1.4 Validity, Truth, Soundness, Strength and Cogency - PowerPoint PPT Presentation

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1.4 Validity, Truth, Soundness, Strength and Cogency

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1.4 Validity, Truth, Soundness, Strength and Cogency Goal: Learn the terms used to evaluate inductive and deductive arguments Deductive Arguments An argument in which ... – PowerPoint PPT presentation

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Title: 1.4 Validity, Truth, Soundness, Strength and Cogency


1
1.4 Validity, Truth, Soundness, Strength and
Cogency
  • Goal Learn the terms used to evaluate inductive
    and deductive arguments

2
Deductive Arguments
  • An argument in which it is claimed that it is
    impossible for the conclusion to be false if the
    premises are true is a deductive argument.
  • That is, it is claimed that the argument it
    valid.
  • In a valid argument, if the premises are true,
    the conclusion must be true too.

3
  • Validity describes the relationship that the
    premises bear to the conclusion in a deductive
    argument.
  • The test for validity Imagine that the premises
    are true and the conclusion is false

4
  • If you cannot imagine that the premises are true
    and the conclusion is false, then the argument is
    VALID.
  • If you can imagine that the premises are true and
    the conclusion is false then the argument is
    INVALID.

5
  • P1. All CCU students are hardworking.
  • P2. Howie is a CCU student.
  • C. So, Howie is hardworking.
  • Valid if the premises were true, the conclusion
    could not be false.

6
  • P1. All CCU students are hardworking.
  • P2. Howie is hardworking.
  • C. So, Howie is a CCU student.
  • Invalid if the premises were true, the
    conclusion could still be false.

7
  • In a good argument the premises support the
    conclusion (validity) AND the premises are true.
  • A good deductive argument is called a sound
    argument. A sound argument is valid and has all
    true premises.

8
  • P1. All CCU students are hardworking.
  • P2. Howie is a CCU student.
  • C. So, Howie is hardworking.
  • Valid if the premises were true, the conclusion
    could not be false.
  • False premise(s)
  • Unsound

9
Sound Valid True Premises
  • P1. If Obama is president then he is Commander in
    Chief.
  • P2. Obama is president.
  • C. Therefore, he is Commander in Chief.
  • Is this argument sound?

10
Sound Valid True Premises
  • P1. All cats have tails.
  • P2. My dog has a tail.
  • C. So, my dog is a cat.
  • Is this argument sound?

11
Inductive Arguments
  • An argument in which it is claimed that it is
    improbably that the conclusion is false if the
    premises are true is an inductive argument.
  • That is, it is claimed that the argument is
    strong.
  • A strong argument is one whose conclusion is
    probably true given the truth of the premises.

12
  • The test for strength Imagine the premises are
    true, and determine whether the conclusion is
    probable given the truth of the premises.
  • Does the truth of the premises make it more like
    that the conclusion is true then had the premises
    been false?

13
  • P1. All past presidents have been men.
  • C. The next president will be a man.
  • Inductive Possible for the conclusion to be
    false
  • Can be made stronger or weaker given more
    information.
  • There are degrees of strength!

14
  • P1. All past presidents have been men.
  • C. The next president will be a man.
  • P1. All past presidents have been women.
  • C. The next president will be a woman.

15
  • P1. Every student I know has a cell phone.
  • C. So, all students have cell phones.
  • P1. Every only child surveyed reported being
    satisfied not having siblings.
  • C. Thus all only children are so satisfied.

16
Cogent Strong True Premises
  • P1. I have lost every hand so far.
  • C. So, I will win the next hand.
  • P1. Korby Ray used Ultra 90 and lost 50.
  • C. So, if I use Ultra 90 I will lose 50.

17
  • Arguments Premises conclusions
  • Factual claim (are the premises true?)
  • Inferential claim (is the argument
    valid/strong?)
  • Deductive Arguments
  • Sound Valid True premises
  • valid if the premises are true, the conclusion
    must be true
  • Inductive Arguments
  • Cogent Strong True Premises
  • strong the conclusion is probably true if the
    premises are true

18
Deductive Arguments
  • 1. All hamburgers are high in fat.
    Quarter-pounders with cheese are hamburgers.
    Therefore, quarter-pounders with cheese are high
    in fat.
  • Sound ___ Valid ___ True premises ___

19
Deductive Arguments
  • 1. All hamburgers are high in fat.
    Quarter-pounders with cheese are hamburgers.
    Therefore, quarter-pounders with cheese are high
    in fat.
  • Sound _ ? __ Valid __?_ True premises _ ? __

20
  • 2. All hamburgers are high in fat. Chicken
    nuggets are high in fat. Therefore, chicken
    nuggets are hamburgers.
  • Sound ___ Valid ___ True premises ___

21
  • 2. All hamburgers are high in fat. Chicken
    nuggets are high in fat. Therefore, chicken
    nuggets are hamburgers.
  • Sound _x__ Valid __x_ True premises _ ? __

22
  • 3. All hamburgers are good sources of vitamins.
    Fresh vegetables and fruit are hamburgers.
    Therefore, fresh vegetables and fruit are good
    sources of vitamins.
  • Sound ___ Valid ___ True premises ___

23
  • 3. All hamburgers are good sources of vitamins.
    Fresh vegetables and fruit are hamburgers.
    Therefore, fresh vegetables and fruit are good
    sources of vitamins.
  • Sound _x__ Valid _ ? __ True premises _x__

24
Inductive Arguments
  • Meatpacking has become the most hazardous
    occupation in the US, with three times the injury
    rate of factory work. Therefore, John who works
    at a meatpacking plant will probably get injured
    on the job.
  • Cogent___ Strong ___ True Premises ___

25
  • 2. I havent gotten sick from eating fast food
    yet, so I will probably get sick if I eat fast
    food any time soon.
  • Cogent___ Strong ___ True Premises ___

26
  • 3. McDs promises, to use only fresh, wholesome
    ingredients when serving you and your family so
    they only use fresh, wholesome ingredients in
    their food.
  • Cogent___ Strong ___ True Premises ___

27
  • 4. Because McDs is the single largest purchaser
    of US beef, they have considerable say over how
    beef is processed in this country. So, the beef
    they buy (and sell) is safe.
  • Cogent___ Strong ___ True Premises ___

28
  • 5. McDs webpage has a statement from Dr. Dean
    Ornish whose diet has been proven to reduce heart
    disease therefore, the foods that McDs serves
    are part of a heart-healthy diet.
  • Cogent___ Strong ___ True Premises ___
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