnegie’s “How to Win Friends and Influence People.
Show respect for the other person’s opinions. Never say, “You’re wrong.”
“Never begin by announcing “I am going to prove so-and-so to you.” That’s bad. That’s tantamount to saying: “I’m smarter than you are. I’m going to tell you a thing or two and make you change your mind.” (p. 152)
“You can tell people they are wrong by a look or an intonation or a gesture just as eloquently as you can in words—and if you tell them they are wrong, do you make them want to agree with you? Never! For you have struck a direct blow at their intelligence, judgment, pride and self-respect. That will make them want to strike back. But it will never make them want to change their minds. You may then hurl at them all the logic of a Plato or an Immanuel Kant, but you will not alter their opinions, for you have hurt their feelings.” (p. 151)
“If a person makes a statement that you think is wrong—yes, even that you know is wrong—isn’t it better to begin by saying: “Well, now, look. I thought otherwise, but I may be wrong. I frequently am. And if I am wrong, I want to be put right. Let’s examine the facts.” (p. 152)