# What Is a Deductive Method?

Daniel Liden
Daniel Liden

The deductive method is an approach to reasoning that is based on deduction, or starting from a general case and, from that general case, drawing a conclusion about something more specific. An argument based on this method may be formulated as such: "All men lie. Dave is a man, therefore Dave lies." Of course, the rightness or wrongness of the specific conclusion is entirely reliant on the correctness of the general claim; if the general claim is wrong, specific conclusions deduced from it are also wrong, or at the very least are incorrectly deduced. In contrast to deductive reasoning, inductive reasoning involves starting from specific cases and, from them, drawing a general conclusion.

Reasoning used in the deductive method can be presented, formally or informally, in a variety of different ways. One of the most common forms of the deductive method is the syllogism, in which two conditional statements are given and from them a conclusion is drawn. For example, a syllogism can take the following form: "If Dave is late for work again, his boss will be angry. If Dave's boss is angry, Dave will not get a raise. Therefore, if Dave is late for work again, Dave will not get a raise." The conclusion that Dave will not get a raise if he is late for work is drawn from the two preceding conditional statements.

Much crime-solving fiction, most notably the Sherlock Holmes stories, is based in the deductive method. In such contexts, this method is a process for solving crimes based on the application of deductive reasoning to criminal cases. A detective may apply some general knowledge about criminal psychology or crime scene investigation to the specifics of the case at hand in order to draw conclusions about the identity and methods of the criminal. The deductive method is actually used in many real-life crime solving situations as well, as many methods of investigation are based in the application of general knowledge applied to specific cases.

Reasoning in day-to-day life, both in professional and personal contexts, is often based on the deductive method. Medical examination, investigation, and treatment, for instance, are all based on the application of general medical knowledge to specific individual patients. Much research in fields as diverse as economics, physics, and biology make use of both deductive and inductive reasoning. Researchers make hypotheses and explain some results based on general rules but often need to make new general theories to explain specific results that do not fit into the existing theoretical framework.

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jessiwan

@Shell: I am the same way too and I think you are absolutely correct. I think it really comes down to the fact that some people are more right brain-oriented and others more left brain.

About the deductive method: I know that if both premises are true, then the conclusion must necessarily be true. However, what if one of the premises is subjective in nature? For example, consider:

"Abortion is wrong.

Jennifer got an abortion.

Therefore, Jennifer did something wrong."

I said something very similar to this in an online discussion with someone, and he told me that using a premise that was subjective in nature made the whole argument "weak". But I thought that my logical form was totally valid. The conclusion might or might not have been sound depending on whether one thought abortion was wrong or not, but I could not find anything wrong with my syllogism.

shell4life

The deductive method was always something I could easily understand in school. I was always pretty good at writing and reading, so I think that gave me a good jump on learning to use deductive reasoning.

However, when the deductive method had to be used to solve certain math problems, I failed at it miserably. I just could not get from point A to point C if numbers were involved, no matter how much logic had to be used.

It amazes me that if the problem had nothing to do with numbers, I could solve it with no issues, but the addition of math just confused my brain to no end. I guess there really is a division between the people who are good at English and the ones who are good at math.

JimmyT

@matthewc23 - Good points. Along with your example, I would also add that deductive reasoning often usually relies on Occam's razor, as well. That can also be referred to as the KIS or the Keep it Simple method. If I wake up in the morning with a headache, my first instinct should not be to think that I have a brain tumor. I could base that on the fact that I haven't had chronic headaches. I may have also bumped my head on something the night before. In other words, there are much simpler and more reasonable explanations for my headache than a brain tumor.

That being said, Occam's razor and the parsimony method usually doesn't work very well in books or movies. Very rarely is the real criminal the person who seems to be the most likely. With that in mind, though, the detectives can use deductive reasoning to put together various pieces of evidence and find the real criminal. I guess real crime-solving is like that at times, but not like it is on TV.

matthewc23

I think whether people realize it or not, we all use some sort of deductive reasoning every day to some extent. If you wake up in the morning and don't feel good, you immediately try to figure out why. If you were drinking alcohol the night before, that would be a pretty obvious explanation. If you didn't have anything to drink, you start narrowing down the possibilities.

What did you eat yesterday? Did you eat anything that you don't normally eat, or did you possibly eat something that was undercooked? Maybe you have food poisoning. Do you know anyone else around you that has been sick recently? Maybe you got whatever they had.

Just the types of symptoms will be big keys in figuring out what's wrong. Whether you are vomiting, coughing, or have a sore neck will point to much different possible ailments.

jcraig

@stl156 - Interesting question. Unfortunately, I have never taken a formal logic class, so I am not sure what the exact definition would be. I can see what you're getting at. It almost seems like there would have to be two different types of those relationships where an outside source is involved.

It is just like real life when you think about it. For example, if the boss's car doesn't start, he might be angry when he gets to work. Depending on who the person is, the anger may or may not affect Dave's chances of getting a raise. It all depends on how that person compartmentalizes the anger. Does the boss realize that Dave didn't cause the anger or not? I am sure there is a much more technical way to describe a scenario like that, but I think that is a good basic explanation.

stl156

I have never heard of a syllogism before, although it is a common form of deductive reasoning. I always like learning the technical terms for these types of things.

I do have a question about that, though. In the example, it talks about Dave being late, making his boss angry, and not getting a raise. Does reasoning like this rely on Dave's boss being angry solely because Dave is late or being angry period? For example, the boss could be angry because his car wouldn't start that morning. Would that also prevent Dave from getting a raise?

I guess what I am really wondering about is the exact definition of syllogism. Does it have to rely on one person's actions affecting someone else, or can any action affect the second person?