Suppose that you have heard, secondhand, or even firsthand, of an instance in which a man was abused or tortured by the police in a hypothetical American community. You tell a friend whom you are trying to persuade that the police are rotten. He doesn’t believe you. He wants firsthand or, if you got it secondhand, at least secondhand testimony. You go to your original source, who has told you the story only because of his absolute trust in you. You want him now to tell a man he doesn’t trust, a friend of the police. He refuses. And he warns you that if you use his name as authority for the
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