CHAPTER 12: LOGIC AND
REASONING
OBJECTIVES
• Distinguish among several different types
of reasoning and recognize faulty or
misleading types.
• Better adapt your use of logic to a
specific audience.
• Analyze your own logic to determine if
your conclusions are valid.
OUTLINE
• 1. Evaluating Ideas: Methods of Reasoning
• 2. Fallacies
• 3. The Ethics of Audience Adaptation
EVALUATING IDEAS: METHODS OF
REASONING
• Inductive Reasoning: in mathematics a technique for proving
theorems in which the mathematician uses certain specific
cases to help prove a general truth.
Specific
General
•
https://www.khanacademy.org/math/trigonometry/seq_induction/deductive-and-inductive-reasoning/v/u12-l1-t3-we1inductive-reasoning-1
TYPES OF REASONING
• Sign Reasoning: We draw conclusions
about a given situation based on physical
evidence.
Example: When the fire alarm is
going off in the kitchen, something is
burning.
ANALOGIES
• Reasoning by Analogy-illustration in which the
characteristics of a familiar object or event are used to explain
or describe the characteristics of less familiar object or event.
They are often used in standardized tests.
Example: Hand is to person as
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•
•
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A. tree is to sky
B. dirty is to laundry
C. foot is to powder
D. paw is to dog
Example: Students are like worker bees because they are constantly
studying and doing work for their classes.
DEDUCTIVE REASONING
• Deductive reasoning moves from generalizations or premises,
to a specific instances.
• Premises: statements on which reasoning is based.
• CATEGORICAL SYLLOGISM
• MAJOR PREMISE: All men are mortal.
• MINOR PREMISE: Socrates is a man.
• CONCLUSION: Socrates is mortal
• All students go to school.
• You are a student.
• Therefore, you go to school.
SHOULD PEOPLE BE ALLOWED TO
BURN THE AMERICAN FLAG?
• The First Amendment says to allow all types of expression.
• Flag burning is a type of expression.
• Therefore, flag burning should be allowed.
• Is violence ever a just response to injustice?
• Just means fair, upright, legal, reasonable
• Justifiable means “excusable”.
FALLACIES
• Fallacies are errors in reasoning or mistaken beliefs.
• Weaken your credibility as a speaker
Avoid them at all cost!
They include the following:
Hasty generalizations
False premises
Circumstantial evidence
Mistaken causality
Misuse of numbers
False analogy
Ignoring the question
Begging the question
HASTY GENERALIZATION
• Definition: The size of the sample is too small to support the
conclusion.
• Examples: Fred, the Australian, stole my wallet. Thus, all Australians are
thieves. (Of course, we shouldn't judge all Australians on the basis of one
example.)
• I asked six of my friends what they thought of the new spending restraints
and they agreed it is a good idea. The new restraints are therefore generally
popular.
• Proof: Identify the size of the sample and the size of the population,
then show that the sample size is too small. Note: a formal proof
would require a mathematical calculation. This is the subject of
probability theory. For now, you must rely on common sense.
• References: Barker: 189, Cedarblom and Paulsen: 372, Davis: 103
26 May 1995
FALSE PREMISE
• A false premise is an error in deduction.
• For example, parents like to tell their children,” All teenager are
irresponsible. You are a teenager. Therefore you are not responsible
enough to leave the house tonight.
CIRCUMSTANTIAL EVIDENCE
• It only suggests a conclusion; the evidence does not prove the
conclusion.
It seems logical that a person holding a smoking gun at
the scene of a murder was the one who committed the crime.
What if that person was trying to defend the person who got
murdered, while the actual murderer ran away?
MISTAKEN CAUSALITY
• To say two events are casually related is to
claim that one event brings about the other.
• Example: One billiard ball strikes another and
causes the second
ball to bounce off the cushion at an angle. There is
a causal relationship because the first ball caused
the action of the second ball.
• If a bus passes a church everyday at noon and the
bells ring at noon, that does not mean the bus made
the bells ring.
PLAYING WITH NUMBERS
• This involves manipulating statistics to
misrepresent facts.
• They numb you with numbers in order to
persuade you to agree with their cause.
FALSE ANALOGY
• Compares two things that are not really the same. (can be
literal or figurative). It assumes that because two things,
events, or situations are alike in some known respects, that
they are alike in other unknown respects.
• example: What's the big deal about the early pioneers killing a few
Indians in order to settle the West? After all, you can't make an omelet
without breaking a few eggs.
• example: Banning "head" shops from selling drug paraphernalia in order
to curb drug abuse makes about as much sense as banning bikinis to
reduce promiscuity.