CS 100 - Week 5 Lecture 1 - 9-18-12

*   Starting Chapter 3! - Basic Logical Concepts
    (introducing deduction and induction)

Course text, p. 53:
*   "In EVALUATING any argument,
    one should always ask TWO KEY QUESTIONS:

    1.  Are the premises TRUE? and

    2.	DO the premises provide GOOD reasons to 
        accept the conclusion?"

*   we'll return to QUESTION 1 -- "Are the premises 
    true?" -- in CHAPTER 8;

    ...in THIS chapter, we focus on QUESTION 2, 
    "Do the premises provide GOOD reasons to accept 
    the conclusion?"

TWO CATEGORIES of arguments
----------------------------
*   TRADITIONALLY, arguments have been divided into TWO types:
    *   deductive
    *   inductive

*   why does it matter?
    ...because the STANDARDS for evaluating each are 
    QUITE DIFFERENT;

*   DEDUCTIVE arguments try to PROVE their conclusions 
    with rigorous, inescapable logic;

*   INDUCTIVE arguments try to SHOW that their conclusions 
    are PLAUSIBLE or LIKELY given the premise(s).

EXAMPLE: deductive reasoning:
 
All humans are mortal.
Socrates is human.
Therefore, Socrates is mortal.

*   but there are a NUMBER of "classic" deductive
    reasoning patterns!

EXAMPLE: inductive reasoning:

Every ruby so far discovered has been red.
So, PROBABLY all rubies are red.

KEY DIFFERENCES between deductive and inductive reasoning
----------------------------------------------------------
*   DEDUCTIVE arguments claim that:		
    *   if the premises are true, then	  	    	  
	  the conclusion MUST be true
    *	The conclusion follows NECESSARILY
	  from the premise
    *	It is IMPOSSIBLE for all the premises
	  to be true and the conclusion false
    *   It is LOGICALLY INCONSISTENT to
	  assert the premises and DENY the
	  conclusion; if you ACCEPT the
	  premises, you MUST accept the
	  conclusion

*   INDUCTIVE arguments claim that:
    *   if the premises are true, then
        the conclusion is PROBABLY true
    *   The conclusion follows PROBABLY
        from the premise
    *   It is UNLIKELY for the premises to be
        true and the conclusion false
    *   Although it is logically consistent to
        assert the premises and deny the conclusion,
        the conclusion is PROBABLY true if 
        the premises are true

FOUR TESTS to HELP with determining whether an argument 
should be regarded as DEDUCTIVE or INDUCTIVE
-----------------------------------------------------------
*   the INDICATOR WORD test
*   the STRICT NECESSITY test
*   the COMMON PATTERN test
*   the PRINCIPLE of CHARITY test

*   indicator word test --
    *   note: these SUGGEST, they don't guarantee

    some common INDUCTIVE indicator words:
    *   probably 
    *   likely
    *   odds are that
    *   chances are that
    *   it is plausible to suppose that
    *   it is reasonable to assume that
    *   one would expect that
    *   it's a good bet that

    some common DEDUCTIVE indicator words:
    *   certainly
    *   definitely
    *   absolutely
    *   conclusively
    *   it necessarily follows that
    *   it logically follows that
    *   this logically implies that

*   strict necessity test

    *   An argument's conclusion either follows with
        strict logical necessity from its premises,
	or it does not;

        IF the argument's conclusion DOES follow with
        strict logical necessity from its premises,
        THEN the argument SHOULD be treated as deductive;

        IF the argument's conclusion does NOT follow
	with strict logical necessity from its premises,
	THEN the argument should NORMALLY be treated as
	inductive

    *   (we'll be discussing some exception to this
        a bit later on in this chapter...)

*   common pattern test 

    *   it turns out that deductive and inductive arguments
        often follow some characteristic patterns;
	... if you recognize one of those patterns,
	    then that is evidence about the type of that
            argument.

    *   we'll be discussing some of these patterns in this
        chapter...

*   principle of charity test

    *   mentioned last week!

    *   when interpreting an argument or passage,
        GIVE the speaker the benefit of the doubt;

        ...don't attribute a WEAKER argument when the
           evidence reasonably permits attributing
	   a STRONGER one;

        ...don't interpret a passage as a BAD argument
           when the evidence reasonably permits
	   attributing it as NOT an argument at all;

    *   this is meant as a principle of interpretation,
        NOT argument repair;