If you really read the fairy tales, you will observe that one idea runs from one end of them to the other—the idea that peace and happiness can only exist on some condition. This idea, which is the core of ethics, is the core of the nursery-tales. The whole happiness of fairyland hangs upon a thread, upon one thread. Cinderella may have a dress woven on supernatural looms and blazing with unearthly brilliance; but she must be back when the clock strikes twelve. The king may invite fairies to the christening, but he must invite all the fairies or frightful results will follow. Bluebeard’s wife may open all doors but one. A promise is broken to a cat, and the whole world goes wrong. A promise is broken to a yellow dwarf, and the whole world goes wrong. A girl may be the bride of the God of Love himself if she never tries to see him; she sees him, and he vanishes away. A girl is given a box on condition she does not open it; she opens it, and all the evils of this world rush out at her. A man and woman are put in a garden on condition that they do not eat one fruit: they eat it, and lose their joy in all the fruits of the earth.
This great idea, then, is the backbone of all folk-lore—the idea that all happiness hangs on one thin veto; all positive joy depends on one negative. Now, it is obvious that there are many philosophical and religious ideas akin to or symbolised by this; but it is not with them I wish to deal here. It is surely obvious that all ethics ought to be taught to this fairy tale tune; that, if one does the thing forbidden, one imperils all the things provided. A man who breaks his promise to his wife ought to be reminded that, even if she is a cat, the case of the fairy-cat shows that such conduct may be incautious. A burglar just about to open some one else’s safe should be playfully reminded that he is in the perilous posture of the beautiful Pandora: he is about to lift the forbidden lid and loosen evils unknown. The boy eating some one’s apples in some one’s apple tree should be a reminder that he has come to a mystical moment of his life, when one apple may rob him of all others.
— All Things Considered (1908).