His world once invented, the highest law that comes next into play is, that there shall be harmony between the laws by which the new world has begun to exist; and in the process of his creation, the inventor must hold by those laws. The moment he forgets one of them, he makes the story, by its own postulates, incredible. To be able to live a moment in an imagined world, we must see the laws of its existence obeyed...Link to The Fantastic Imagination:
George MacDonald in this article is talking about the creation of Imaginary Worlds, though in reference to the creation of Fairytales, of which he wrote some wonderful examples. I highly recommend reading the article as it provides some very good advice for Gamesmasters who might want to create Worlds that are works of Art. I also even more highly recommend reading George MacDonald's works, as he was a fantastic writer!