Have you heard the saying that truth is in the eye of the beholder?

What does this say about truth and perception? For example: If you believe that there is a troll outside your door, and you do not check to see if there really is, but you still strongly believe that there is, does the troll exist at all? Even if its in your own mind only, does the very act of having the thought or belief give it some amount of truth value, no matter how small or unlikely the chance may be?