SBM Thinking Critically

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SBM - Thinking Critically
Analyzing Arguments
Chapter 1 Section D
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What well learn

• Inductive argument
• Deductive argument
• Strength
• Validity
• Soundness

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Arguments

• An Inductive Argument makes a case for a general
  conclusion from a more specific premise.
• A Deductive Argument makes a case for a specific
  conclusion from a more general premise.

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Example

• Deductive
• Premise All politicians are married (general)
• Premise Vern Ehlers is a politician (general)
  (House of Representatives or Senate, Republican
  or Democratic?)
• Conclusion Vern Ehlers is married (specific)

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Example

• Inductive
• Premise Birds fly up into the air but eventually
  come down for food (specific)
• Premise Golf balls fly for a while, then end up
  back on the ground (specific)
• Conclusion What goes up, must come down
  (general)

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Strength

• Formally speaking, an inductive argument has to
  have strength.
• a strong argument is better than a weak argument
• The grass is green because I said so.
• Even though this is true, it is weak

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The two questions

• On deductive arguments, we must ask two
  questions
• Does the conclusion follow necessarily from the
  premise?
• Are the premises true?
• If both answers are yes, then they have validity

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Validity

• Remember the political example? We must think
  that the question is valid if its conclusion
  follows necessarily from its premise.
• Note validity is concerned only with the
  logical structure of the argument it involves no
  personal judgement and has nothing to do with the
  truth of the premise or conclusions.

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Soundness

• If a deductive argument is valid and the premise
  are true, then we can say the argument is sound.
• If the argument is sound, we know that the
  conclusion is true beyond all doubt.

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Key distinctions

• Inductive Arguments
• A conclusion is formed by generalizing from a set
  of more specific premises

• Deductive Arguments
• A specific conclusion is deduced from a set of
  more general (or equally general premises.

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Key distinctions

• An inductive argument can be analyzed only in
  terms of its strength. Evaluating strength
  involves personal judgement about how well the
  premises support the conclusion.

• A deductive argument can be analyzed in terms of
  its validity and soundness. It is valid if its
  conclusion follows automatically from its
  premises. It is sound if it is valid and its
  premises are true.

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Key distinctions

• An inductive argument cannot prove its conclusion
  true at best, a strong inductive argument shows
  that its conclusion probably is true.

• Validity concerns only logical structure for
  example, a deductive argument can be valid even
  when its conclusion is blatantly false. But a
  sound deductive argument provides definitive
  proof that

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Key distinctions

• Its conclusion is true. (However, evaluating
  soundness often involves personal judgement.)

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Venn Diagram Test of Validity

• To test the validity of a deductive argument on a
  Venn Diagram
• Draw a Venn Diagram that represents all the
  information contained in the premise.
• Check to see whether the Venn Diagram also
  contains the conclusion. If it does, then the
  argument is valid. Otherwise, the argument is
  not valid.
• See page 56-57 for diagrams.

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Analyze the following

• Denying the Consequent
• Premise 1 A narcotic is habit forming
• Premise 2 Heroin is not habit-forming
• Conclusion Heroin is not a narcotic
• Since Heroin is habit forming, the premise is
  false.but the argument is still valid due to the
  conclusion following the premises.

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Examples

• A chain of conditionals
• If elected to the school board, Maria Lopez will
  force the school district to raise academic
  standards, which will benefit my childrens
  education. Therefore, my children will benefit
  if Maria Lopez is elected.
• Change to Conditionals

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Maria Lopez

• Premise If Maria Lopez is elected to the school
  board, then the school district will raise
  academic standards
• Premise If the school district raises academic
  standards, then my children will benefit.
• Conclusion If Maria Lopez is elected to the
  school board, then my children will benefit.

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Examples

• Chores
• Make p you shop, q I make dinner, r you
  take out the trash.
• Premise If p, then q
• Premise If r, then q
• Conclusion If p, then r
• Not true, because there is no chain from p to r.

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Homework

• 1D s 1 4, 9 11, 15 17, 20, 23 26,
  30, 33 37, 41, 42, 45, 46.
• You must refer to your notes and book to answer
  these homework questions.