Suppose you are a young man whose best friend has a sister whom you have heard about but never met.  Your friend describes her jet-black hair, and lovely eyes, her outgoing vivacious personality and her fun-loving disposition.  The more you hear about her, the more interested you become.  Week after week, he tells you about her, and you find yourself wanting to meet her.  So you say, "How about introducing me to her?"  And he replies, "I don't think you believe she really exists."  You reply, "That's dumb; of course, I believe she exists.  Remember, you showed me her picture!"   Should that happen, you would begin to wonder about your friend's sanity.