Story logic comes from two sources: external and internal. Both are equally important. Let’s first define what I mean by each.
External logic deals with how pieces of information, such as historical and scientific facts that relate to the external world setting of your story integrate with the story’s plot.
Internal logic deals with how various pieces of information fit together so that they make sense within the story.
Now that I’ve probably totally confused you, let me give some examples to clear up what I mean. Let’s say you’re writing a modern-day detective story and your main detective does everything by the book. I’ll call him BTB-D.
The story’s EXTERNAL logic dictates that what BTB-D does should follow the processes used by real detectives, which means the author will have to do some research to get the facts straight.
The story’s INTERNAL logic dictates that BTB-D not only follow regular logical practices but also act in a manner consistent with his/her personality.
If you’ve ever watched the TV police series Blue Bloods, you’ll have a good idea of what I’m talking about. For those who haven’t seen the series, it involves a family of police officers, with the daughter being an assistant district attorney in New York City. The father (played by Tom Selleck) is the current police commissioner, and his father is the retired PC. The oldest son is the sometimes hothead (and often not by the book), while the youngest son is more by the book. You can imagine the conflict that can arise, especially with the daughter being an assistant DA and being generally forced to go by the book.
Because this series is set in a real-world situation, external logic plays a huge part in that the story elements need to reflect real-world facts. The internal logic means that the characters must be consistent to their natures and personalities. However, while the logic of their personalities must be consistent, it cannot violate the external logic of the setting. In NYC the mayor is elected, but the police commissioner is appointed by the mayor, not elected. That’s how it works in NYC in real life, and that represents both the external and internal logic of the story.
If your story is set in a different town or city where the head of the police department is elected, then your story’s logic must follow that situation. And here it’s important to get your facts and external logic straight. In America, police chiefs are usually appointed, while sheriffs are usually elected officials. While there could be exceptions in your setting location if it’s fictitious, if you’re using a real city as your foundation, then your story’s logic demands that you obey the real-world logic. This holds whether your story is set in the present day or in the historical past.
Here’s an example of a logic issue. I won’t go into any details of the plot, but an author I know had a particular birth-control pill being sold over the counter and on the shelves in various stores, including small grocery stores. The logic problem is that birth-control pills require a prescription to purchase them and would have to be purchased at the pharmacy counter. Some of these stores in the author’s story were mom-and-pop type stores that did not have a pharmacy counter, the pills were on the regular shelves. Astute women readers in particular would likely spot this error. This demonstrates the importance of being sure you have your facts straight to avoid such logic errors.
Now, what if you’re setting is an alternate-history one or a futuristic one (or set on another world)? In that case, you can make up your own rules, but two caveats go with that:
(1) You must be consistent in following your rules.
(2) Your rules must make sense in your world and follow some logic in the real world.
Yes, I hear your objections to point (2). “But, Rick, if they’re my rules in my fictional world, why do they need to make sense in the real world?”
When it comes to scientific principles, there are places where you can take liberties and others where you can’t. Star Trek stretched a lot of scientific principles, but the writers were still careful to stay within the bounds of believable and consistent extensions of the science (for the most part).
Where I see writers get into trouble is making up things without knowing the science. I recall a while back seeing one romance author refer to a chemical compound named something like magnesium sodium. Any chemist knows that this name makes no sense, and when I read it, I immediately knew that the author just pulled two chemical elements and put them together in an attempt to be creative and to sound intelligent. The result was the opposite because it sounded stupid.
Let’s look at an example of good internal story logic. Another author I know writes about supernatural and shape-shifting characters and throws angels and demons into the mix. This author has created quite a complex world of the characters. On top of that, the author throws several anomalies into the mix to stir things up. The fact that this is both urban fantasy AND set in our modern world presents challenges for any writer, but so far, I have not run into any logic errors. The author has created a credible setting that is both internally and externally consistent.
Therefore, as you review and revise your story, be sure that ALL its elements have internal and external logical consistency that fit the setting. The more attention you pay to your story’s logic, the better the chances that you won’t alienate and lose knowledgeable readers.