PHL 105Y September 19, 2005

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PHL 105YSeptember 19, 2005

• For Wednesdays class, finish reading the Weston
  book
• If you are here for the first time, welcome. Take
  a syllabus from the desk at the front, as well as
  the reading assignment sheet. You will need to
  sign up for a tutorial section by Friday follow
  the instructions on the website.
• Tutorials will begin Friday, September 23. There
  is a one-page practice assignment to complete and
  take to tutorial on Friday.
• TUT0201Fri 1100 1200 NE 266    
• TUT0202Fri 1200 1300 NE 266    
• TUT0203Fri 1000 1100 NE 152    
• TUT0204Fri 1400 1500 NE 152    
• TUT0205Fri 1500 1600 NE 143    
• TUT0206Fri 1200 1300 NE 152

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PHL105Y Logic unit

• Our aim to continue our brief survey of some
  techniques for analyzing the form of arguments,
  and to apply these techniques to a few arguments

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Westons rules

• Distinguish premises and conclusion
• Present your ideas in a natural order
• Start from reliable premises
• Be concrete and concise
• Avoid loaded language
• Use consistent terms
• Stick to one meaning for each term (avoid fallacy
  of equivocation)

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Westons rules, continued

• Arguments by example
• Give more than one example
• Use representative examples
• Background information is crucial
• Consider counterexamples
• Arguments by analogy
• Analogy requires a relevantly similar example

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Westons rules, continued

• Arguments from Authority
• 13. Sources should be cited.
• 14. Seek informed sources
• 15. Seek impartial sources
• 16. Cross-check sources
• 17. Personal attacks do not disqualify a source
  (avoid the ad hominem fallacy)

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Westons rules, continued

• Arguments about causes
• 18. Explain how cause leads to effect
• 19. Propose the most likely cause
• 20. Correlated events are not necessarily related
• 21. Correlated events may have a common cause
• 22. Either of two correlated events may cause the
  other
• 23. Causes may be complex

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Westons rules, continued Valid Deductive
Arguments

• In a VALID deductive argument, IF the premises
  are true, the conclusion MUST also be true. A
  valid deductive argument CANNOT simultaneously
  have true premises and a false conclusion
• In calling an argument valid we are saying that
  IF you accept the premises, you must accept the
  conclusion (a valid argument can have a false
  conclusion, as long as it has at least one false
  premise)
• An example of a valid argument
• Either the butler or the maid disposed of the
  body.
• If the maid disposed of the body, her shoes would
  have been muddy.
• The maids shoes were not muddy.
• The butler disposed of the body.

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VALIDITY, continued

• If there is any logically possible way that the
  premises could be true, and the conclusion false,
  then the argument is deductively INVALID
• INVALID
• If Germany does not win the medal, then either
  France or Kenya will win.
• Kenya will not win the medal.
• So France will win the medal.
• Fred is a Canadian.
• 99 of Canadians have telephones.
• Fred has a telephone.

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Validity and invalidity whats counter-intuitive

• Perfectly valid arguments can look quite strange.
  You might find a valid argument with premises
  you would reasonably feel very reluctant to
  accept, but what matters in evaluating validity
  is just whether if you could bring yourself to
  accept those premises, you would then be forced
  to accept the conclusion as well. So this
  argument is valid (believe it or not)
• VALID
• If Paul Martin is a penguin, then he lives on
  Mars.
• Paul Martin is a penguin.
• Paul Martin lives on Mars.
• Likewise, deductively invalid arguments can look
  quite appealing (and causal or statistical
  arguments can be plausible without being
  deductively valid). We dont REQUIRE deductive
  validity in a causal or statistical argument. We
  do require it in an argument that puports to
  offer a logical demonstration of its conclusion.

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SOUNDNESS

• A SOUND argument is a deductively VALID argument
  with all TRUE premises.
• Soundness is a higher standard than deductive
  validity all sound arguments are deductively
  valid, but not all deductively valid arguments
  are sound
• (Which deductively valid arguments are unsound?)

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Westons rules, continued Valid Deductive
Arguments

• 24. Modus ponens
• If p then q
• p
• Therefore, q

• 25. Modus tollens
• If p then q
• Not q
• Therefore, not p
• Tip put in a whole sentence for p and another
  for q keep each the same throughout a given
  argument

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Two fallacies to avoid affirming the consequent,
denying the antecedent

• Affirming the consequent
• If p then q
• q
• Therefore, p
• Example If Robert steals, then he is breaking
  the law. Robert is breaking the law. Therefore
  he is stealing.

• Denying the antecedent
• If p then q
• Not p
• Therefore, not q
• Example if Jane is taking chemistry, then she is
  a student. Jane is not taking chemistry.
  Therefore, Jane is not a student.

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Westons rules, continued Valid Deductive
Arguments

• 26. Hypothetical syllogism
• If p then q
• If q then r
• Therefore, if p then r

• 26. Disjunctive syllogism
• p or q
• Not p
• Therefore, q

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Westons rules, continued Valid Deductive
Arguments

• 26. Dilemma
• p or q
• If p then r
• If q then s
• Therefore, r or s

• 27. Reductio ad absurdam
• To prove p
• Assume the opposite not-p
• Argue that this assumption leads to a
  contradiction or absurdity
• Conclude p must be true after all

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Variation on a theme the constructive dilemma

• Heres a standard dilemma argument
• p or q
• If p then r
• If q then s
• Therefore, r or s

• Heres a constructive dilemma argument
• p or q
• If p then u
• If q then u
• Therefore, u
• How does that work?

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Westons rules, continued Valid Deductive
Arguments

• 30. Deductive arguments can operate in steps
• - a single argument can employ many of the forms
  we have learned, in sequence.

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Test yourself what arguments forms are at work
here?

• If OPEC increases production, then gas prices
  will fall. If gas prices fall, then the
  government will not cut taxes. So, if OPEC
  increases production, the government will not cut
  taxes.
• Today is either Saturday or Sunday. Today is not
  Sunday. Therefore, today is Saturday.
• If Rhonda has taken Statistics, she has satisfied
  her quantitative analysis requirement. Rhonda
  has not satisfied her quantitative analysis
  requirement. Therefore, Rhonda has not taken
  Statistics.

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Test yourself what arguments forms are at work
here?

• Either you are alive or you are dead. If you are
  alive, you do not feel the pain of death. If you
  are dead, you do not feel anything at all.
  Therefore, either you do not feel the pain of
  death, or you do not feel anything at all.
• Assume that my client committed this crime. We
  know that he was in Montreal at 10 am, and that
  the crime was committed in Miami at 11 am. It is
  not possible to travel from Montreal to Miami in
  one hour. Therefore, my client did not commit
  this crime.
• Practice composing arguments in each of the valid
  forms we have learned.

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Back to real philosophical arguments

• NOTE Real philosophical arguments arent always
  deductively valid
• Sometimes, this is because the philosopher has
  made a stupid mistake.
• Sometimes, this is because the philosopher isnt
  trying to give a deductive argument. For
  example, arguments by analogy or example are not
  deductively valid they may still be persuasive,
  if the analogy is good, or the examples
  sufficient.

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Analyzing arguments

• What point is the author attempting to make?
  (sort premises from conclusions)
• And what kind of argument(s) has he or she
  attempted to offer here?
• Notice that forms can be combined a single
  passage can exhibit a causal argument, two rounds
  of modus ponens and a hypothetical syllogism

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Sample Argument for discussionWhat is the
analogy here and how appropriate is it?

• How men, whose plentiful fortunes allow them
  leisure to improve their understandings, can
  satisfy themselves with a lazy ignorance, I
  cannot tell but methinks they have a low opinion
  of their souls, who lay out all their incomes in
  provisions for the body, and employ none of it to
  procure the means and helps of knowledge who
  take great care to appear always in a neat and
  splendid outside, and would think themselves
  miserable in coarse clothes, or a patched coat,
  and yet contentedly suffer their minds to appear
  abroad in a piebald livery of coarse patches and
  borrowed shreds, such as it has pleased chance,
  or their country tailor (I mean the common
  opinion of those they have conversed with) to
  clothe them in.
• --John Locke, An Essay Concerning Human
  Understanding 4.20.6

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Sample Argument for analysisWhat is the
conclusion? What is the overall form?

• 4. Let no one be slow to seek wisdom when he is
  young nor weary in the search thereof when he is
  grown old. For no age is too early or too late
  for the health of the soul. And to say that the
  season for studying philosophy has not yet come,
  or that it is past and gone, is like saying that
  the season for happiness is not yet or that it is
  now no more. Therefore, both old and young ought
  to seek wisdom, the former in order that, as age
  comes over him, he may be young in good things
  because of the grace of what has been, and the
  latter in order that, while he is young, he may
  at the same time be old, because he has no fear
  of the things which are to come.
• --Epicurus, letter to Menoeceus

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Grasping the underlying form of the complex
sample argument

• You are either young or old.
• If you are young, you ought to seek wisdom (so as
  not to fear the things which are to come).
• If you are old, you ought to seek wisdom (so as
  to enjoy the grace of what has been).
• You ought to seek wisdom.