Transcript Logic - CinnamonPhilosophy

Intro to Logic: the tools of the trade
You need to be able to:
Recognize an argument when you see one
(in media, articles, people’s claims).
Organize arguments into a proper
argument outline.
Identify characteristics of an argument:
premise, conclusion, assumptions, and
consequences.
Know the difference between deductive
and inductive argument forms.
Know some common informal fallacies.
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Round One
Definitions:
Argument
Proposition
Premise
Conclusion
Conditional statement
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http://www.youtube.com/watch?v=kQFKtI6gn9Y
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http://www.youtube.com/watch?v=kQFKtI6gn9Y
• Monty Python Argument Clinic.
An argument is not just contradicting
your opponent. (A contradiction an automatic
gainsaying of any statement that another person makes).
An argument is a connected series of
statements intended to establish a
proposition.
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Vaughn’s definition of an argument:
A combination of statements in which
some of them [the premises] are
intended to support another one of them
[the conclusion].
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Deductive Logic/
Inductive logic/
Informal fallacies
Argument intended
To guarantee
The truth of its conclusion.
IF the premises are true,
Then the conclusion is
GUARANTEED
To be true.
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Example of an argument:
It is good to take a philosophy class. It
develops your critical thinking skills.
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Example of an argument:
“It develops your critical thinking skills”
is intended to support, or give reason
for:
It is good to take a philosophy class.
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Example of an argument:
Premise: Taking philosophy develops your
critical thinking skills.
-----------------------------------Conclusion: It is good to take a philosophy
class.
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Check for unstated assumptions.
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Example of an argument:
Premise: It is good to develop your critical thinking
skills. (Unstated Assumption)
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Outlining an argument..
Premises and conclusions are written as propositions:
statements that are either true or false.
Propositions consist of the subject clause, and predicate
clause (or property about that subject).
I.e. “All cats are grey.”
“all cats” is the subject, “are grey” is the predicate.
“Philosophy classes help you think well.” can be true or
false.
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Conditional “IF Then” statements.
“If you want to develop critical thinking skills, then you
should take a philosophy class.”
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P1. “If you want to develop critical thinking skills, you should
take a philosophy class.”
P2. you want to develop critical thinking skills
C. You should take a philosophy class.
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Outlining an argument
P1. “If you want to develop critical thinking skills, you should
take a philosophy class.”
P2. you want to develop critical thinking skills.
C. You should take a philosophy class.
A= “you want to develop critical thinking skills.”
B= “You should take a philosophy class.”
P1: If A then B
P2: A
C: Therefore, B
Valid, modus ponens
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Which are premises?
Which is the conclusion?
It must have rained last night.
The road is wet.
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P: If the road is wet, then it rained last night (conditional
inference statement)
P: The road is wet (empirical evidence).
--------------------------
C: It must have rained last night (conclusion).
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P1. If the road is wet, then it rained outside.
P2. The road is wet
C. Therefore, it rained last night.
-----------------This argument can be written
P1. If A then B
P2. A
C. Therefore, B
Where A = “the road is wet,”
And B= “it rained last night”
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Premises
P 1. If the road is wet, then it
rained last night (implied or
assumed)
P 2. The road is wet (empirical
evidence).
Conclusion
C. It rained last night
(conclusion).
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How to recognize an argument:
•Indicator words for
premises:
--because
--since
--for
--given that
--the reason being
Indicator words for
conclusions:
Consequently
Thus
Therefore
As a result
Hence
Vaug
hn.
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Identifying arguments in articles.
A thesis statement is the author’s conclusion.
It is what the author will convince you about.
Usually articles have two or three premises
to support the conclusion.
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Creating arguments
Your own essays will have one conclusion/
thesis statement about two arguments
supporting the thesis.
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Commercials
American Express
http://www.youtube.com/watch?v=UwXO22U8jEs
Direct TV (slippery slope)
http://www.youtube.com/watch?v=7udQSHWpL88
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P1. If you want to be like Sean White, then use American
Express.
P2. (assumed). You want to be like Sean White.
C. Use American Express.
A= you want to be like Sean White
B= you use American Express.
If A then B
A
Therefore B.
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c
In Deductive format:
P1: If you buy cable, you’ll end up in a roadside ditch.
P2. You don’t want to end up in a roadside ditch.
Conclusion: Don’t buy cable.
P1. Either buy cable or Direct TV.
P2. Don’t buy cable.
C. Buy Direct TV.
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Slippery slope.
P1. If you buy cable, you’ll end up in a roadside ditch.
Step A. When you use cable, they put you on hold
B. You get angry and blow off steam
C. when you blow off steam, accidents happen
D. When accidents happen, you get an eye patch
E. When you get an eye patch, people think you’re tough
F. when people think your tough, they want to see how
tough.
Z. When people want to see how tough, you end up in a
roadside ditch.
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Critiquing arguments
1. Check to see if the reasoning is good (does the
conclusion follow from the premises?).
2. Check for soundness (e.g. are the premises true?).
3. Check for strong or weak sample sizes (if it is an
inductive argument).
4. Check for unstated assumptions in the argument.
5. Check for unwanted or absurd consequences of an
argument (i.e. assume the argument is sound).
6. Check for informal fallacies.
Wason Selection task
http://www.philosophyexperiments.com/wason/Default.aspx
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Validity vs. Soundness
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The FORM or REASONING of an argument can be
separated from the CONTENT.
Even if the premises are true, the reasoning can be
bad.
And, even if the premises are false, the reasoning
can be good.
Validity = form of argument
Soundness= truth of propositions
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Two valid deductive structures:
Modus ponens:
If A, then B,
A,
Modus tollens:
If A, then B,
Not B,
Therefore, not A.
Therefore B.
Two Invalid structures:
Affirming the consequent:
If A, then B,
B,
Therefore A.
Denying the antecedent.
If A, then B,
Not A,
Therefore Not B.
False premises but the reasoning is good.
Wason Selection Task
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False premises, good reasoning.
P1. All pigs are pink.
P2. This is a pig.
C. Therefore, this pig is pink.
(Valid modus ponens; unsound)
False premises, but good reasoning.
P1. If the sun rises in the east, then pigs fly.
P2. The sun rises in the east.
C. Therefore pigs fly.
True premises, bad reasoning
P1. If Bill Gates lives in Washington, then Bill Gates lives in
the USA.
P2. Bill gates lives in the USA.
C. Therefore, Bill Gates lives in Washington.
(Invalid: Affirming the consequent).
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True premises, bad reasoning.
P1. If Jones stands in the heavy rain without an umbrella, then
Jones will get wet.
P2. Jones is wet.
C. Jones was standing in the heavy rain without an umbrella.
P1. All bathrooms have toilets in them.
P2. This room has a toilet in it.
C: This room is a bathroom.
P1. all toilets are in bathrooms
P2. this room has a toilet in it
C this is a bathroom
P1. All bathrooms have toilets in them.
P2. This room has a toilet in it.
C: This room is a bathroom.
------------------------
P1: All A’s are B’s
P2: B
C: A
Counterexample: Home Depot has a toilet in it, but it
doesn’t mean that it’s a bathroom.
Critiquing arguments
1. Check to see if the reasoning is good (E.g. does the
conclusion follow from the premises?).
2. Check for soundness (e.g. are the premises true?).
3. Check for strong or weak sample sizes (if it is an
inductive argument).
4. Check for unstated assumptions in the argument.
5. Check for unwanted or absurd consequences of an
argument (i.e. assume the argument is sound).
6. Check for informal fallacies.
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