Inductive vs. Deductive Reasoning

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Inductive vs. Deductive Reasoning
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Deductive Reasoning

• Starts with a general rule (a premise) which we
  know to be true. Then, from that rule, we make a
  true conclusion about something specific.

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Deductive Reasoning

• The process of reasoning from known facts to
  conclusions. When you reason deductively, you can
  say therefore with certainty. If your facts
  were firm to begin with, then your conclusions
  will also be firm.

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Deductive Reasoning
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Deductive Reasoning

• From vague
• To specific

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A deductive argument

• is one in which it is impossible for the premises
  to be true but the conclusion false.
• It is supposed to be a definitive proof of the
  truth of the claim (conclusion).
• Premise ? All men are mortal.
• Premise ? Socrates was a man.
• Conclusion ? Socrates was mortal.
• If the premises are true (and they are), then it
  simply isn't possible for the conclusion to be
  false.
• If you have a deductive argument and you accept
  the truth of the premises, then you must also
  accept the truth of the conclusion.

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A deductive argument
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Deductive Reasoning

• Example
• Smith owns only blue pants and brown pants.
  Smith is wearing a pair of pants today. So,
  Smith is wearing either blue or brown pants today.

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Inductive Reasoning

• The process of going from observations to
  conclusions.
• This type of conclusion is sometimes called an
  inference.

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Inductive Reasoning

• Observing that something is true many times, then
  concluding that it will be true in all instances
• Using the data to make a prediction

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Inductive Reasoning
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Inductive Reasoning

• From specific
• To vague

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An inductive argument

• is one in which the premises are supposed to
  support the conclusion.
• If the premises are true, it is unlikely that the
  conclusion is false.
• The conclusion probably follows from the
  premises.
• Premise ? Socrates was Greek.
• Premise ? Most Greeks eat fish.
• Conclusion ? Socrates ate fish.
• Even if both premises are true, it is still
  possible for the conclusion to be false (maybe
  Socrates was allergic to fish).
• Words which tend to mark an argument as inductive
  include probably, likely, possibly and reasonably.

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A inductive argument
True Premise
True Premise
Probably True Conclusion
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Inductive Reasoning

• Example
• January has been cold here in Siberia. Today is
  January 14, so it is going to be another cold day
  in Siberia.

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Which conclusion below shows evidence of
deductive reasoning?

1. One time an employee used store computers to
   download a harmful virus, so employee computer
   use is a security threat.
2. Many customers complained about employees using
   computers rather than working, so computer misuse
   was harming the companys sales.
3. An employee on a break distracted other employees
   by using a computer in a public area for private
   business, so that employee was given a warning.
4. Several employees left customer sales information
   visible on computer screens, so those employees
   were instructed to always close windows with
   personal information.

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Lets Practice

• The word wall is Inductive
• The windows are Deductive

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1

• Premise Everything made of copper conducts
  electricity.
• Premise
• This wire is made of copper.

• Conclusion This wire will conduct electricity.

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2

• Premise Socrates was Greek.
• Premise Most Greeks eat fish.

• Conclusion Socrates probably ate fish.

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3

• The local branch of Wachovia Bank was robbed
  yesterday. Jenny needed money to pay off her
  gambling debts. She just bought a gun two days
  ago, and I saw her hanging around the local
  Wachovia Bank yesterday morning. Today the
  bookies goons stopped looking for Jenny. So
  Jenny robbed Wachovia Bank yesterday.

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4

• Premise All turtles have shells.
• Premise The animal I have captured is a turtle.

• Conclusion I conclude that the animal in my bag
  has a shell.

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5

• Sherlock Holmes and Watson were on a camping
  trip. They had gone to bed and were lying there
  looking up at the sky. Holmes said, Watson,
  look up. What do you see?
• I see thousand of stars.
• And what does that mean to you?
• I guess it means we will have another nice day
  tomorrow. What does it mean to you, Holmes?
• To me, it means someone has stolen our tent.

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6

• Premise Tonya is seen walking from her car to
  her home with a set of golf clubs.
• Premise Tonyas husband Jeff loves golf and
  tomorrow is his birthday.

• Conclusion Tonya has bought the set of golf
  clubs for Jack.