Truth

Truth is relative to your own perception.
i.e. two blind men standing on either side of an elephant trying to describe it to the other.--

To detect the truth is difficult do to each persons perception. But to detect a liar is just about impossible until it's too late... If there is a pattern of lying then forever their honesty will be in question. If you have a pattern of honesty then your deception will come to fruition. But, it will only come to fruition at the cost of your reputation, and the trust you've earned may be forever lost.