First guess today. Got lucky as I really only had a vague notion
Karl Popper's "Paradox of Tolerance". If we tolerate intolerance, intolerance will flourish. In other words: if a society is tolerant without limit, its ability to be tolerant is eventually destroyed by the intolerant. Tolerating bigotry allows bigotry to spread. Exclusion isn't always a dirty word. Tell your local Nazi to F off today.