“The only mainstream writer to whom Heinlein acknowledges a debt is Sinclair Lewis, and it is not for literary style. Lewis laid out extensive backgrounds for his work which did not directly appear in the story. That way he understood how his characters should react in a given situation, since he knew more about them than the reader did. In Heinlein, this ultimately grew beyond the bounds intended by Sinclair Lewis, whose characters performed against a setting with which the reader might be familiar. The Sinclair Lewis method couldn’t work for science fiction unless an entire history of the future was projected: then individual stories and characters in that series could at least be consistent within the framework of that imaginary never-never land.

“In following just this procedure, Robert A. Heinlein inadvertently struck upon the formula that had proved for successful for Edgar Rice Burroughs, L. Frank Baum, and, more recently, J. R. R. Tolkien. He created a reasonably consistent dream world and permitted the reader to enter it. Heinlein’s Future History has, of course, a stronger scientific base than Burroughs’s Mars, Baum’s Oz, or Tolkien’s land of the ‘Rings,’ but is fundamentally the same device.”

— Sam Moskowitz, Seekers of Tomorrow: Masters of Modern Science Fiction (New York: Ballantine, 1967). 194.