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What is Deductive Reasoning?

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Deductive reasoning is one of the two basic forms of valid reasoning. It begins with a general hypothesis or known fact and creates a specific conclusion from that generalization. This is the opposite of inductive reasoning, which involves creating broad generalizations from specific observations. The basic idea of deductive reasoning is that if something is true of a class of things in general, this truth applies to all members of that class. One of the keys for sound deductive reasoning, then, is to be able to properly identify members of the class, because incorrect categorizations will result in unsound conclusions.

Truth and Validity

For deductive reasoning to be sound, the original hypothesis or generalization also must be correct. A logical deduction can be made from any generalization, even if it is not true. If the generalization is wrong, though, the specific conclusion can be logical and valid but still can be incorrect.

Examples

One can better understand deductive reasoning by looking at examples. A generalization might be something such as, "All wasps have stingers." The logical conclusion of a specific instance would then be, "That is a wasp, so it has a stinger." This is a valid deduction. The truth of the deduction, however, depends on whether the observed insect is, indeed, a wasp.

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People often use deductive reasoning without even knowing it. For example, a parent might say to a child, "Be careful of that wasp — it might sting you." The parent says this because he or she knows that wasps have stingers and, therefore, that the observed wasp has a stinger and might sting the child.

Inductive Reasoning

Inductive reasoning would work in the opposite order. The specific observation would be that a particular wasp has a stinger. One could then induce that all wasps have stingers. Many scientific tests involve proving whether a deduction or induction is, in fact, true. Inducing that all cats have orange fur because one cat has orange fur, for example, could be easily disproved by observing cats that do not have orange fur.

Syllogism

One of the most common and useful forms of deductive reasoning is the syllogism. A syllogism is a specific form of argument that has three easy steps: a major premise, a minor premise and a logical conclusion. For example, the premise "Every X has the characteristic Y" could be followed by the premise "This thing is X," which would yield the conclusion "This thing has the characteristic Y." The first wasp example could be broken up into the major premise "Every wasp has a stinger," the minor premise "This insect is a wasp" and the conclusion "This insect has a stinger." Creating a syllogism is considered a good way for deductive reasoning to be tested to ensure that it is valid.

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Discuss this Article

anon306507
Post 28

Technically, the both of you are using deductive reasoning. Therefore, if one of you is correct, nether one of your answers can be believed to be correct except by yourselves and others who share your opinion.

But no matter how hard you try, you will not be able to prove your answer as logical or correct, so the logical answer is that there is no logical answer. -menefese k.c

anon263018
Post 26

Funny how people come to these conclusions. If any of you were actually using logic, you would know racism is acquired through interactions with people, and discrimination is taught, so you can't really prove these hypothetical instances.

What it comes down to is logic is a contradiction in itself. It attempts to explain reason without a purpose and purpose without a reason. Ask yourselves any question and put purpose or reason in the sentence, and you'll find that you have two very different answers. Logic cannot defeat logic!

anon244853
Post 25

There are about 150 examples of logical inferences installed with a software program that can make deductions based on meaning.

anon227133
Post 23

@anon200865: Your premise is flawed. I have spoken to white Americans who made it clear they weren't racist, hence, some white Americans aren't racist.

But you still make a good point. A bad premise will result in a bad conclusion.

anon200865
Post 22

All the white people I have met admitted to me that they were racists. This person is white, so he is most likely a racist.

Since American society has been historically controlled by white people and all the white people I have met have admitted to me that they were racists, this means that the American society is ran and controlled by white racists.

Since most law enforcement agencies, lawyers and judges are predominately controlled from the top by white people and all the white people I have met, admitted to being racists, then all black people in prisons are victims of a white, racist legal and police system, controlled by racist white people.

See the problem with this kind of reasoning?

anon195104
Post 21

@anon143318: That reasoning is only descriminatory due to the shifting of variables (which you may or may not have intended for the sake of your statement). The following is a corrected version of your deductive reasoning scenario:

1. 50-plus percent of criminals in jails are black people.

2. He is a criminal in jail.

3. Therefore, he is more likely to be black.

Notice that any ethniticity could replace 'black' and this would remain true. The only potential issue is whether statement no. 1 is fact. If not, then a fallacy is being comitted resulting in statements no. 2 and 3 being misleading or false.

Discrimination only comes into play when faulty deductive reasoning is utilized.

anon179157
Post 19

@anon143318: Of course that deductive reasoning fails as a result of the fact that most black people are not criminals.

I'd say rather that racists using corrupted "deductive reasoning" to argue their beliefs were the root of discrimination, and not the method of deductive reasoning itself.

anon161938
Post 18

Death is only an inductive illusion where deductive motives can not infer or concur where why how what and when. Therefore, life and death do not matter, rather, they are a series of reactions within a system. Ultimately, the only thing that separates known species is how humans can reason and think.

anon151845
Post 17

@Shag: in an attempt to sound intellectually superior, you actually got your deduction wrong before flying off on that meaningless tangent.

shagspur
Post 16

General inductive comment: the quality of reasoning in most of these comments is very poor; therefore, I suspect that most users of this site are poorly educated and not intelligent.

Inductive comment: Inductive vs. deductive reasoning has always confused me but after inductively reading the facts presented on this page, the fog has lifted.

The essential problem of deducing the meaning of deduction is tautological: we may use deductive reasoning to say that all planets revolve around a sun, and since the earth revolves around a sun, it must be a planet, but how in God's name did we come to define any planet? (chicken/egg problem) Logically, trying to untangle the reasoning knot between the inductive/deductive understanding of planets would be endless.

How do we even know the earth is a planet? One can easily deduce that there was a time when no human being understood that we live on a planet; empirically, it is easy then to induce that it took thousands of years for the human race to reason this out, using both inductive and deductive logic in a constantly oscillating sequence.

anon143318
Post 15

Deductive reasoning is the root of discrimination.

An example of how deductive reasoning is abused as a form of discrimination:

Syllogism:

1. 50-plus percent of criminals in jails are black people.

2. He is a black person.

3. Therefore he is more likely to be a criminal.

anon119216
Post 14

I'm a student and after my teacher lectures and even by reading these examples i can't differentiate between the two terms deductive and inductive, because deductive reasoning is from general to specific and inductive reasoning is from specific to general, but most examples do not satisfy these terms.

anon80296
Post 11

Inductive:

An inductive report involves moving from the specific issues, as outlined in the discussion, to the more general, summarized information.

Example:

1. Syllogism: Earth is a planet; it revolves around the sun.

Conclusion: All planets revolve around a sun.

2. Syllogism 1. This fire warms

Syllogism 2. And this fire warms.

Syllogism 3. Also this fire warms.

Conclusion: therefore every fire warms.

3. Syllogism 1. I am a man.

Syllogism 2. I must die.

Syllogism 3. All men must die

Conclusion: die

Deductive

This type of order is effective when faced with an audience who does not have time to read the whole document, but can access the conclusions and recommendations

Example:

1. Conclusion: all planets revolve around the sun.

Earth is a planet; it revolves around the sun.

2. Conclusion: All men must die.

I am a man

I must die.

anon76913
Post 10

I think deductive reasoning comes in handy a lot

for arguments that need reasoning. It should be learned by everybody.

anon65673
Post 8

I think deductive reasoning is the anticipation of the future and the the general understanding of the environment.when it come to mathematical deductive reason will allow the person to react faster.

karamage, student in finance department; school of finance and banking.

anon54114
Post 7

Deductive reasoning is something that is actually nice to learn (not learn about, but learn), given that the problems we need to think on are more complex and exciting than normal everyday events.

anon52887
Post 6

Death is only an inductive illusion where deductive motives can not infer or concur where why how what and when. Therefore, life and death do not matter, rather, they are a series of reactions within a system. Ultimately, the only thing that separates known species is how humans can reason and think.

anon50838
Post 5

who really even cares about deductive reasoning? Gosh, who wants to teach it? Who wants to learn it? No one. All we need is the basics. I'm not a bum -- i just don't want to be confused. If it's something we already use in our everyday lives then why learn it in a more complex way during class?

anon49501
Post 4

I do believe that deductive reasoning is a necessity. We use it under a daily basis but we do not realize it. I highly disagree with teachers not teaching this. I am a student and just because the answer is sometimes false does not mean they should hold back on us.

anon44361
Post 3

This is incorrect. Some examples of deductive reasoning are given, but these examples cover only a subset of what deductive reasoning is. As a teacher, I am told to avoid teaching deductive reasoning because of the misconception that it consists only of dry material such as this. Deductive reasoning is something we use every day and we neglect at our peril. The best examples of deductive reasoning are in Socratic dialogues when Socrates encourages others to think deductively.

malena
Post 1

To add....deductive reasoning is one of two types of formal logic -- the other being inductive reasoning. Formal logic is primarily based on abstract rules whereas its counterpart, informal logic, or critical thinking, is based on analyzing verbal arguments to see if they are true -- or an even weaker goal -- to see if they are probably true. Generally, formal logic is a necessary precursor to informal logic.

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