There is a story told of C. S. Lewis, as a small boy — about six or seven, I think. One day he announced to his father,
“Daddy, I have a prejudice against the French.”
“Why?” asked his father, not unreasonably.”
“If I knew that,” replied the precocious youngster triumphantly, “it wouldn’t be a prejudice.”
He was quite right, of course. The point about a prejudice is that it’s what you have when you are “pre-judging” a case: making your mind up before you know the facts.
Now of course there are many halfway stages between naked prejudice and completely well-informed opinion. Frequently we back up our prejudices by finding out just enough facts that support our case, and conveniently ignoring the rest. Bad historians, clever politicians, and lazy theologians do that all the time.

Acts for Everyone, p.161-162