"Children are capable, of course, of literary belief, when the story-maker's art is good enough to produce it. That state of mind has been called "willing suspension of disbelief." But this does not seem to me a good description of what happens. What really happens is that the story-maker proves a successful "sub-creator." He makes a Secondary World which your mind can enter. Inside it, what he relates is "true": it accords with the laws of that world. You therefore believe it, while you are, as it were, inside. The moment disbelief arises, the spell is broken; the magic, or rather art, has failed. You are then out in the Primary World again, looking at the little abortive Secondary World from outside. If you are obliged, by kindliness or circumstance, to stay, then disbelief must be suspended (or stifled), otherwise listening and looking would become intolerable. But this suspension of disbelief is a substitute for the genuine thing, a subterfuge we use when condescending to games or make-believe, or when trying ... to find what virtue we can in the work of an art that has for us failed."
We've all heard it before - don't mess with the reader. Don't break the rules of the world you're creating, and don't arbitrarily add new rules late in the game in order to try and make sense of what your characters are doing. We are not artists when we do that. We are merely finger-painters slopping colors without regard onto slick paper.
Tolkien knew the difference between belief and suspension of disbelief, and that's why he has such a legion of fans - they all believe.
Here's hoping that, as writers, we can become artful sub-creators and cast that unbreakable spell of belief in our reader's minds.