SPOTTING FAULTY LOGIC
Judging deceptive or faulty arguments in persuasive texts
DO YOU BELIEVE EVERYTHING YOU SEE?
DO YOU BELIEVE EVERYTHING YOU READ?
Advertisements are designed to get you to buy
things. Do you buy what you see on every ad?
 Newspaper and magazine editorials are written
to get readers to share the author’s point of view.
Do you agree with everything you read?
 Letters to the editor are citizen’s responses to what
they’ve read in print media. Do you believe
everything every stranger writes?
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DO YOU BELIEVE EVERYTHING YOU READ?
If you answered to no to each of the previous
questions (and I hope you did!), why did you
answer that way?
 If you think about people’s reasons for advertising
and writing you’re off to a great start. However,
if you begin to look at specific ways people to try
to persuade you to buy, feel, think, or do certain
things, you’ll be able to make more informed
decisions.
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WHY MIGHT AN AUTHOR OR SPEAKER BE
DISHONEST?
 They may earn money by getting you to buy
something or pay for a service.
 They may gain power or influence by having you
as a follower.
 Their cause may gain publicity the more followers
they acquire.
 They may want you to believe as they do because
they are convinced their belief is correct.
 They may want you to do something for them.
WORDS TO KNOW
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Anecdote
Analogy
Appeal
Deceit/ deceitful
Deceptive
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Fallacy
Faulty
Logic
Premise
Specious
MAKING A LOGICAL ARGUMENT
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Deductive arguments are supposed to  The classic example of a
be water-tight. For a deductive
deductively valid argument is:
argument to be a good one (to be
“valid”) it must be absolutely
(1) All men are mortal.
impossible for both its premises to be
true and its conclusion to be false.
(2) Socrates is a man.
Any deductive argument that fails to
Therefore:
meet this (very high) standard commits
(3) Socrates is mortal.
a logical error, and so, technically, is
a fallacy.
This includes many arguments that we  It is simply not possible that
would usually accept as good
both (1) and (2) are true and
arguments, arguments that make their
(3) is false, so this argument is
conclusions highly probable, but not
certain. Arguments of this kind,
deductively valid.
arguments that aren’t deductively
valid, are said to commit a “formal
fallacy”.
TYPES OF FAULTY ARGUMENTS
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Ad hoc, ergo propter ad
hoc
Ad hominem
Ad ignorantiam
Appeal to wrong
authority
Card-stacking
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Circular reasoning
Either/or fallacy
False analogy
False authority
Inconsistency
Personal incredulity
Straw man
AD HOC, ERGO AD PROPTER HOC
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Also called “false causation”
This fallacy follows the basic format of: A preceded B,
therefore A caused B, and therefore assumes cause and
effect for two events just because they are temporally
related (the Latin translates to "after this, therefore
because of this").
Example:
“My alarm goes off every morning at sunrise, therefore the
sun rises because my alarm goes off.”
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AD HOMINEM ATTACKS
An ad hominem argument is any
that attempts to counter another’s
claims or conclusions by attacking
the person, rather than addressing
the argument itself.
Example:
"Andrea Dworkin has written several
books arguing that pornography
harms women. But Dworkin is an
ugly, bitter person, so you shouldn't
listen to her."
 Dworkin's appearance and
character have nothing to do with
the strength of her argument, so
using them as evidence is fallacious.
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Ad hominem attacks are also
known as “mud-slinging”.
AD IGNORANTIAM
An argument from ignorance (“ad ignoratiam” in
Latin).
 The argument from ignorance basically states that
a specific belief is true because we don’t know
that it isn’t true.
Example:
"Of course telepathy and other psychic phenomena
do not exist. Nobody has shown any proof that
they are real."
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APPEAL TO WRONG AUTHORITY
YouTube - Mike Huckabee Ad: "Chuck Norris
Approved“
 This ad also uses a testimonial wherein the fact that
Chuck Norris is a celebrity is supposed to overrule the
fact that he is not an authority on politics.
 Chuck Norris uses glittering generalities to discuss
Huckabee’s qualifications to be president. Glittering
generalities are words of praise for a product or
person; using nice words like ‘goodness’ or
‘patriotism’.
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CARD-STACKING
Manipulating information to make a product
appear better than it is often by unfair
comparison or omitting facts.
YouTube - New Mac Ad: Viruses
This can also be considered an example of a straw
man argument or selective evidence.
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CIRCULAR REASONING
Circular reasoning occurs when the reasoner begins with what
he or she is trying to end up with.
Example:
A satisfied citizen says: “Richardson is the most successful
mayor the town has ever had because he's the best mayor
of our history.”
 The second part of this sentence offers no evidence — it
simply repeats the claim that was already presented. Don’t
be fooled into believing that using the word “because” in
an argument automatically provides a valid reason. Be
sure to provide clear evidence to support your claims, not a
version of the premise (the initial statement in an argument).
EITHER/ OR FALLACY
An either/or fallacy occurs when a speaker makes
a claim (usually a premise in an otherwise valid
deductive argument) that presents an artificial
range of choices.
Example:
“Well, it is time for a decision. Will you contribute
$10 to our environmental fund, or are you on the
side of environmental destruction?”
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FALSE ANALOGY
In an analogy, two objects (or events), A and B are
shown to be similar. Then it is argued that since A has
property P, so also B must have property P. An
analogy fails when the two objects, A and B, are
different in a way which affects whether they both
have property P.
Example:
Students and nails are the same. As it is necessary to hit
nails on the head in order to make them work, the
same must be done with students.
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FALSE AUTHORITY
Appealing to someone who is not an authority to
give an expert opinion. You cannot use yourself as
your own authority with total certainty, either.
Example:
A doctor is more qualified to diagnose your
shoulder pain than you are; your teachers are
better qualified to evaluate your performance
than a student.
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FALSE PREMISE
The premise (proposition, or assumption) is not
correct, the conclusion drawn may be in error.
Example:
If the streets are wet, it has rained recently.
(premise)
The streets are wet. (premise)
Therefore it has rained recently. (conclusion)
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INCONSISTENCY
Applying criteria or rules to one belief, claim,
argument, or position but not to others.
 The fallacy occurs when we accept an inconsistent
set of claims, that is, when we accept a claim that
logically conflicts with other claims we hold.
Example:
“I’m not racist. Some of my best friends are white.
But I just don’t think that white women love their
babies as much as our women do.”
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STRAW MAN
Arguing against a position which you create
specifically to be easy to argue against, rather
than the position actually held by those who
oppose your point of view.
Example:
"Opposition to the North American Free Trade
Agreement amounts to nothing but opposition to
free trade." (Someone can believe in free and
open trade and yet still oppose NAFTA.)
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PERSONAL INCREDULITY
I cannot explain or understand this, therefore it
cannot be true.
Example:
I don’t understand what the fossil record is or means,
therefore dinosaurs must be made up.
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TYPES OF ARGUMENTS THAT CAN BE
MANIPULATED
These types of arguments and evidence can be used, but you should be careful to use
them sincerely and carefully
ANECDOTAL EVIDENCE
This kind of
evidence involves
telling stories of
individuals
suffering because
of a problem
related to the topic,
or individual
examples of
incidents that
support a claim.
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Examples:
Story of a woman murdered by a paroled
killer in a state that doesn't have the death
penalty.
Story of a teenage girl who died of a heroin
overdose and who started using marijuana at
age 12.
Story of someone who died of a treatable
disease after waiting several months for a
routine procedure in a country with nationalized
health care.
Story of a college student in a country with a
low drinking age who does not abuse alcohol.
APPEALS TO LOGIC USING EXPERT OPINION
Intentionally failing to use information suspected of
being relevant and significant is committing the
fallacy of suppressed evidence. This usually occurs
when the information counts against one’s own
conclusion.
 Perhaps the arguer is not mentioning that experts
have recently objected to one of his premises. The
fallacy is a kind of fallacy of selective attention--improperly focusing attention on certain things and
ignoring others.
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APPEALS TO LOGIC USING STATISTICS
73% of all statistics are made up.
 Insufficient Statistics--drawing a statistical conclusion
from a set of data that is clearly too small.
Example:
A pollster interviews ten London voters in one building
about which candidate for mayor they support, and upon
finding that Churchill receives support from six of the ten,
declares that Churchill has the majority support of London
voters.
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EMOTIONAL APPEAL
YouTube - Christine O'Donnell ad
 This ad also uses a “Plain Folks” appeal which
appeals to regular people's values like family,
patriotism, and their sense of distrust of the
powerful and influential.
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THERE ARE MANY MORE TYPES OF DECEPTIVE, ILLOGICAL,
AND POORLY CONSTRUCTED ARGUMENTS
It is your job as a careful read and a thoughtful writer to
identify them when someone is working to persuade you
and avoid using them when you are trying to persuade
others.