Inductive vs deductive reasoning

Inductive vs.
Deductive
Reasoning

1

Deductive Reasoning
 Startswith a general rule (a
premise) which we know to be
true. Then, from that rule, we
make a true conclusion about
something specific.

2

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.

3

Deductive Reasoning

4

Deductive Reasoning
From vague

To specific

5

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.
6

A deductive argument

True True True
Premise Premise Conclusion

7

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.

8

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

9

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

10

Inductive Reasoning

11

Inductive Reasoning
From specific

To vague

12

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.
13

A inductive argument

True True Probably
Premise Premise True
Conclusion

14

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.

15

Which conclusion below shows
evidence of deductive reasoning?
A. One time an employee used store computers to download
a harmful virus, so employee computer use is a security
threat.
B. Many customers complained about employees using
computers rather than working, so computer misuse was
harming the company’s sales.
C. 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.
D. Several employees left customer sales information visible
on computer screens, so those employees were instructed
to always close windows with personal information.

16

Let’s Practice –
 The word wall is Inductive
 The windows are Deductive

17

#1
 Premise:  Conclusion:
Everything made This wire will
of copper conduct
conducts
electricity.
electricity.
 Premise:

This wire is made
of copper.
18

#2
 Premise:  Conclusion:
Socrates Socrates
was Greek. probably ate
 Premise: fish.
Most Greeks
eat fish.
19

#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
bookie’s goons stopped looking for
Jenny. So Jenny robbed Wachovia
Bank yesterday.

20

#4
 Premise: All  Conclusion: I
turtles have conclude that
shells. the animal in
 Premise: The my bag has a
animal I have shell.
captured is a
turtle.
21

#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.”

22

#6
 Premise: Tonya is  Conclusion:
seen walking from Tonya has
her car to her home
bought the set of
with a set of golf
clubs. golf clubs for
 Premise: Tonya’s Jack.
husband Jeff loves
golf and tomorrow
is his birthday.

23

Inductive vs deductive reasoning

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Inductive vs deductive reasoning