Neil Gaiman calls Terry Pratchett a "science fiction writer." Perhaps that's the real reason I've always liked him.
Terry was a science fiction writer. It was the way his mind worked: the urge to take it all apart and put it back together in different ways, to see how it all fit together. It was the engine that drove Discworld, not a “what if” or an “if only” or even an “if this goes on…” It was the far more subtle and dangerous “if there was a really a —, what would that mean? How would it work?”