Life lurches. Genre offers predictability within certain constraints, but then, you have to ask yourself, what’s genre? It’s not subject matter. It’s not tone.

Genre, it had always seemed to me, was a set of assumptions, a loose contract between the creator and the audience.

An American film professor named Linda Williams wrote an excellent study of hard-core pornographic movies, back in the late eighties, called Hard Core, subtitled Power, Pleasure and the “Frenzy of the Visible,” which I read more or less by accident (I was a book reviewer and it landed on my desk to review) as a young man, and which made me rethink everything I thought I knew at the time about what made something genre.

I knew that some things were not like other things, but I didn’t know why.

Professor Williams suggested in her book that pornographic films could best be understood by comparing them to musicals. In a musical you are going to have different kinds of song—solos, duets, trios, full choruses, songs sung by men to women and by women to men, slow songs, fast songs, happy songs, love songs—and in a porn film you have a number of different kinds of sexual scenarios that need to be gone through.

In a musical the plot exists to allow you to get from song to song and to stop all the songs from happening at once. So with a porn film.

And furthermore and most importantly, the songs in a musical are, well, they’re not what you’re there for, as you’re there for the whole thing, story and all, but they are those things that if they were not there you as a member of the audience would feel cheated. If you’ve gone to a musical and there are no songs, you are going to walk out feeling that you did not get your musical money’s worth. You are never going to walk out of The Godfather going, “There weren’t any songs.”

If you take them out—the songs from a musical, the sex acts from a porn film, the gunfights from a Western—then they no longer have the thing that the person came to see. The people who have come to that genre, looking for that thing, will feel cheated, feel they have not received their money’s worth, feel that the thing they have read or experienced has broken, somehow, the rules.

And when I understood that, I understood so much more—it was as if a light had been turned on in my head, because it answered the fundamental question I’d been asking since I was a boy.

I knew there were spy novels and that there were novels with spies in them, cowboy books and books that took place among cowboys in the American West. But before that moment I didn’t understand how to tell the difference and now I did. If the plot is a machine that allows you to get from set piece to set piece, and the set pieces are things without which the reader or the viewer would feel cheated, then, whatever it is, it’s genre. If the plot exists to get you from the lone cowboy riding into town to the first gunfight to the cattle rustling to the showdown, then it’s a Western. If those are simply things that happen on the way, and the plot encompasses them, can do without them, doesn’t actually care if they are in there or not, then it’s a novel set in the old West.