They have found nothing in their experience that better translates for them their native sense of life's mysteriousness into a form that dispels the darkness and resolves the ambiguity that always lurks beneath the surface of life.
Mud is much like the island he's stuck on — elusive and whimsical but ultimately alone and unfulfilled — and McConaughey fantastically portrays that charismatic mysteriousness without diving into hyperbole.