You’ve probably heard people say things like this.
“It didn’t really make sense when the character did that…but it is a story about dragons, after all.”
Or, “I thought there were some leaps in logic, but what can you expect from a story about talking mice?”
This is wrong. When people say things like that directly to me, I argue, and I’ll tell you why.
A fantasy element and a logical flaw are two completely different things. A story set in a world with dragons is a story in which dragons are logical. A character who acts out of character is out of character in any world, whether they live in Brooklyn, or in a cave with dragons.
Men can’t fly. But if Superman flies past a cry for help without stopping, the problem here is not that he’s flying.
If anything, I think fantasy and science fiction books have to be even more firm in their logic, in their reasoning, in their premise and their characters, because they are already trying to make the reader accept something which is impossible. The reader is invited into an impossible world, and everything that goes on inside of it needs to be internally consistent and internally reasonable, or the story won’t be believable–and not because there are dragons.
A writing teacher discussed this once. She explained certain options in what you have happen in your story:
There are impossible probabilities: men can’t fly, but if one could, we find it probable that he would use that ability to help people (or to take over the world, depending on what character we’re looking at). It’s impossible, but once you accept the impossible, the rest is probable.
There are improbable possibilities: it’s possible for someone who has previously shown an inclination to help people to ignore someone in need–nothing in the laws of physics prevents it–but we’d find it improbable.
You can also have possible probabilities, those are well-written stories set in the real world, and improbable impossibilities, which are fantasy stories that don’t make sense.
The point is that a good fantasy has to make sense too. Having a fantastical element in a story doesn’t excuse that story from being any less well-reasoned than the most down to earth story there is.
I think the reason this “it’s not logical but it’s a fantasy” reasoning frustrates me is that it implies that fantasy are not as well put-together as other stories. That they’re somehow excused from having good characters and a logical plot–which is just a hop and a leap from saying that fantasies tend not to have those things.
There are plenty of fantasies out there that have solid characters and believable plots. They’re just impossible probabilities.
Tales of the Marvelous 
I generally don’t have a problem believing anything within a story, and can deal with a serious amount of technobabble. But sometimes they cross that line where even I have to cry foul.
I went to see the new X-Men movie with my sisters and cousins. It was good, but I raised a concern with them. There is a girl in the movie that has little dragonfly wings and… she can fly with them. I couldn’t deal with it. No way this girl can fly with little dragonfly wings; they wouldn’t support her. They didn’t have a problem and reminded me that there was a guy using his voice to to glide. I could accept the guy gliding, because it made sense. He screams at the ground, the compression waves bounce back, get caught in his glider suit, and lift him up. I don’t even know if that’s possible, but it seems possible. Girl flying with dainty wings… no. Odd thing about it… I wouldn’t have a problem if she was flying without wings. I’d accept that.
That’s a really good point too–are the fantastical elements plausible? Is the science or magic convincing? I was mostly thinking about unrelated logic elements that people somehow want to give a pass just because there’s also magic, but whether the fantasy/sci-fi is logical is a good question too. I can take most things and most amounts of technobabble, but there has to be some amount of logic to it. Some of it depends on the style of the story as well–I think X-Men tries to be fairly serious about its sci fi, which gives them a lot to live up to in their logic. L. Frank Baum comes to mind as a fantasy writer who does have some unlikely elements (lunch boxes growing on trees, or a woman who exchanges heads) but he writes a much more whimsical story, so I’ll accept it in his world.
You are so right in your argument. Internal consistency in the story and characters is key for believability in any writing genre. Fantasy stories would follow that rule like any others.
Exactly–internal consistency, and consistency with broader rules like logic and human nature.