Philosophy 1100
Title:
Critical Reasoning
Instructor:
Paul Dickey
E-mail Address: pdickey2@mccneb.edu
Website:http://mockingbird.creighton.edu/NCW/dickey.htm
Today: Editorial Essay #1 Due.
Next class: Midterm Exam
Chapters 1-3
Homework: AT LEAST TWO HOURS ON
QUIA ACTIVITIES !!!
1
Chapter Two, cont.
Two Kinds of Reasoning
How Do Premises Support Conclusions?
For an Deductive argument, premises prove or
demonstrate a conclusion based on if the premises
make the conclusion certainly true.
Consider the argument:
(P1) If it’s raining outside, the grass near the
house gets wet.
(P2) It’s raining outside.
_________________________
The grass near the house is wet.
In a Deductive argument, premises prove a
conclusion based on the logical form of the
statement or based on definitions. It would be a
contradiction to suggest that the conclusion is
false but the premises are true.
What is Logical Form?
Consider the following argument:
A good God cannot exist. There is evil in the world and
any God who is good would not permit evil to exist.
This argument can be stated as follows:
(Premise 1) There is evil in the world.
(P2) A God who is good would not permit
evil to exist.
____
(Conclusion) A God who is good does not
exist.
What is Logical Form?
Note that we can symbolize this argument with
variables. In this case, say for example, this
argument could be represented as:
G = A good God exists, E= There is no evil in the
world.
This argument is of the form:
If G  E
~E
_____
~G
Thus, it is a valid deductive argument. This is
the deductive rule of Modus Tollens. EVERY
argument that can be represented in this form
is valid, regardless what G and E represent.
How Do Premises Support Conclusions?
For an Inductive argument, premises support
(never prove) a conclusion based on how strongly
the premises provide evidence for the conclusion.
Consider the argument (Variation One):
(P1) When it rains outside, the grass near the house
only gets wet when the wind is blowing strongly
from the North.
(P2) The wind usually blows from the South in
Omaha.
________________________
Even though it is raining, the grass near the house is
not wet.
How Do Premises Support Conclusions?
For an Inductive argument, premises support
(never prove) a conclusion based on how strongly
the premises provide evidence for the conclusion.
Consider the argument (Variation Two):
(P1) When it rains outside, the grass near the house
always gets wet when the wind is blowing strongly
from the North.
(P2) In Omaha, the wind usually blows from the
South.
(P3) Today the wind is blowing from the North.
________________________
The grass near the house is wet.
How Do Premises Support
Factual vs. Normative Conclusions?
In regard to evaluating Inductive support for Factual
vs. Normative Conclusions, I would suggest the
following two tips to keep in mind
1) Only Factual Premises support Factual
Conclusions. That is, if the conclusion is factual (or
descriptive), ALL premises must be factual.
2) A Normative Premise is always needed to
support a Normative Conclusion. That is, if the
conclusion is normative (or prescriptive), there must
be at least one normative premise. Of course, there
may or may not be factual premises!
Chapter Three:
Vagueness
& Ambiguity
9
Vagueness
•
A vague statement is one whose meaning is
imprecise or lacks appropriate or relevant detail.
“Your instructor wants everyone to be
successful in this class.”
“Your instructor is bald.”
•
Vagueness is often evident when there are
borderline cases. Problem is not so much what the
concept is but what is the scope of the concept.
(e.g. baldness)
•
Some assertions may be so vague that they are
essentially meaningless (e.g. “This country is
morally bankrupt,” but most concepts though
vague can still be useful.
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Vagueness
•
A critical thinker will first want to clarify what is
being asserted, even before asking about what are
the reasons to believe or what is the evidence.
•
The more precise or less vague a statement is the
more relevant information it gives us.
a. Rooney served the church his entire life.
b. Rooney has taught Sunday School in the
church for thirty years.
a. The glass is half full with soda pop.
b. I poured half of a 12 oz can of soda pop into the
empty glass.
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Vagueness
•
What detail is appropriate depends on audience
or the issue. It can be difficult to determine.
Compare your friend calling you after reading an
article in the paper about mortgage rates and
telling you that you should expect to pay higher
rates vs. your bank calling you and telling you
that your mortgage rate is going up.
You are at your neighbor’s for a BBQ and you
ask him, “So how big is your yard? How far does
the property line go to? “ He says “Oh, right
behind the trees.” This is probably a good
answer. But now you are thinking about buying
his home and you ask the real estate agent the
same question. You will not be satisfied with his
“vague” answer.
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Vagueness
•
Vagueness at times is intentional and useful.
1) Precise information is unavailable and any
information is valuable.
“This word just in. There has been a shooting at the
Westroads Mall and there may be fatalities. More
information will be forthcoming as soon as
available.”
2) Precise information will serve no useful function in
the context (yes, even in a logical argument!)
Rarely, if ever, at a funeral, does a minister remind the
grieving family that their father only attended church
infrequently and showed no interest in his family
attending. Ministers who would do such a thing would
probably be considered jerks.
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Vagueness in a Logical Argument
•
The bottom line in the context of analyzing or
proposing a logical argument, a claim is vague
when additional information is required to
determine whether or not a premise is relevant.
•
Such vagueness is always a weakness and
effort must be taken to avoid it. It is generally
considered to be “hiding the evidence” when it
is done intentionally.
•
You remove vagueness by adding the relevant
detail.
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Ambiguity
•
A statement which can have multiple
interpretations or meanings is
ambiguous.
•
Examples:
“Lindsay Lohan is not pleased with our textbook.”
“The average student at Metro is under 35.”
“Jessica rents her house.”
“Alice cashed the check.”
“The boys chased the girls. They were giggling.”
15
Ambiguity
•
Of course, ambiguities can be obvious
(and perhaps rather silly)
“The Raider tackle threw a block at the
Giants linebacker.”
“Charles drew his gun.”
•
In these cases, we are not likely to be
confused. The context tells us more or
less what is meant. However, it should
be understood that it is often not good to
assume our audience will always have
the same knowledge, orientation, and
background that we do.
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Ambiguity
•
Carmen's Swimsuit Switcheroo
Frequently a logical argument is
sabotaged by a person switching
meanings in the middle of an
argument. This is known as
equivocation.
We all know what we mean by
“subjective” in this class, but we need
to be sure that the term is used
consistently and not switch to one of
the other meanings in the middle of a
a discussion.
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Ambiguity
•
•
Ambiguities can also be quite subtle, e.g. “We heard
that he informed you of what he said in his letter.”
•
One ambiguity here is whether the person (the
“you” in question) received a letter at all. Did “he”
inform “you” of what he said but only we saw a
letter to that affect, thus “we heard in his letter (to
us),” or did “we hear” that within a letter “you”
were informed and we heard that you were
informed by means of a letter to “you”?
•
Such a point might seem tedious, but could in
fact legally be very significant.
Actually, Bill Clinton had a point when he said “It
depends on what the meaning of is is.” e.g. Are you
having a fight with your husband?
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Ambiguity
•
Keep in mind that ambiguity, like
vagueness, is at times intentional and
often is useful.
1) Clever uses of “double meaning” can
catch our attention and entertain us or
provoke us to consider the claim more
carefully.
“Tuxedos cut ridiculously.”
“You can’t pick a better juice than
Tropicana.”’
“Don’t freeze your can at the game.”
“We promise nothing.”
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Ambiguity in a Logical Argument
•
The bottom line is that in the context of
analyzing or proposing a logical argument,
ambiguity is always a weakness and effort
should be taken to avoid it.
•
If you use it for “effect,” you should be
absolutely sure that the claim and your
premises are clear to your audience.
20
Ambiguity
•
Please note that while with the case of
vagueness, we resolved it by adding
information that clarified meaning, with the
case of ambiguity what we are interested in
is to eliminate the suggestion of the potential
alternate meaning that we do not desire.
•
“The Raider tackle threw a block at the
Giants linebacker.”
We want to eliminate the possibility that one
could think that one is “throwing a block (of
wood?)” Thus, we can say “ The Raider
tackle blocked the Giant’s linebacker.”
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Ambiguity
•
Let’s discuss three kinds of ambiguity.
1. Semantic ambiguity is where there is an
ambiguous word or phrase, e.g. “average” price.
-- When Barry Goldwater ran for president, his
slogan was, "In your heart, you know he's right."
In what way is this ambiguous?
2. Syntactic ambiguity is where there is ambiguity
because of grammar or sentence structure, e.g.
--“Players with beginners’ skills only may use
Court #1.”
3. Grouping ambiguity is ambiguous in that the
claim could be about an individual in the group or
the group entirely,
-- Baseball players make more money that
computer programmers.” (fallacy of division)
22
Defining Your Terms
•
Defining terms helps one avoid vagueness
and ambiguity.
Video
•
Sometimes you need to use a stipulating
definition if perhaps you are using a word in
an argument in a different way than it is
usually understood or it is a word in which
there is itself some controversy.
•
It is frequently quite reasonable in a logical
argument to accept a stipulating definition that
you would not yourself have chosen, but does
not pre-judge the issue and allows the
discussion to precede without distractions.
23
Defining Your Terms
•
Most definitions are one of three kinds:
1.
2.
3.
Definition by example.
Definition by synonym.
Analytical definition.
•
Any of these might be appropriate.
•
Be careful of “rhetorical” definitions that use
emotionally tinged words to pre-judge an
issue.
•
Do not allow someone in an argument to
use a “rhetorical definition” as a stipulative
definition. If you do, the argument will likely
be pointless and subjective.
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