Understanding and Evaluating Arguments

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Understanding and Evaluating Arguments
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The Anatomy of Arguments

• Arguments consist of premises (or reasons) which
  support a conclusion
• Dont cheat on your test because if you cheat
  youll get caught.

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Good and Bad Arguments

• A good argument provides grounds for accepting
  its conclusion
• A bad argument does not
• Arguments can be better or worse

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Valid and Invalid Arguments

• The truth of the premises of a valid argument,
  absolutely guarantee a true conclusion
• Example
• All human beings are mortal Nicole is a human
  being so Nicole is mortal
• An invalid argument does not absolutely guarantee
  its conclusion

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Invalid Argument

• If there is water falling from the sky then I get
  wet
• I get wet
• So there is water falling from the sky
• You could get wet in the shower

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No Guarantees but

• It is possible that an invalid argument has both
  true premises and a true conclusion.
• For example
• Nicole plays the oboe
• Nicole likes cookies
• Not all oboe players like cookies
• See the problem with the argument? The truth of
  the premises does nothing to guarantee the truth
  of the conclusion.

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IMAGINATION

• You have to imagine the premises are true and
  then see if the conclusion would have to be true.
• It doesnt matter if the premises are true.
• It doesnt even matter if the conclusion is true!

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Is the Following Valid or Invalid?

• If Im a purple fairy then I have wings
• Im a purple fairy
• Therefore, I have wings.

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IMAGINATION

• You have to imagine the premises are true and
  then see if the conclusion would have to be true.
• It doesnt matter if the premises are true.
• It doesnt even matter if the conclusion is true!

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Once Again

• An arguments being valid does not depend on its
  premises being true, only that its conclusion
  absolutely follows from the premises (if the
  premises were true the conclusion would have to
  be true!)

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Soundness

• If the premises of a valid argument are all true
  premises it is called a sound argument
• For example
• I am a teacher
• Teachers teach
• Therefore, I teach

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Strong and Weak Arguments

• Invalid arguments may be strong or weak,
  depending on the likelihood of the conclusion
  being true
• An argument is strong if, on the assumption that
  its premises are true, its conclusion is probably
  true
• A weak arguments conclusion is more likely to be
  false even if its premises are true

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Valid and Invalid Weak and Strong

• An argument is either valid or it is not
• An argument is either stronger or weaker. It
  depends on how much support for the conclusion
  the premises provide

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Deduction and Induction

• Deductive arguments are either valid or those
  their author hopes are valid.
• Inductive arguments are invalid and were not
  supposed to be valid, but they can be quite strong

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Inductive Argument Example

• I have hair
• Billy has hair
• Fred has hair
• Marga has hair
• Therefore, most of the people in this classroom
  have hair.

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Another Example

• Every president of the university since it was
  started over two hundred years ago has been male.
• Therefore, the next president will probably be
  male.

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Unstated Premises

• Sometimes premises are not explicitly stated but
  must be assumed for an argument to be either
  valid or strong.

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Example

• Nicoles dog is a bloodhound therefore, Nicoles
  dog has a keen sense of smell
• If we assume all bloodhounds have a keen sense
  of smell the argument is valid
• If we assume most bloodhounds have a keen sense
  of smell it is strong

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Identifying Unstated Premises

• After you figure out what premise is missing you
  should ask yourself if that premise requires a
  questionable assumption. If the argument can be
  made neither valid nor strong without a
  questionable assumption, reject the argument.

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Consider Again

• Nicoles dog is a bloodhound therefore, Nicoles
  dog has a keen sense of smell
• Should we assume all bloodhounds have a keen
  sense of smell? If not the argument is not
  valid.
• Should we assume most bloodhounds have a keen
  sense of smell? If not the argument is not
  strong.

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To Better Understand Arguments

• First find the premises and conclusion.
• Exclude the irrelevant sometimes people are
  trying to convince by persuasion rather than
  argument!
• Finally, if it is an argument, ask yourself how
  are the premises related to the conclusion.

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• Is it valid or strong, if it is valid is it
  sound?
• If its weak or invalid, toss it.
• If it is unsound, toss it.
• You dont need to ask yourself if an invalid
  argument is unsound to know it is not a good
  argument.

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To put this another way

• Ask yourself if the premises support the
  conclusion. If they do, then the argument is
  either (a) valid or (b) relatively strong. If
  not, toss it.
• Then, ask yourself if the premises are
  reasonable?
• Are they from a credible source?
• Do they conflict with any of your background
  information or other credible sources?
• Are they vague, ambiguous, or otherwise unclear?

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Lets Practice

• Smurfs live in mushrooms
• Smurfs are little blue men
• Some little blue men live in mushrooms.
• Is this valid or invalid, strong or weak?

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Another

• Books are made of paper
• Paper is made of trees
• Books are made of trees
• Is this valid or invalid, strong or weak?
• If it is valid is it sound or unsound?

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Yet Another

• Yesterday when I was on campus other people were
  on campus too.
• Every time I am on campus there will probably be
  other people on campus too.
• Is this valid or invalid, strong or weak?
• If it is valid is it sound or unsound?

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One More

• Everyday Ive ever been on campus other people
  have been on campus too.
• Today I am on campus
• Today there are probably other people on campus
  too.
• Is this valid or invalid, strong or weak?
• If it is valid is it sound or unsound?

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Your Job

• Think of an ethical dilemma in a business
  situation. Create an argument which tells a
  person what to do in that situation. Is it valid
  or invalid, strong or weak, sound or unsound?
• Come up with a valid argument that is sound and
  one that is unsound. Then come up with an
  invalid argument. Finally, create a strong
  argument and a weak argument. If you want you
  can make up these arguments about the same
  situation.

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• Example
• Sarahs boss has told her to fix the books at her
  firm she has to decide whether to fix the books
  or not
• If Sarah fixes the books she will be breaking a
  just law.
• Sarah shouldnt break a just law.
• Sarah shouldnt fix the books.