Deductive Versus Inductive Appeals to Reason

1
Deductive Versus InductiveAppeals to Reason

• Ms. O Shea
• Riverside High School
• English IV

2
DEDUCTION

• Formal Logic
• It orders ideas from the general assertion to
  specific reasons and support that culminate in a
  certain, inevitable conclusion. GENERAL TO
  SPECIFIC
• Deductive reasoning employs the syllogism which
  moves from a major statement/premise, through a
  minor premise, to a conclusion. The premises are
  presumed to be true.
• In a valid, deductive argument, if the premises
  are true, then the conclusion necessarily
  follows.

3
ARISTOTLES FAMOUS SYLLOGISM

• MAJOR PREMISEAll men are mortal.
• MINOR PREMISESocrates is a man.
• CONCLUSION Therefore Socrates is mortal.
• The conclusion is said to be entailed in, or
  contained in, the premises.
• The Law of the Sea treaty states that any vessel
  beyond a 12 mile limit is in international
  waters. The treaty also states that any vessel in
  international waters cannot be legally stopped or
  boarded. Therefore, when the U.S. Coast Guard
  intercepts boats coming from Cuba or Haiti more
  than 12 miles from the U.S. coast, it is
  violating the Law of the Sea.

4
sample Venn diagramof a deductive argument
tortoises
vegetarian animals
Thus, Bessie must be a vegetarian
All tortoises fall in the circle of animals that
are vegetarians
Bessie falls into the circle of animals that are
tortoises
Bessie
5
DEDUCTION

• Deductive reasoning is either valid or invalid,
  that is completely correct or completely
  incorrect, nothing in between. This type of
  reasoning does not yield new knowledge it
  clarifies concepts we know something about, and
  it is based on the truth of its premises.
• If the reasoning employed in an argument is valid
  and the arguments premises are true, then the
  argument is said to be sound.
• valid reasoning true premises sound
  argument

6
More Deductive Arguments

• Suppose the following statements are all true
• Person L is shorter than person X
• Person Y is shorter than person L
• Person M is shorter than person Y
• What additional piece of information would be
  required to conclude that Person Y is shorter
  than Person J?

1. Person L is taller than J
2. Person X is taller than J
3. Person J is taller than L
4. Person J is taller than M
5. Person M is taller than Y

Solution Answer C M lt Y lt L lt X So, if J is
taller than L, Y must be shorter than J
7
More Deductive Arguments

• Suppose every place in the world that people live
  is represented by the blue space inside the
  rectangle. Suppose the long pink oval represents
  all the wooden houses in the world. And, suppose
  the green circle represents Canada. The most
  logical conclusion one can draw from the figure
  is

1. all wooden houses are found in Canada
2. Everyone lives in a wooden house
3. Some Canadians live in wooden houses
4. No one lives in Canada

8
The Sophism

• If one of the premises in a syllogism is unsound,
  the conclusion will be invalid. This creates a
  SOPHISM.
• Example
• All women are wise.
• Kate is a woman.
• Therefore, Kate is wise.

9
SOPHISM

• If faulty reasoning and not an invalid premise
• creates the sophism, then the result will be one
  of the fallacies youve studied. Which do you
  recognize here?

10
Enthymeme

• The enthymeme is a syllogism in which one of the
  premises is implied, rather than being directly
  stated.
• Wasps are insects. Therefore wasps have six
  legs.
• What premise is unstated?

11
INDUCTION

• commonly known as informal logic, or everyday
  argument. It is embodied in the scientific
  method.
• Induction moves from the SPECIFIC (examples) to
  the GENERAL (conclusions), thereby reversing the
  process of deduction.
• involves drawing inferences from
  experimentation,examples,observations, and
  experiences.
• the conclusions reached are probable, reasonable,
  plausible, and believable.

12
INDUCTION EXAMPLES

• A sample of fifty motorists who were stopped by
  the CHP at a sobriety checkpoint on a Saturday at
  midnight revealed that one in four drivers were
  either uninsured, intoxicated, or both. Thus, if
  you get involved in an accident on the freeway
  there is a 25 chance the other motorist will be
  drunk or uninsured.
• Boss to employee Biff has a tattoo of an anchor
  on his arm. He probably served in the Navy.
• Observation Tonya is seen walking from her car
  to her home with a set of golf clubs.Observation
  Tonyas husband Jeff loves golf and tomorrow is
  his birthday.Conclusion (inference) Tonya has
  bought the set of golf clubs for Jeff.

13
INDUCTION

• Arguments based on experience, experiment or
  observation are best expressed inductively, while
  arguments based on laws, rules, or other widely
  accepted principles are best expressed
  deductively.
• Inductive reasoning helps us to infer probable
  and likely conclusions and to gain new knowledge
  by examining statistical patterns. It also helps
  us to identify cause and effect. The analogy is
  an example of this type of reasooning.

14
INDUCTIVE REASONING

• Inductive reasoning enjoys a wide range of
  probability it can be plausible, possible,
  reasonable, credible, etc.
• the inferences drawn may be placed on a continuum
  ranging from cogent at one end to fallacious at
  the other.
• Cogent (believable)_____________Fallacious
  (false)

15
Deduction Versus Induction

• Deductive reasoning is commonly found in the
  natural sciences or hard sciences, less so in
  everyday arguments.

• Inductive reasoning is found in the courtroom,
  the boardroom, the classroom, and throughout the
  media
• Most, but not all everyday arguments are based on
  induction
• Examples The reasonable person standard in
  civil law, and the beyond a reasonable doubt
  standard in criminal law

16
SOURCES
Adapted from commfaculty.fullerton.edu/rgass/.../D
eduction20Vs.20Induction.ppt