PHL 105Y September 20, 2004

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PHL 105YSeptember 20, 2004

• For Wednesdays class, finish reading the Weston
  book
• Tutorials will begin Friday, September 24. You
  will be given an assignment on Wednesday to
  complete and take to tutorial on Friday.
• 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. Read the syllabus
  carefully. The most important piece of advice on
  the syllabus is to do the reading before coming
  to class. Its also important to read the
  warning about plagiarism. I eat plagiarists for
  breakfast. My teeth are sharp. I have no mercy.
• Announcements we need two class representatives
  -- an EPUS volunteer (full time students) and an
  APUS volunteer (part-time). As a volunteer for
  these important student groups you have to attend
  one meeting a year (maybe two) you get to speak
  up for your fellow students on issues like
  parking, class availability, and ..fill in your
  favourite issue here. You get a certificate
  signed by the Principal and the Dean for being a
  class rep.
• AMSA is hosting a forum on human rights in
  religion tomorrow night 7-10 at the Kaneff
  Centre. Speakers from various religions will
  present their views.

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Dont take these powerpoint slides too seriously

• The slides are a mere starting point, a bare
  outline of questions we will consider they do
  not contain the contents of the lectures.
• If you do feel compelled to take them seriously,
  remember you dont have to write down everything
  you see on the screen
• The slides are posted (within a week) to the
  course website
• http//www.utm.utoronto.ca/jnagel/105.htm
• You do not need to have the PowerPoint Software
  to see the slides at home they run fine on the
  free internet browser Internet Explorer.
• You are entitled and encouraged to use the
  computers in the Library. (Also try the
  Humanities Computer Lab, North Building 161)

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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 3

• 3. 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 4

• 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 sample argument 4

• 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.

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Sample Argument 5

• 5. Throw several pieces of steel together,
  without shape or form they will never arrange
  themselves so as to compose a watch. Stone, and
  mortar, and wood, without an architect, never
  erect a house. But the ideas in a human mind, we
  see, by an unknown, inexplicable economy, arrange
  themselves so as to form the plan of a watch or
  house. Experience, therefore, proves, that there
  is an original principle of order in mind, not in
  matter.
• --David Hume, Dialogues Concerning Natural
  Religion

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Sample Argument 6

• 6. When reprisal comes as soon as possible after
  the event, it immediately bars the way against
  people who are strongly inclining towards making
  use of their badness while it is in full flow.
  For deferment of the debt of punishment is not
  only more debilitating to the injured partys
  hopes and more depressing than deferment of any
  other kind of debt, but it is also the best boost
  to the wrongdoers daring and audacity on the
  other hand, retaliation which wastes no time in
  challenging audacity not only deters future
  crimes, but is inherently also the greatest
  possible consolation to the victim.
• --Plutarch, On Gods slowness to Punish