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.
A commonly given example of inductive reasoning is saying that the sun will rise tomorrow, with some underlying assumption that it is said based on the inductive reasoning that it always has risen, therefore it will tomorrow. But if a person knows the appearance of sunrise is based on the earth spinning on its axis, and all data shows there is nothing going to destroy earth or sun overnight, then it is closer to deductive reasoning to say it will rise again tomorrow, no?
True, but I didn’t want to get into what Hume calls ‘the problem of induction’. Basically, the assumption that inductive reasoning is reliable when it comes to things like the sun rising are themselves products of inductive reasoning.
There are a few other types of induction that people use too. What makes induction reliable is if you get enough evidence all pointing in the same direction.
Thanks for the comment! Keep them coming!
Oh wait, just read the end of your comment correctly.
The point of deductive reasoning is that is it logically impossible for the conclusion to not be true if the premises are true. You can have “certainty” in the colloquial sense with strong inductive reasoning, but not absolute “philosophical certainty”. This is what people mean when they say science can’t “prove” stuff. You can “prove” things well enough to fly a plane or a space ship, but you don’t have the iron-clad certainty of deductive reasoning.
For all intents and purposes in real life though, strong evidence based certainty and deductive certainty can be treated the same. (Now some angry logician is gonna throw rocks at me)
Then I will present my New Law of Inductive Logic Philosophy (original with me unless I read it somewhere before and don’t remember, or someone else already said it and I never heard it): One can formally Deduce that one can Never Deduce any future event. So:
“All people are mortal.
Steven Zuber is a person.
Conclusion: Steven Zuber will die.”
is not deductive logic, but inductive. In fact, you are hoping the logic IS flawed, eh?
The logic is solid, but I think the premises aren’t sound. I should have made that distiction in the post was well. A sound argument is one in which the premises are true, thus proving that the conclusion is true. A perfect deduction can be wrong, and thus not sound. For example:
All men are immortal
Socrates is a man
Therefore Socrates is immortal
I guess I had to “OK” the reblog. Sorry that took so long. Thanks!!
Pingback: Deductrive vs. Inductive Reasoning from Think That Through | Dan McCormick
Stevenator, please don’t forget that cops use essentially abbreviated methods to get at suspects because stupid political issues accellerate their timeline. Cops generally do good work. The proof of that theorem is this: we are not in an anarchic society.
What are similarities between the two reasoning?
Thanks for the question!
The similarities are that both attempt to draw new knowledge from given knowledge. Deduction does this by proving (or attempting to prove) conclusions, while induction uses probabilities.
For most practical purposes in daily life, all of our reasoning is inductive. Unless your a mathematician, all us muggles can do with deduction is come up with meaningless tautologies, or force ourself to see the relationship between two things in a more solid way.