There was a class advertised at work: "How to deal with unreasonable people." Many people showed up. At the start, the instructor asked, "How many of you -- show of hands please -- have had to deal with unreasonable people ?" Everyone raised their hand.
There were a number of case studies of folks in conflict. Of course, the choice of case studies supported the bottom line.
I thought it a good -- no, excellent bottom line.
And that was, if you're dealing with an unreasonable person, look in the mirror.
That was ten years ago, and has served me well.