Truth.

If a truth is man made then it is impossible for it to be absolute. If a truth is universal though, then it is absolute. You see a man made truth is always corrupted with the bias of a side and can work much like opinions. A non-fiction book may be true, but it always will carry a bias of one side over another. Where as a fiction book is obviously fake but they generally contain a lesson of truth. I think Neil Gaiman said it best, "Fairy tales are more than true, not because they tell us dragons exist. But because they tell us dragons can be beaten."