- The famous champion of reason. In the stories, and this movie, Holmes says he uses deductive reasoning, but it’s really inductive. I’m not sure why they always get that wrong on TV.
I’m going to do a few posts on logical fallacies! In a nutshell, a logical fallacy is an error in reasoning in which the premises of an argument do not actually support the conclusion. But first, I think it is important to give an overview of how good arguments work. That way, it will be easier to see how a fallacious argument doesn’t work. Let’s get started using a famous example:
Premise 1: All people are mortal.
Premise 2: Socrates is a person.
Conclusion: Socrates is mortal.
This is an example of a deductive argument. This is one of the two branches of reasoning. In a deductive argument, the premises guarantee the conclusion. This means that if the premises are true, the conclusion necessarily must also be true. By ‘necessarily’, I mean ‘can’t not be’. Look at the argument above and you’ll see that if the premises are in fact true, the conclusion has to be true. It’s a really cool thing, when you think about it. Unfortunately, most things in the world can’t be contained in deductive thinking alone; there are too many variables.
Inductive reasoning is the alternative. It’s the type of reasoning we are all most familiar with because we all use it all the time. This is the type of thinking that cops use to pin down suspects. (Or it should be. If half of the stories I hear from ex cops are true, then most police work really is as shaky and stupid as it is portrayed on TV.) There are many types of inductive reasoning that I don’t want to pick apart. They are all the same in that they use arguments to support conclusions, not to logically prove them. Using the cop example, if someone has means, motive, and opportunity to commit a crime and there is tons of evidence at the scene that points to him/her, than he/she is a reasonable candidate for suspicion. With induction, the more evidence you have (or the more supporting arguments) the more confident you can be in the conclusion.
I hope that explanation hit right in the target zone between “too brief” and “too long”. The first zone means that I didn’t thoroughly explain myself and the second means that I went on for so long that it got boring. Let me know in the comments! I’ll release a few logical fallacy posts per week until I feel like I’ve done enough of them.
