Critical Thinking
Ch 6
Logical Fallacies II
Fallacies of Insufficient Evidence
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Lecture Notes © 2008 McGraw Hill Higher
Education
The Fallacies of Insufficient
Evidence
1.
2.
3.
4.
5.
6.
7.
8.
9.
Inappropriate Appeal to Authority
Appeal to Ignorance
False Alternatives
Loaded Question
Questionable Cause
Hasty Generalizations
Slippery Slope
Weak Analogy
Inconsistency
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Education
Inappropriate Appeal to
Authority


…occurs when an arguer cites an authority who, there is
good reason to believe, is unreliable.
Ways we can question reliability:


Are they an authority/expert?
Are they biased?
• Usually “someone having something to gain” is reason to doubt their
claim; however it can’t affect the soundness of their argument.





Are their observations questionable?
Are they generally reliable?
Are they citing their source correctly?
Does the authority disagree with expert consensus?
Lecturebe
Notes settled
© 2008 McGraw Hill
Is it an
byHigher
an authority?
3 issue that can
Education
Appeal to Ignorance

…occurs when someone claims that, the failure
to prove something false, entails that it is true.


e.g., There must not be intelligent life on other
planets. We have never found any.
Exceptions:
Fruitless Searches: If a search is exhaustive (we
looked everywhere), or extensive (we tested for
years), then a lack of evidence can be sufficient
evidence.
 Special rules: e.g., innocent until proven guilty.

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False Alternatives

Insisting that there are less choices than there actually
are.


Usually such arguments are in the form of “either/or” (like
the one above). But they can also present multiple options:



e.g., You can either vote Republican or Democrat, but you
don’t want a Democrat, so you must vote Republican.
e.g., when an argument says there are only three options
when there are actually four.
e.g. You can either work or study.
They also can be in the form of an “if then.”

e.g., If we don’t elect a Democrat the economy will go down
the tubes (sewer pipes) and we don’t want that! So we
should elect a Democrat.
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Loaded Question



A loaded question is a question that contains a
presupposition such that, either way you
answer it, you will appear to endorse that
assumption.
Examples:
 Have you stopped cheating on your exams?
When someone asks you a loaded question
the appropriate response is—not to answer it
but—have them clarify what question they
want answered (or clarify for them, and
answer each individually).
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Questionable Cause

…occurs when one claims, without sufficient evidence, that
one thing is the cause of something else.
 The post hoc fallacy: suggesting that A causes B just
because A came before B.
• e.g., I drank the ginseng tea and I was better by the next day. The tea
must have made me better.

Correlation fallacy: suggesting that the constant
conjunction of A and B entails that they are causally related.
• e.g., Every time I go to the washroom, the phone rings.

Oversimplified cause fallacy: suggesting that A is the
cause of B when clearly B has many causes.
• e.g., Employment has been weak. Government stimulus must have
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not 7been sufficient.
Education
Hasty Generalization


…occurs when one draws a general conclusion from a sample
that is biased or too small.
 Biased sample: I polled 1000 patrons from 100 malls, only 5% of
them don’t drive. I guess most Malaysians drive.
 Too small of a sample: I asked my professors if they liked MJ,
and only one did. I guess professors don’t like MJ.
If it doesn’t have a “general conclusion,” then it’s not a
generalization .
 That biker with the Terminator sunglasses and brass knuckles will
probably crush me up if I talk to him.
 Since this argument draws a conclusion about one biker, and not
all (or most) of them, it is not a “generalization” at all.
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The Slippery Slope

…fallacy is committed when one claims,
without sufficient evidence, that a
seemingly harmless action will lead to a
terrible one.

e.g., Dr Friedman has proposed that we let
AIG and GM go bankrupt. No sensible
person should listen to such an proposal. If
we allow AIG and GM to fall, the whole US
economy will collapse.
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Weak Analogy


…occurs when an arguer compares two (or
more) things that aren’t really comparable in
the relevant respect.
 e.g., Lettuce is leafy and green and good
on burgers. Poison Ivy leafy and green.
It would be good on burgers too.
Exception:
 e.g. Yingluck lives in a mansion and she
is rich. Abhisit lives in a mansion. Abhisit
is probably rich too.
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Inconsistency

…the fallacy of inconsistency is committed
when an arguer espouses two logically
contradictory claims.


e.g., Nobody goes there anymore; it’s too
crowded.
The only exception to this rule:

Laden is dead even though he isn’t.
• If you mean “he’s emotionally dead, even though he
isn’t physically dead” then you are not contradicting
yourself… you are just being unclear (by being
ambiguous).
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Tutorial
All marked ones
 Exercise 6.1, Parts 1 and 2

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Lecture Notes © 2008 McGraw Hill Higher
Education