One of the most basic things you’ll learn in philosophy/logic will be something called conditionals. There are three different kinds of conditionals but for the sake of this post I will only address one: the strict conditional.
A strict conditional means it must be true in every possible world, it necessarily must be, or is always the case.
For example: if Glitter Death is a purple unicorn, then she is a unicorn.
There is absolutely no possible world where Glitter Death could be a purple unicorn without there being a unicorn.
Another example: 3 + 3 = 6
Again, there is no possible world where the number three plus the number three will not equal the number six.
What isn’t a strict conditional: If George Washington was president of a country, then he was president of the United States of America.
It is not necessarily the case that George Washington would be the president of the U.S.A. in every possible world. Therefore, this does not qualify as a strict conditional.
When I was first taught about strict conditionals, I was told, “if you aren’t sure, turn it into a movie.” By that my professor meant that pretty much anything goes in a movie and people believe it, because in most cases, there’s a possible world where that could happen.
If you see something in a movie, like the number three plus the number three equals seven, regardless of what kind of movie it is, you would be confused because it just doesn’t make any sense.
I don’t think you’ll come across too many situations that are strict conditionals and you’re threatening to violate, but this leads into the next part.
Be consistent in your world/mythology. A lack of consistency creates a lack of believability.
Many fiction writers create their own worlds, even if it’s just planet Earth where a few things are different than the norm or traditional rules; vampires sparkling in the sun for example.
Actually, let’s continue with that. Stephanie Meyer, the author of the Twilight Series, wrote her own take on vampires and decided that instead of the traditional, “burning to ash in the sun”, they would instead have an aura that enhanced their beauty, and thus we have a mythology for sparkling vampires.
Ms. Meyer established this mythology and used it consistently throughout her writing. Now, if she had decided to have one of the vampires erupt into flames and disintegrate in the sunlight to up the action or because it would be cool, without any plausible explanation, she would have violated her personal mythology of the story. The stories would have been inconsistent in that regard and the believability of Ms. Meyer’s vampires would have tanked.
I can’t stress enough the importance of being consistent with your mythology, or even the rules of your world that are created as a byproduct of taking a certain approach. (You want “Situation A” but as a consequence you would also have “Situation B”.) With a lack of consistency/believability, the reader becomes more and more distant from your story.
If it helps you keep your content in order, you may consider writing down your mythos and referencing it as you write. Pay attention to where you have wiggle room with your creation and watch out for where there isn’t any.
Your readers will appreciate the effort you put into creating a believable, consistent world.