"It is necessary to create constraints, in order to invent freely. In poetry the constraint can be imposed by meter, foot, rhyme, by what has been called the "verse according to the ear."... In fiction, the surrounding world provides the constraint. This has nothing to do with realism... A completely unreal world can be constructed, in which asses fly and princesses are restored to life by a kiss; but that world, purely possible and unrealistic, must exist according to structures defined at the outset (we have to know whether it is a world where a princess can be restored to life only by the kiss of a prince, or also by that of a witch, and whether the princess's kiss transforms only frogs into princes or also, for example, armadillos)."