Transcript Introduction to Logic

Introduction to Logic
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The two types of logos
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Definition: compiling
of evidence/reasons/
examples that support
an argument
Induction
Example: “Wow! My dog
smells terrible! She was
outside all night, and
around midnight I
smelled skunk at the back
of the house. I heard her
barking loudly, too.
That’s definitely a skunky
smell on her. She must
have tangled with that
skunk.”
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Definition: deductive
logic is built on
premises, or statements
upon which all parties
agree, which, when
considered logically,
lead to a strong
conclusion
Deduction
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Example: syllogism
(formal statement of
deductive proof)
Major premise: All
humans are mortal.
Minor premise:
Socrates is human.
Conclusion: Socrates is
mortal.
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Syllogisms are rarely found in their simplest
form.
Instead, they often appear as enthymemes.
Simply put, an enthymeme is a claim
supported by a reason expressed as a because
clause.
Syllogisms
“I hear you and Elizabeth are getting married.”
“Yes, that’s true.”
“Well, now that you’ve got a woman to cook for
you, maybe you could invite me over for dinner
sometime.”
“Why do you think Elizabeth will be doing the
cooking?”
“Because she’s a woman.”
Turn this enthymeme into a syllogism.
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Enthymeme example
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Premise: Women do all
the cooking in a family.
Premise: Elizabeth is a
woman.
Conclusion: Elizabeth will
do all the cooking. (And
presumably, her husband
will be eating better meals,
so his friend wants to be
invited over.)
Enthymeme to syllogism
“I’d better close the windows because the sky is
getting darker.”
What is the major premise, minor premise and
conclusion contained in this statement?
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Enthymeme example
Major premise: Dark
skies mean it will rain.
 Minor premise: The sky
is getting darker.
 Conclusion: It will rain.
(And presumably, I don’t
want wet carpet, so I will
close the windows.)
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Enthymeme to syllogism
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1. He must be happy because he’s smiling all
the time.
2. A nuclear war is inevitable, for our sworn
enemy, North Korea, has a hydrogen bomb.
3. He would not take the crown. Therefore, ‘tis
certain he was not ambitious. (Julius Caesar)
4. Since you didn’t speak up in my defense at
the meeting, you must be as much against me
as the rest of them are.
Enthymeme practice
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Thomas Jefferson, primary author of the
Declaration of Independence, was an educated
man well schooled in logic.
The Declaration is an extended enthymeme (it
contains premises that lead to a logical
conclusion).
After reading the Declaration of Independence,
work with your group to write the syllogism
that is the basic idea behind the document.
Remember to write it in the form of premisepremise-conclusion.
Test on syllogisms