1
errare
humanum
est
(to err is human)
FALLACIES
PART 1:
INDUCTION
vs.
DEDUCTION
4
INDUCTION
(S  G)





think “increase” (induction, increase)
from specific/particular instance or instances
to generalization
quantity: (#) sufficient # of items or people asked,
sufficient data (stats)
quality: reliable source (authority), accurate &
in-depth observation
*PROBABILITY, not certainty*
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INDUCTION
BAD EXAMPLE:
 You compare the prices of 4 textbooks at
store #1 with those at store #2 and notice
that the prices are higher at the former
than at the latter. You conclude that store
#1 is more expensive.
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INDUCTION
GOOD EXAMPLE:

A television documentary focuses on the issue of
unwed teenage mothers in a particular city
neighborhood. Four girls are interviewed and
followed for several days. Then, a noted and
respected sociologist who has studied thousands
of unwed teenagers is interviewed, and she claims
these four girls are representative of the many.
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INDUCTION
*INDUCTIVE FALLACIES:

wrong use of data—


insufficient sample (not enough people interviewed)
ignore evidence (other possibilities)
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DEDUCTION



(G  S)
think “decrease” (deduction, decrease)
from inductive generalization
to conclusion
9
DEDUCTION
3 Propositions of a Deductive Syllogism

1) MAJOR PREMISE:



inductive generalization
“All”
All humans are mortal.
*(“syllogism”: Greek, to calculate using logic)*
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DEDUCTION
3 Propositions of a Deductive Syllogism

2) MINOR PREMISE:



statement about a specific member of that
group
“This”
My English professor is a human.
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DEDUCTION
3 Propositions of a Deductive Syllogism

3) CONCLUSION:

Major Premise + Minor Premise = Conclusion




1+2=3
S  G, G  S
“Therefore”
Therefore, my English professor is mortal.
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DEDUCTION




IF the MAJOR premise = absent or faulty
induction
IF the MINOR premise = faulty observation
concerning the individual at issue
THEN conclusion = faulty, invalid.
HOWEVER, if the opposite is true, then the
conclusion is a valid, strong one — stronger than
induction.
*CERTAINTY, validity, truth*
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DEDUCTION
*BAD EXAMPLE:

A man is sitting opposite you on a train. He
has what appears to be chalk dust on his
fingers, and you conclude that he is a
teacher.
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DEDUCTION
*BAD EXAMPLE:



Major premise = “All men with chalk on
their fingers are school teachers.”
Minor premise = “This man has chalk on
his fingers.”
Conclusion = “Therefore, this man is a
school teacher.”
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DEDUCTION
*BAD EXAMPLE:

Faulty Conclusion:

other occupations:



draftsmen, carpenters, tailors, artists, chalk maker
coach, janitor, field crew, gymnast, father, …
other powders:




flour, confectioner’s sugar/powdered doughnut, talcum
pixie dust, cocaine or heroin, anthrax or ricin
sulfate of potash, chlorine, lye, powdered milk
laundry detergent powder, hygiene powder, anhydrous salts
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DEDUCTION
*DEDUCTION & INTRODUCTIONS*






“Funnel Effect,” “Inverted pyramid”
GS
syllogism, conclusion of syllogism = thesis statement
Major Premise - Generalization = “Reducing
awareness of social differences is a desirable goal for
the school.”
Minor Premise - Narrowing = “A uniform dress code
would help to achieve that goal.”
Conclusion - Thesis = “Therefore, students should be
required to dress uniformly.”
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DEDUCTION
*DEDUCTIVE FALLACIES:




failure to follow the logic of a series of
statements
an error in one premise or both premises
other possibilities exist that were not taken
into consideration
reliance upon support other than facts
END
PART 1