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Deductive Reasoning. By. Mr. Kass. Deductive Reasoning Is…. Deductive reasoning is when you start from things you assume to be true, and draw conclusions that must be true if your assumptions are true. . For Example All dogs have a tail. Buddy is a dog. Therefore Buddy has a tail.

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deductive reasoning is
Deductive Reasoning Is…
  • Deductive reasoning is when you start from things you assume to be true, and draw conclusions that must be true if your assumptions are true.

For Example

All dogs have a tail.

Buddy is a dog.

Therefore Buddy has a tail.

deductive vs inductive
Deductive Vs. Inductive
  • Induction is usually described as moving from the specific to the general, while deduction begins with the general and ends with the specific.
  • Arguments based on laws, rules and accepted principles are generally used for Deductive Reasoning. Observations tend to be used for Inductive Arguments.
still don t get it
Still don’t get it?
  • Man approaches to greet a new neighbor who is just moving into the house next door and asks what he does for a living.
  • Neighbor 1: I am a professor at the University, I teach deductive reasoning.
  • Man: Deductive reasoning? What is that?
  • Neighbor 1: Let me give you an example. I see you have a dog house out back. By that I deduce that you have a dog.
  • Man: That's right.
  • Neighbor 1: The fact that you have a dog, leads me to deduce that you have a family.
  • Man: Right again.
  • Neighbor 1: Since you have a family I deduce that you have a wife.
  • Man: Correct.
  • Neighbor 1: And since you have a wife, I can deduce that you are heterosexual.
  • Man: Yup.
  • Neighbor 1: That is deductive reasoning.
sherlock holmes
Sherlock Holmes
  • Sherlock Holmes would use deductive reasoning to help solve crimes.


Sherlock Holmes could help solve a mystery by making inferences. If Holmes saw a pack of cigarettes by a victim (the victim did not smoke), Holmes can make the assumption that the killer is a smoker.

Andrew Ault

  • Deductive reasoning may not be the most accurate way of solving a problem, cause we all know that assumptions can be wrong.
other faults of deductive reasoning
Other faults of deductive reasoning
  • All Graduates of M.I.T. are Engineers

George is not from M.I.T.

Therefore George is not an Engineer

Everybody from Texas is a cowboy

Scott is from Texas

Scott is a cowboy

My sociology teacher told us that those of us who grew up in America in the twentieth century, with all of its greed, bigotry, and abuse of power inherited a sickness. Well, if I’m sick, I guess I don’t have to go to school today.

how can deductive reasoning be applied in school
How Can Deductive Reasoning Be Applied In School?

“Ugggh, I don’t know…”

(My Hero. Ralph Wiggum from the Simpsons)



Using laws and rules to make assumptions

The law of gravity means everything that goes up must come downI threw a baseball in the airThat means the baseball must come down.

Social Studies

To be elected President you must obtain at least 270 electoral votesGeorge Bush won 287 electoral votesTherefore George Bush is the President.



Scott has a case of soda in his house since there are 13 cans of soda left I deduce that Scott has drank 11 cans of soda.


When I see like in a sentence, I deduce that it is a simile.




And many more...



  • These professions tend to ask a lot of questions to try to solve problems or to prove a point.
  • Often they would have to make assumptions to solve problems.
  • They would use rules and widely accepted beliefs to prove their argument.

An attorney states that his client is innocent because the crime victim was hit by a car. Since his client does not have a license. He can deduce that his client is innocent.