Logically, of course, examples cannot stand as proof, but psychologically they work with argument to gain acceptance. If you wanted to argue that all truth is equally valid but not equally valuable, you might use an analogy to get your audience to accept what you are saying. A penny and a dollar bill are both genuine, you may point out, but they are not of equal worth. Therefore we must distinguish between penny- and dollar-truth.

source: Haddon Robinson, Biblical Preaching, p 152 tags: Truth, Preaching