Be very careful never to show your own bias to anyone who is giving you information, or passing it on to you. Once he sees that you have a particular inclination he will instinctively tend to tell you what he thinks will suit you, and enhance your opinion of him.

B. H. Liddell Hart (1895-1970) English soldier, military historian (Basil Henry Liddell Hart)

This Expanding War, “Intelligence Problems” (1942)