The fable and the fairy tale are things utterly distinct. There are many elements of difference; but the plainest is plain enough. There can be no good fable with human beings in it. There can be no good fairy tale without them.
Aesop, or Babrius (or whatever his name was), understood that, for a fable, all the persons must be impersonal. They must be like abstractions in algebra, or like pieces in chess. The lion must always be stronger than the wolf, just as four is always double of two. The fox in a fable must move crooked, as the knight in chess must move crooked. The sheep in a fable must march on, as the pawn in chess must march on. The fable must not allow for the crooked captures of the pawn; it must not allow for what Balzac called “the revolt of a sheep.” The fairy tale, on the other hand, absolutely revolves on the pivot of human personality. If no hero were there to fight the dragons, we should not even know that they were dragons. If no adventurer were cast on the undiscovered island—it would remain undiscovered. If the miller’s third son does not find the enchanted garden where the seven princesses stand white and frozen—why, then, they will remain white and frozen and enchanted. If there is no personal prince to find the Sleeping Beauty she will simply sleep. Fables repose upon quite the opposite idea; that everything is itself, and will in any case speak for itself. The wolf will be always wolfish; the fox will be always foxy.
– Introduction to Aesop’s Fables (1912).