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)