CS 100 - Week 6 Lecture 2 - 9-27-12

Chapter 5 - Logical Fallacies, Part 1

logical fallacy - an argument that contains a mistake in reasoning
 
...a fallacious argument contains one or more logical fallacies;

*   we'll be discussing MANY (not all) of the most common types
    of logical fallacies;

*   why learn about these?
    *   they are FREQUENTLY committed;
    *   unfortunately, they are often psychologically persuasive --
        people may very be swayed by them,
	even though the argument hasn't ACTUALLY proven its claim;

*   one (of several) ways to classify/categorize these is into
    TWO broad groups:
    *   fallacies of RELEVANCE  <-- discussed in Chapter 5
    *   fallacies of INSUFFICIENT EVIDENCE <-- discussed in Chapter 6

    *   fallacies of relevaance are mistakes in reasoning
        that occur because the premises are LOGICALLY IRRELEVANT 
        to the conclusion
    
    *   fallacies of insufficient evidence are mistakes in reasoning
        that occur because the premises, though relevant to the
        conclusion, FAIL to provide SUFFICIENT EVIDENCE to support
        the conclusion;

SO -- focusing of fallacies of relevance,
let's start with: what is relevance?
------------------------------------------
*   logically, to say that one statement is RELEVANT to another
    is to say that it COUNTS either FOR or AGAINST that statement;
    or, if provides at least SOME reason for thinking that the second
    statement is TRUE or FALSE;

*   THREE ways that a statement can be relevant or irrelevant to another
    statement:
    *   positively relevant - counts in FAVOR of the other
    *   negatively relevant - counts AGAINST the other
    *   irrelevant - counts NEITHER for NOR against the other

*   positively relevant -
    counts in FAVOR of the other,
    one statement that DOES provide reason to think that another is true;

    *   Dogs are cats.
        Cats are felines.
        So dogs are felines.
        *   this pair of premises DO provide reason to support the conclusion

        *   this pair of premises is positively relevant to the
            conclusion;

        *   (note, you CAN have positive relevance even in an
            unsound or uncogent argument)

        *   (note, the CONTEXT can make a difference --
	    this PAIR of premises, together, are positively relevant
	    to this conclusion --
	    the FIRST alone -- if that is all there were:
	    Dogs are cats. So dogs are felines. <-- premise ISN'T
                                                    positively relevant here;

*  negatively relevant
   *   one statement counts against the other,
       one statement being true provides reason to think another
       is actually false;

   Marty is a high-school senior. So, Marty probably has a Ph.D.

   ...this premise actually provides REASON to think that this 
      conclusion is false;

*   irrelevant
    *   counts NEITHER for NOR against;
    *   premise doesn't provide reason to think the conclusion
        is true OR false;
  
    The earth revolves around the sun. Therefore, colas should contain
    less caffeine.

    I dreamed the Rangers won the pennant. Therefore, the Rangers
    will win the pennant.
    
    in BOTH cases, the premise really doesn't provide evidence
    for thinking the conclusion is true or false;

*   SO -- note!
    in Fallacies of RELEVANCE, the arguer offers reasons that are
    LOGICALLY IRRELEVANT to his/her conclusion;

    ...chapter describes ELEVEN!! 11 of these;

1.  Personal attack (ad hominem)
    *   when one rejects someone's argument or claim by attacking
        the PERSON rather than the person's argument or claim.

    ex:   X is bad. Therefore, X's argument must be bad.

2.  Attacking the motive 
    *   when one criticizes a person's motivation for offering a
        particular argument or claim, rather than examining the
        worth of the argument or claim itself

    ex: X is biased or has questionable motives -- therefore,
        X's argument or claim should be rejected.

3.  Look who's talking (tu quoque)
    *   when you reject an argument or claim because the arguer
        fails to practice what he/she preaches.

4.  Two wrongs make a right 
    *   when an arguer attempts to justify a wrongful act
        by claiming that some other act is bad or worse