In her review of Susanna Clarke’s new collection, Ursula Le Guin writes:
Those who dislike fantasy dismiss it as inconsequential: Inconsequentiality is in fact fatal to it. Explanation is irrelevant, but coherence, inner consistency, is essential to the flight of imagination. Gabriel García Márquez does not explain where the man with enormous wings flew from; Jorge Luis Borges does not explain the mysterious appearance of the Aleph; J.R.R.Tolkien does not explain the existence of Middle Earth: They simply tell, and the narrative is both explanation and justification, because it is strictly consequential. The imaginary act has inevitable consequences; the fantastic threat is carried out; the uncanny cause has its ineluctable effect. A fantasy lacking consequence, like a spell spoken wrong, is mere nonsense.
Which is lovely and has the added benefit of being true. (Le Guin, btw, is not convinced that Clarke meets this challenge and the review is positive overall but with reservations.)