Ya gotta love Mr. Clemens (aka Twain) and his wonderfully dry sense of humour, because this particular aphorism is bang on for writers. Unlike the strange tale of our daily lives, where things too often seem to be without any sense whatsoever (although as Will says, things are not always what they seem), when we read fiction, we expect --- actually, demand might be a better word --- that the events we see chronicled will, by and large, make sense.
I’m very grateful to CinemaSins of YouTube fame (“no movie is without sin”) for graphically demonstrating this to me. I quite enjoy watching their critiques of various films, ferreting out the things that don’t make sense, and counting those sins as they go (ding!). Sometimes, sure, they get just a tad too nit-picky, but by and large, I find that the things they deal with are spot on. It’s actually made me a better writer, because when I write nowadays, I try to make sure that the things going on in my story make sense and that there are no gaping logic holes that the mellifluous voice of Jeremy Scott, the fellow who usually narrates the CinemaSins videos, could exploit. (My nerd confession for today is that I can at times “hear” his commentary in my mind as I write a scene --- complete with the occasional bleeped out obscenities he resorts to when particularly disgusted. Hey, we’re all equal and entitled to our own craziness.)
A long time ago (but in this galaxy, thanks very much) science fiction writer Harlan Ellison coined a term relating to this issue which I like a great deal: interior logic. Any story’s got to have it --- unless you’re into the theatre of the absurd or want to write companion pieces to Alice in Wonderland, that is… and a good part of Alice’s charm lies in its absolutely nonsensical nature.
What is interior logic? I look at it like this: it’s making sure that the world you create in your writing does not look like a painting by Picasso or Dali. Fine artists, both of them, but not big on logic and reality in their paintings. Or in terms of writing: you want a dragon in your world? Great. But that dragonfire it’s fond of spewing has to use some sort of fuel or something, so it can’t just belch forth flame and holocaust endlessly. It’s gotta stock up on kerosene or something from time to time. Things must make sense… or you’re going to be featured on the next CinemaSins. In other words, deus ex machinas need not apply… except maybe once in a blue moon.
Does this mean that the things characters do in a story must always make sense? Of course not. We wacky humans occasionally/sometimes/often/always --- take your pick, using your own life as a metric --- do things that make no sense whatsoever. (Parents are usually really good at pointing these things out to their teenagers, for example, because something appears to happen when children hit puberty, something that seems to completely disconnect a child’s prefrontal lobe from the rest of its brain so that, by and large, teenagers act irrationally most of the time.)
So have a critical look at what you’re writing from time to time. Are there holes in the plot big enough to drive a truck through? Better do something about them… or you may hear the dreaded faint and far off ding! of a literary sin counter.