Bigotry, bias and prejudice are labour saving devices. It saves a lot of time if you can make up your mind about someone without bothering to meet them. A lot of effort can be avoided if you know that you dislike some idea without finding out what it actually is.
Labour saving devices are welcome and pleasant additions to our lives when they wash dishes or beat meringues. But in the case of history, bigotry, bias and prejudice are less benign. The labour they save us is essential; they remove the basic caution that enormous quantities of data should instil in us. To simplify out all the things that we do not wish to acknowledge is in a way pleasant, but it is a royal road to distortion.