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curses cause disease, then you’re writing fantasy.
    You must inform your reader as quickly as possible after the beginning of your story whether it’s going to be fantasy or science fiction. If it’s science fiction, and you signal this to the reader, then you have saved yourself enormous amounts of effort, because your reader will assume that all the known laws of nature apply, except where the story indicates an exception.
    With fantasy, however, anything is possible. And where anything can happen, who cares what actually occurs? I mean, if your hero can get into

    Trouble and then wish his way out, so what? Why worry about him? Why

    care?

    The truth is that good fantasies carefully limit the magic that’s possible. In fact, the magic has to be defined, at least in the author’s mind, as a whole new set of natural laws that cannot be violated during the course of the story. That is, if at the beginning of the story you have established that your hero can make only three wishes, you better not have him come up with a fourth wish to save his neck right at the end. That’s cheating, and your reader will be quite correct to throw your book across the room and carefully avoid anything you ever write in the future.
    All speculative fiction stories have to create a strange world and introduce the reader to it-but good fantasy must also establish a whole new set of natural laws, explain them right up front, and then faithfully abide by them throughout.
    Having said all this, I must now point out that there are numerous exceptions. For instance, by this definition time travel stories in which the hero meets himself and stories that show spaceships traveling faster than light should all be classed as fantasy, because they violate known laws of nature-and yet both are definitely classed as science fiction, not fantasy.
    Why? One explanation is that people were writing these stories as science fiction before the relevant laws of science were widely known, and so these tales remain science fiction under a sort of grandfather clause. Another explanation is that there was no commercial publishing category of fantasy until the 1960s, so a lot of fantasy came to live quite comfortably within the tent of science fiction and, when the fantasy publishing category came into existence, nobody bothered to move them from one category to the other. They were already conventional.
    But to all these explanations I say “bunk.” Time travel and faster-than light (FTL) starships respect the real boundary between fantasy and science fiction: They have metal and plastic; they use heavy machinery, and so they’re science fiction. If you have people do some magic, impossible thing by stroking a talisman or praying to a tree, it’s fantasy; if they do the same thing by pressing a button or climbing inside a machine, it’s science fiction.
    So in a sense even science fiction stories have to define the “rules of magic” as they apply in the world of the tale, just as fantasies do. If FTL travel is possible in your science fiction universe, you have to establish that fact early on. If you want time travel, you must either make the story
    be about time travel or establish immediately that time travel is commonplace in the world of the story.
    Still, the difference remains: If a story is perceived as fantasy, the reader must be told as soon as possible the “natural laws” that apply in this fantasy world, whereas if the story is perceived as science fiction, the reader will assume that the natural laws of this universe apply until he is told otherwise.
    Note that this applies only to the beginning of the story. Your “fantasy” might end up with all seeming magic explained away as perfectly natural phenomena; your “science fiction story” might end up being a tale of witchcraft or vampirism in space. Indeed, this is exactly what Sheri Tepper did in her nine-volume True Game series. The story deals with people who spend their lives acting