Informal Logic, Famous Fallacies

1
Informal Logic, Famous Fallacies

• Dont Be Fooled by Bad Arguments (Part 2)

2
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 15. Begging the Question (Petitio Principii)
• In formal debate, the question is the very
  issue being debated, and while debaters can
  request a concession of certain points, they
  cannot ask a concession of the very point being
  debated!
• Question Does God Exist?
• Question begging argument
• 1. Prophets inspired by God wrote the bible
• 2. The Bible says God exists
• 3. God exists.
• Parts taken from Hurleys A Concise Introduction
  to Logic

3
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• Begging the Question, cont.
• That differs starkly from media uses of the term
  Heat Wave Begs the Question Global Warming?
• Media should say, Invites the Question Global
  Warming?

4
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 16. Complex Question
• Please answer with either Yes or No. Have you
  stopped beating your wife?
• This question is complex.
• What is implied if you answer yes?
• What is implied if you answer no?

5
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• Complex Question, cont.
• Differs from a leading question, which simply
  suggests a given answer
• Why do you dislike the defendant? (Complex)
• Do these jeans make my butt look big? (Leading)

6
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 17. Faulty Dilemma (False Dichotomy, False
  Bifurcation, Black and White Thinking, Either/Or
  fallacy, False Choice)
• You must either support the war, or be labeled a
  traitor!
• There is a tertium quid, a third thing/option
  you could seek to end the war for what appear to
  be good reasons!

7
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• Faulty Dilemma, cont.
• There are also Trilemmas, Tetralemmas, Etc.
• A famous religious argument
• 1. Jesus must be either Lord, Lunatic, or Liar.
• 2. Cant be Lunatic (teachings are too
  impressive)
• 3. Cant be liar (hes a paradigm of morality!)
• 4. Jesus must be Lord then, QED!
• Can you think of a forth option that would show
  this trilemma faulty?

8
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• Are these genuine dilemmas?
• Either creation or evolution is true.
• You are either with us, or against us (said to
  Bobsled teammate who showed up wearing a parka
  with a huge hood).
• In the age of terrorism, you must choose between
  your rights and your safety.

9
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 19 Equivocation
• Generating a faulty conclusion based on using a
  single term twice, but with a different meaning
  each time
• Homosexuality violates the laws of nature
• Violating laws is a crime
• Therefore, homosexual conduct is a crime against
  nature
• Which term is being used with 2 meanings?

10
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 19 Equivocation
• A mouse is an animal
• Therefore, a large mouse is a large animal.
• Jordy hates things that smell.
• Cooks smell things every day.
• Jordy must hate cooks!

11
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 20. Amphibole
• The police report says he was shot in a secure
  area. I hope he can still have children!
• Amphiboles occur because of grammatical errors,
  missing comas, dangling modifiers, ambiguous
  antecedent or pronoun, etc.

12
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• Amphibole, cont.
• John told Henry he made a mistake. It follows
  that John at least has the courage to admit his
  own mistakes.
• Jane said shes giving a lecture about drug use
  in her office. Lets go for pizza we all know no
  one uses drugs in her office!

13
Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 21. Composition
• Mistakenly attributing a feature of the parts to
  the whole
• 1. Every page of this book is light.
• 2. This book is light.
• No Fallacy
• 1. All these legos are red.
• 2. This dinosaur made with them is red.

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Fallacies of Presumption, Ambiguity, and
Grammatical Analogy

• 22. Division
• Mistakenly attributing a feature of the whole to
  its parts
• 1. We drafted players from the national
  championship team.
• 2. We drafted the best players.
• No Fallacy
• 1. This quartet sings on key.
• 2. Joe, the tenor in the quartet, sings on key.

15
Quick Quiz

• Every sentence in that paragraph is well-written.
  Therefore, that paragraph is well-written.

16
Quick Quiz

• Why are you holding a grudge against your high
  school classmates?

17
Quick Quiz

• George said he was interviewing for a job
  drilling oil wells in the supervisors office. We
  can conclude that that supervisor has a very
  dirty office.

18
Quick Quiz

• Emeralds are seldom found in this country, so you
  should be careful not to misplace your emerald
  ring.

19
Quick Quiz

• Tom is super good at getting dogs to follow his
  commands!
• How do you know that?
• Well, because hes such a good dog trainer!

20
Arguments v. Explanations

• In both arguments and explanations, there is at
  least one premise, and at least one conclusion.
  The crucial difference between them is, the
  premises of an argument are better known than the
  conclusion, and in an explanation, the conclusion
  is better known than the premises.
• The terms premise/conclusion arent used for
  explanations their parts are called
  explanans/explanandum, respectively.

21
Arguments v. Explanations

• If an arguments premises guarantee its
  conclusion, the argument is deductive.
• 1. Edgar ran across the yard.
• 2. Edgar ran.
• Notice the premise is more specific than the
  conclusion, illustrating why it is a mistake to
  think all deductive arguments move from the
  general to the particular.
• Consider is the premise better known than the
  conclusion?

22
Arguments v. Explanations

• If an arguments premises just make the
  conclusion likely, the argument is inductive.
• 1. The lights are on.
• 2. My parents are home.
• (perhaps the lights are on because a burglar
  turned them on, or your parents left them on by
  mistake, etc. there is no guarantee your
  parents are home from the lights being on.)

23
Arguments v. Explanations

• Okay, Dad, I swerved to miss a cat.
• The back end of the car swung around.
• The car is a mess.
• (for the father, the conclusion is well-known,
  but the premises are dubious this, then, is
  an explanation, not an argument.)