Transcript Philosophy 103 Linguistics 103 Introductory Logic

Philosophy 103
Linguistics 103
Even More
Introductory Logic:
Critical Thinking
Dr. Robert Barnard
Last Time:
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Basic Concepts:
Deductive/Inductive
Valid/Invalid
Strong/Weak
Sound
Cogent
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Started Informal Fallacies
Plan for Today
Try to wrap up Informal Fallacies
Time Permitting: Some Philosophical Issues
Fallacies Review
• Fallacy =df “A fallacy is a mistake in an
argument which consists in something other
than merely false premises”
• Fallacies of Relevance
– Appeal to Force (Ad Bacculum)
– Appeal to Pity (Ad Misericodium)
– Appeal to the People (Ad Populum)
4. Argument against the
Person (Ad hominem)
Attacking the source of an argument
instead of the argument itself.
4. Argument against the Person (Ad
hominem) (2)
Ad Hom. Abusive: John Kerry says that we
should spend more state revenue on
education because doing so would result in a
more productive workforce. But Kerry is a
bleeding-heart, liberal Yankee from
Massachusetts -- so you know that his
opinion is worthless.
4. Argument against the Person (Ad
hominem) (3)
Ad Hom. Circumstantial: Barnard says
that we should spend more state
revenue on education. But Barnard is a
professor who wants a better salary -so you know that his opinion is
worthless.
4. Argument against the Person (Ad
hominem)(4)
Ad Hom From Hypocrisy: You've claimed that
smoking is bad for one's health; but you
smoke too.
Think about:
4. Argument against the Person (Ad
hominem) (5)
Think about:
Credibility: if a person with low credibility asserts something
without supplying evidence for it, then we should withhold
judgment.
Reasons: if the person does supply reason for the claim, then we
still need to look at those reasons and evaluate whether they
support the conclusion in question.
Contradictory beliefs. If we can show this, then we have indeed
supplied a good reason to believe that the person is confused.
People can change their minds. Changing your mind is fine;
contradictory beliefs are not.
5. Accident
Applying a general rule to a case it was not
designed to cover.
Example: Killing is bad: therefore, it was wrong
for us to go to war against the Nazi's.
6. Straw Man
Attacking an oversimplified version of an
opponent's actual position.
Example: Those who support gun control are
wrong; they believe that no one should have
the right to defend themselves in any situation.
7. Missing the Point/Red
Herring
Two closely related fallacies, which involve
diverting the listener's attention by changing the
subject or drawing a slightly different conclusion
than the one that should be drawn.
Example: The death penalty is the only way to
punish criminals. Why? Because the justice system
in this country has gone straight to hell -- what
with murderers, rapists and robbers getting off
scot-free! It has got change!
Another (Common) Red Herring:
Tom: Johnson wants the government to pay its
bills, and not borrow anymore money.
Joe: But Johnson is a tax-and-spend biggovernment politician. Anything he says must
be wrong!
Note: Red Herring bleeds into Ad Hom.
Fallacies of Weak Induction
With these sorts of fallacies, the problem is
that the premises provide extremely weak
support for the conclusion. They often disguise
this fact by involving an emotional appeal of
some sort.
1. Argument from
Unqualified Authority
Arguing for a conclusion based on the
testimony of someone who is not qualified to
speak on the relevant subject.
Example: Be careful and look out for lions
when you go hunting next weekend; Coach O
says that lions migrate south during the winter
in the United States.
2. Appeal to Ignorance
Drawing a conclusion based on a premise which states
that nothing has been shown.
Example: No one has ever proven that ghosts don't
exist. Therefore, they obviously do.
What I saw on “Meet the Press”
Russert: So can you produce actual
evidence that there were WMDs in Iraq?
Talking Head: Can you prove there weren’t?
Russert’s Problem: HE DID NOT CALL THE TALKING
HEAD ON THE FALLACY!!!
Exception: Arguments from Ignorance
For Ignorance
Example: You have consistently failed to
demonstrate your knowledge of the material
on the exam. Therefore, I don't think you
know the material.
(Ignorance IS evidence of ignorance…)
3. Hasty Generalization
A very bad inductive generalization.
Example: All three of the Ole Miss students I've
met so far have been from Mississippi; so there
must be no out-of-state students here.
4. False Cause
Stating that there is a causal connection when one probably
does not exist. There are different types:
Arguing from Coincidence: Example: When I've used my lucky
pen before, I've passed the test; therefore I'll fail if I don't use
that pen. (Post Hoc Ergo Propter Hoc)
Oversimplifying the Cause: Example: Our society is filled with
violence and there is a lot of violence on TV. It is obvious that
the violence in society is caused by people watching television.
.
4. False Cause (2)
Slippery Slope: This is a very common variety of the
false cause fallacy; it involves believing without
any supporting reasons that X will lead to Y.
“Slippery Slope” Example
Legalizing marijuana will lead to the legalization
of cocaine. If you legalize cocaine, you'll be
able to buy crack and every other drug at your
local 7-11.
(In this argument, it is asserted that the legalization of
marijuana will lead to (by degrees) the legalization of
every drug. Once one accepts the legalization of
marijuana, then one is assumed to be on the slippery
slope towards the legalization of every drug)
4. False Cause (3)
Sometimes slippery slope arguments are
justifiable: for instance, if the reasons to
accept x are just the same reasons that
would lead you to accept y, then in accepting
x, one should also accept y.
BUT, this is because the reasons for X actually
support Y!
5. Weak Analogy
Making a weak analogy, or unfairly
comparing one thing to something else. It
is very difficult to evaluate analogies with
any degree of precision.
Example: Philosophy 101 is a philosophy class
and has a lot of discussion; Logic is a
philosophy class. So, it must also have a lot of
discussion
An example of Weak Analogy
Fallacies of Meaning and
Ambiguity
Fallacies of this variety turn upon
mistakes of and imprecision in
language
1. Begging the Question
This can occur in several different forms. Essentially,
this fallacy occurs when the key premise of an
argument is unsupported. Here are some varieties of
this common fallacy:
Circular reasoning: Murders have lost the right to live
because anyone who takes the life of another person
has given up that right.
1. Begging the Question (2)
• Concealed Premise: Murder is always
wrong. Therefore, the death penalty is
wrong. (The concealed premise: The
death penalty is murder).
1. Begging the Question (3)
• Wishful Thinking: Of course there is life
after death; if I didn't believe that, life
would be too depressing.
Dilbert begs the question…
2. Complex Question
Sometimes called a "loaded question". A
question which contains a hidden assumption
or condition.
Often, complex questions are such that no
matter how you answer them, you may be
acknowledging something you might not want
to acknowledge.
Complex Question Examples
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When did you stop lying to your friends?
When are you going to give up being a Nazi?
When did you stop beating your dog?
How long has it been since your last illicit
affair?
3. False Dichotomy or False
Dilemma
Presents as a premise two alternatives as if
they were the only two available when in fact
there are more. Often the conclusion is only
implied and not stated.
Example: Either we elect Mr. X or the economy
goes down the tubes. The choice should be
obvious.
Avoid the False Dichotomy!!
The way to avoid falling into this trap:
Before you accept X because Y is false, make
sure there isn't some other alternative that
allows you to reject X as well.
4. Suppressed Evidence:
An inductive argument which ignores overriding
evidence which would prove a different
conclusion.
• This is common is advertising.
Example: Rent-to-own: the cheaper way to
buy!
• Quoting out of context can also lead to this
fallacy, as can ignoring current events.
5. Equivocation
• Where the conclusion of the argument
depends on the fact that a word is being used
in two different senses due to semantic
ambiguity (one word having two or more
definitions).
Example: Every child is a special person. Every
person should vote against the school bond.
Therefore, every child should vote against the
school bond.
6. Amphiboly:
• Where the conclusion of the argument depends on
the fact that a sentence is syntactically ambiguous.
(i.e. the sentence allows for more than one
interpretation of its meaning)
• Ambiguity Example: John attacked the man with a
knife.
• Fallacy Example: Norris said he operates a small car
repair shop. Therefore, you can't take your Cadillac to
him.
(This can be a real problem in legal documents).
7. Composition:
• Mistaking properties of the parts for
properties of the whole.
• Example: Every member of the team is a
winner; therefore the team is a winner.
• Not every instance of this type of reasoning is
bad.
• Additive Quality Example: “Each one of these
stamps is valuable. Therefore, the collection of
stamps as a whole is valuable.”
Even a Viking can do it…
8. Division:
• Mistaking properties of the whole for properties of
the parts.
• Examples:
– The Congress is based in Washington D.C.
therefore each member of Congress is from D.C.
– This football team is the best in the conference;
therefore the quarterback is the best in the
conference.
• Again, not every instance of this reasoning is bad.
Pay attention to the context and the details.
Is the sun coming up,
or going down….?
The “Laws of Thought”
1) Identity
2) Non-Contradiction
3) Excluded Middle
Are They all both General and
Necessary?
The Law of Identity
“A true statement is true.”
“All A is A.”
“Everything is what it is, and not
something else.”
“Everything is self-identical.”
The Law of Non-Contradiction
“ Nothing A is not A” (a form of ‘identity’?)
“No statement is both true and false at the same
time.”
“Nothing is both F and not-F at the same time.”
“Opposite qualities are incompatible.”
“Everything F is not not-F.”
The Law of Excluded Middle
“Every statement must be either true or false.”
“If something is F then it is not not-F.”
“Either F or not-F.”
Logic and Psychology
• Where do the laws of thought come from?
• Are they generalizations upon experiences?
• Could we arrive at their general correctness
without having a variety of experiences?
Logic and Rhetoric
Rhetoric is the art of Persuasion
Logic can be a part of Persuasion
Informal Fallacies are persuasive cases that
violate logic.
Logic Teaches us to know the difference.