Unit 1D

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Unit 1D

• Analyzing Arguments

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TWO TYPES OF ARGUMENTS
Arguments come in two basic types

• Inductive
• Deductive

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INDUCTIVE ARGUMENT
An inductive argument makes a case for a general
conclusion from more specific premises. In other
words, a conclusion is formed by generalizing
from a set of more specific premises.
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EXAMPLE
Premise Sparrows are birds that
fly. Premise Eagles are birds that
fly. Premise Hawks are birds that
fly. Premise Larks are birds that
fly. Conclusion All birds fly.
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NOTES ON INDUCTIVE ARGUMENTS

• An inductive argument cannot proveits conclusion
  true.
• An inductive argument can be evaluated only in
  terms of its strength.
• The strength of an inductive argument is a
  measure of how well the premises support the
  conclusion. Clearly, this is subjective (a
  personal judgment).

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DEDUCTIVE ARGUMENT
A deductive argument makes a case for a specific
conclusion from more general premises. In other
words, a conclusion is deduced from a set of
general premises.
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EXAMPLE
Premise All college professors have masters
degrees. Premise Ms. Gomez is a college
professor. Conclusion Ms. Gomez has a masters
degree.
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NOTES ON DEDUCTIVE ARGUMENTS

• A deductive argument can be evaluated in terms of
  its validity and soundness.
• A deductive argument is valid if its conclusion
  follows necessarily from its premises. Validity
  is concerned only with the logical structure of
  the argument. It has nothing to do with the truth
  of the premises or the conclusion.
• A deductive argument is sound if it is valid and
  its premises are all true.
• A sound deductive argument provides definitive
  proof that its conclusion is true. (However, this
  often involves personal judgment.)

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A VENN DIAGRAM TEST OF VALIDITY
To test the validity of an argument with a Venn
diagram

• Draw a Venn diagram that represents all the
  information contained in the premises.
• 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.

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CONDITIONAL DEDUCTIVE ARGUMENTS
A conditional deductive argument has a
conditional statement for its first premise.
There are four basic conditional arguments
1. Affirming the hypothesis (antecedent).
2. Affirming the conclusion (consequent).
3. Denying the hypothesis (antecedent). 4.
Denying the conclusion (consequent).
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AFFIRMING THE HYPOTHESIS
Structure If p, then q. p is true q is
true Validity Valid This argument is also
known as modus ponems.
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EXAMPLE
Premise If one gets a college degree, then one
can get a good job. Premise Marilyn has a
college degree. Conclusion Marilyn can get a
good job.
VALID
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AFFIRMING THE CONCLUSION
Structure If p, then q. q is true p is
true Validity Invalid
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EXAMPLE
Premise If one gets a college degree, then one
can get a good job. Premise Marilyn gets a good
job. Conclusion Marilyn has a college degree.
INVALID
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DENYING THE HYPOTHESIS
Structure If p, then q. p is not true q
is not true Validity Invalid
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EXAMPLE
Premise If one gets a college degree, then one
can get a good job. Premise Marilyn does not
have a college degree. Conclusion Marilyn
cannot get a good job.
INVALID
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DENYING THE CONCLUSION
Structure If p, then q. q is not true p
is not true Validity Valid This argument is
also known as modus tollens.
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EXAMPLE
Premise If one gets a college degree, then one
can get a good job. Premise Marilyn does not
have a good job. Conclusion Marilyn does not
have a college degree.
VALID
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DEDUCTIVE ARGUMENTS WITH A CHAIN OF CONDITIONALS
1. Structure If p, then q. If q, then r. If
p, then r. Validity Valid
2. Structure If p, then q. If r, then
q. If p, then r. Validity Invalid