timelets

“Fairy tales operate according to several other fundamental principles of magical thinking besides natural magic and animist vitality: animal metamorphosis and changeable bodies on the one hand, and the binding power of promises and curses on the other, govern the logic of the plots—although logic is hardly the mot juste, since magic springs continual surprises that break all the rules of probability. The implied, ever-present possibility of transmogrification means that fairytale protagonists...may be changed, sometimes literally, sometimes figuratively. A stroke of fate will raise them high or lay them low.

Although magic operates according to fundamental principles, its manifestations differ from culture to culture, and era to era, which adds spice and variety to fairy tales.”

...
The stress falls on the binding power of words: the father must keep his promise to the Beast, the beauty will sleep for a hundred years, according to the letter of the spell.
...

“Prophecies—and curses—march on unstoppably. One message of fairy tales is ‘Beware what you wish for.’ Another would be ‘Beware what you promise.’ Yet another would be ‘Beware what you utter.’ You can’t take back what you say. There’s a profound respect in the genre for what words do in the world, as well as in the stories.”

--Warner, Marina;. “Once upon a Time.”

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Promises create a stable structure, while everything else is changeable. This way "what is" and "what ought to be" ultimately are the same.