Job must have done something, they decide, even though they know as well as we do that this isn't the case. The narrator was very clear on this point. "This man was blameless and upright," the story says, "he feared God and shunned evil." Those are the rules of this story. But those rules are terrifying. If a "blameless" man can suddenly lose his home, wealth, health and family, then none of us is safe. So Job's friends spend the first half of the play blaming the blameless man.
43 minutes ago