Download Class #4 - 1/8/14

Philosophy 1010
Class #4
Title:
Introduction to Philosophy
Instructor:
Paul Dickey
E-mail Address: [email protected]
Today:
Submit your two-page Socratic Dialogue “play.”
Submit your Movie Mini-essay.
Assignments for 10/2/13:
1 Read Velasquez, Philosophy: A Text With Readings,
Chapter 3, Sections 3.1 – 3.5
2. Re-read Logic Appendix. Send me an email and ask
specifica questions on what you don’t understand. (I
expect an email from everyone.)
2. Exercises at end of Logic Reading Assignment
(page 70)
Two Kinds of Good Arguments
•
1) A good deductive argument is one in
which if the premises are true, then the
conclusion necessarily (I.e. has to be) true.
•
Such an argument is called “valid” and
“proves” the conclusion.
•
For example – Julie lives in the United States
because she lives in Nebraska.
All men are mortal.
Socrates is a man.
____
Socrates is mortal.
•
A sound argument is a valid, deductive
argument in which the premises are in fact true.
How Do Premises Support Conclusions?
For a Deductive argument, premises prove a
conclusion based on the logical form of the
statement.
Consider the argument:
(P1) If it’s raining outside, the grass is wet.
(P2) It’s raining outside.
_________________________
(Conclusion) The grass is wet.
In this case, the premises support the conclusion
fully simply by what the premises say. It would
be a contradiction to suggest that the conclusion
is false but the premises are true.
Deductive Arguments:
A. Categorical Arguments
•
Categorical Logic is logic based on the
relations of inclusion and exclusion among
classes.
•
That is, categorical logic is about things
being in and out of groups and what it
means to be in or out of one group by being
in or out of another group.
•
The following is a categorical syllogism:
(Premise 1) All Americans are consumers.
(Premise 2) Some consumers are not Democrats.
(Conclusion) Some Americans are not Democrats.
B. Hypothetical Arguments
“If it’s raining outside, the grass is wet. It’s raining
outside. Thus, the grass is wet.”
We often use variables to represent statements to
analyze arguments. In this case, say for example,
R = It’s raining outside; W = The grass is wet.
and “->” as if/then,
1) Thus we have an argument of the form:
R -> W
R
_____
W
This is the rule of modus ponens.
2)
“If it’s raining outside, the grass is wet. The
grass is not wet. Thus, it is not raining.”
R -> W
~W
_____
~R
This is the rule of Modus Tollens.
C. Disjunctive Arguments
“Either it’s raining outside or the grass is dry. The
grass is not dry. Thus, It’s raining outside.”
A before, we use variables to represent statements to
analyze arguments. In this case, say for example,
R = It’s raining outside; D = The grass is dry.”
and “v” as either/or” and “~” as not.
1) Thus we have an argument of the form:
RvD
~D
_____
R
D. Chain Arguments
“If it’s raining outside, the grass is wet. If the grass
is wet, then our toddler will slip and fall. Thus, if it
is raining outside, our toddler will slip and fall.”
R -> W
W -> S
_____
R -> S
Ten Minute Break!
Two kinds of good arguments, cont.
•
2) A good inductive argument is one in
which if the premises are true, then the
conclusion is probably true, but not
always. The truth of the premises do not
guarantee the truth of the conclusion.
•
Such an argument is called “strong”
and supports the conclusion.
•
For example: Craig lives in
Nebraska and he loves football, so
he is a Nebraska Cornhusker fan.
If offered to me before class today, I would
have made a bet with my wife that each of you would
sit in the same seat in class that you did last
Wednesday. If she would have taken the bet, would I
have won more money than I would have lost?
How Do Premises Support Conclusions?
For an Inductive argument, premises support
(never prove) a conclusion based on how good the
premises provide evidence for the conclusion.
Consider the argument:
(P1) If it’s raining outside, the grass near the house
gets wet when the wind is not blowing strongly
from the North (which doesn’t often occur).
(P2) It’s raining outside.
_________________________
The grass near the house is wet.
Note: It would not be a contradiction to suggest
that the conclusion is false but the premises are
true.
How Does Sometimes
Our Thinking Crash?
Rhetoric
We are often influenced by rhetoric, language that is
psychologically persuasive but does not have pertinent
logical force.
There are many kinds of rhetorical deceptions or “devices”,
including:
hyperbole,
proof surrogates,
image rhetoric, and
euphemisms
Subjectivism
•
The view that “one opinion is as good as another,” or
“whatever is true is only what you think is true” is
subjectivism.
•
For some things, this makes sense. Does Miller taste
great?
•
To tell if something is subjective, ask yourself: “If Curtis
says “A” is true and Alicia says “A” is not true, can they
both be right?
•
One cannot give an argument either for or against a
subjective position.
•
Now, do you really believe that whether God exists is
subjective? What about other philosophical issues? Is
what is real dependent on what your friend thinks it is?
When you reach out to catch a ball, do you “really”
believe whether your friend believes the ball is not real
matters?
Logical Fallacies
are “Screw-ups” in Reasoning
Logical Fallacies can be Formal or Informal.
A formal fallacy is something like: All mothers are
women. Janice is a woman. Thus, Janice is a
mother.
This is a formal fallacy because its logical form is
invalid.
An informal fallacy is something like: Janice
believes in God. Janice is not good at algebra.
Thus, God does not exist.
That is, an informal fallacy are errors in logic
usually because the “premises” of the
argument either are ambiguous or irrelevant
to the claim.
Informal Fallacies often occur
when the purported premise is
not even relevant. (These are
known as “the fallacies of
relevance”)
They include:
Appeal to Emotion/Authority
Ad Hominems
Argument from Ignorance
Begging the Question
Wishful Thinking
The Ad Hominem Fallacy
•
Maybe the most common of all logical mistakes.
•
The Ad Hominem Fallacy mistakes the qualities of the
argument itself with the qualities of the person making
the claim. Most Ad Hominem arguments are negative.
•
In an ad hominem, a person attacks the proponent of an
argument rather than analyzing the argument itself.
http://www.youtube.com/watch?v=9emz5hpxkrw
Misplacing the Burden /
Argument from Ignorance
•
The burden of proof in an argument rests on the person
making the claim. It is her responsibility to give the
premises and the reasons to believe her claim is true.
•
To try to shift the burden of proof onto the person who is
listening to your argument and trying to make him show
that you are wrong is called misplacing the burden of
proof.
•
A particular example of this logical error is the appeal to
ignorance which suggests that we should believe
something because no one has proven or shown it to be
wrong.
Begging the Question
•
Begging the question is assuming as true the claim that
is at issue and is to be supported.
For example, God exists because the Bible says so and
we should believe what the Bible says because it was
written by God.
Another example:
An old gold miner’s joke:
One gold prospector asks the other: Why do you get
two pieces of gold for every one I get. The second
answers “Because I am the leader.” The first then
replies but why are you the leader? The second
responds: “Because I have twice the gold you do.”
Wishful Thinking
•
Our hopes, desires and personal needs can delude
us and make us vulnerable to the fallacy of wishful
thinking.
•
We should always be able to recognize when analyzing
an argument what we want to believe and be sure that
our desires are not overriding our critical thinking and
making us come to conclusions simply because of what
“we want to believe.”
•
We may want to believe, for example, that God exists so
that we might feel more secure or happy. We must thus
separate that wish from the reasons that can serve as
premises for our claim that God exists.
•
You probably don’t want to believe this, but it is likely
true: http://www.scholarspot.com/video/11916/4415/Media-MultitaskersPay-Mental-Price
Informal Fallacies also occur when it
is not recognized that the purported
premise is ambiguous. (These are
known as “fallacies of ambiguity”)
These include:
Equivocation
Amphiboly
Composition/Division
1. Equivocation: words or phrases change
meaning between premises and conclusion.
(semantic confusion)
All banks are beside rivers.
Therefore, the financial institution where I
deposit my money is beside a river.
2. Amphiboly : change of meaning due to
grammar (syntactical confusion)
One morning, I shot an elephant in my pajamas.
Thus, elephants wear pajamas.
3. Composition/Division: The confusion is in
attributing the characteristics of part (or
whole) to the whole (or part).
All the books in this library are good. Thus, this
is as a good library. (Composition)
This is a good library. Thus, you can be sure
that
all the books in this library are good. (Division)
Class Discussion – Homework
Assignment
1.
Be sure you understand the argument.
What is the claim? What are the premises
for the claim?
2.
Determine if the argument is deductive or
inductive and apply the appropriate test
either for validity or strong support.
3.
Identify and weed out any logical fallacies,
rhetoric, subjectivity, or irrelevancies. Clarify
any vagueness or ambiguity.
4.
Examine the truth of the premises. If the
argument is inductive, evaluate the
evidence.