Diligence beyond a mere good faith effort to fulfill an obligation. See also good faith and diligence. n.The best evidence rule rule that, to prove the contents of a writing, recording, or photograph, the original is required unless it is not available for some reason other than the serious fault of the party trying to prove the contents thereof. If the original is unavailable, the testimony of the person who created the original or the person who read it (if a writing), listened to it (if a ...