Transcript Logical Fallacies - Website Staff UI

Logical Fallacies
(Adopted from Steve Richardson, George Mason
University)
Zainal A. Hasibuan/Siti Aminah
Fakultas Ilmu Komputer
Universitas Indonesia
Definitions
• Logic is the set of rules by which one can
formulate convincing arguments
• It is "the science of argument."
• When presenting an argument, one takes a
set of premises that are proven to be true,
and uses logic to show how they prove a
certain "foregone conclusion."
• Logical errors in scientific writings are known
as fallacies
Logical errors in scientific writings
• If an argument contains a fallacy, then
the conclusion will not necessarily be
proven
• Some fallacies are just accidental, but
they can also be used to trap an unwary
listener or reader into believing faulty
conclusions
Three general categories of logical fallacies
• Material fallacies
– Deal principally with a premise and its evidence.
– When the premises of an argument, or its evidence, contain material
fallacies, the conclusion is not sufficiently proven.
– "Material fallacies arise out of the fabric (or 'material') used to express an
argument."
• Fallacies of relevance
– Deal principally with the relationship between the premise/evidence, and the
conclusion of the argument.
– For example, someone who tries to prove a point using emotion, or who
proves the wrong point, commits a fallacy of relevance.
– The point that is proved is usually "an issue about which people have strong
opinions, so that no one notices how their attention is being diverted" from
the real issue.
• Verbal fallacies.
– Deal principally with the misusage of words.
– An argument which contains "improper or ambiguous use of words" is
invalid.
Material Fallacies
• False Cause
– Assuming that one event is caused by another, just because one
happens after the other, is the fallacy of false cause. The two
events could have both been caused by another event, or they
could be totally unrelated.
– For example: "More people die in hospitals than anywhere else.
Therefore, going to a hospital causes death
• Hasty Generalization
– A hasty generalization is a general rule that is formed from only a
few examples, or examples that are really exceptions.
– For example: "A bear lives at the zoo, therefore, all bears live at
zoos.“
• Misapplied Generalization
– Generalizations applied to cases that are exceptions to the rule are
said to be misapplied.
– For example: "Tools are useful, therefore this hammer will be
useful." One may not need a hammer, or the hammer may be
broken.
Material Fallacies
• False Dilemma
– When an argument overlooks alternative possibilities, it creates a false
dilemma.
– For example: "America: Love it or Leave it."
• Compound Question
– A compound question is one which is phrased in such a way so as to
unfairly limit the possibilities of one's answer.
– For example: "Are you still as selfish as you used to be?" Even if one
answers "no," one would still be admitting that one had been selfish in the
past.
– One subset of the compound question fallacy is the persuasive definition.
– Redefining the terms of an argument to make them support the conclusion
is the persuasive definition fallacy.
• False Analogy
– When an analogy is drawn between dissimilar objects or ideas, it is called a
false analogy.
– Comparing "apples and oranges" is a well known example of a false
analogy.
Material Fallacies
• Contradictory Premises
– A conclusion which is drawn from premises which cannot both
be true at the same time is the fallacy of contradictory
premises.
– For example: "'What would happen if an irresistible force met
an immovable object?' (One student's answer: 'An
inconceivable smash!')"
• Circular Reasoning
– An argument which contains the fallacy of circular reasoning
uses its conclusion as support of its premises.
– It uses "the original thesis as proof of itself."
– For example: "C. S. Lewis was a good author, because he
wrote good books. I know he wrote good books because he
was a good author."
Material Fallacies
• Insufficient or Suppressed Evidence
– Someone who uses the fallacy of insufficient evidence draws
a conclusion from only a few unrepresentative examples.
– For example: "That type of car is poorly made; a friend of
mine has one, and it continually gives him trouble."
– An argument that uses the fallacy of suppressed evidence
uses as evidence only the facts that support the conclusion,
disregarding the rest of the pertinent facts.
– This fallacy illustrates how the conclusion was formed before
all the evidence for it was gathered, or even in spite of it.
– In scientific writing, this fallacy is seen in "a failure to look for
evidence that will confirm or deny a proposed hypothesis,"
and it is also seen "when one believes an alternate
explanation refutes another explanation without a
comparison of the merits between the two explanations."
Fallacies of Relevance
• Fallacies of relevance deal principally with the
relationship between the premise/evidence,
and the conclusion of the argument.
• For example, someone who tries to prove a
point using emotion, or who proves the wrong
point, commits a fallacy of relevance.
• The point that is proved is usually "an issue
about which people have strong opinions, so
that no one notices how their attention is
being diverted" from the real issue.
Fallacies of Relevance
•
•
Irrelevance
– An argument is irrelevant if it proves or disproves the wrong point.
– This fallacy is really a broad category that includes almost all of the
fallacies of relevance.
– "In a discussion of the relative safety of different makes of car, for
instance, the issue of which cars are made domestically and which
are imported is a red herring.“
Personal Ridicule
– Someone who ridicules his opponent instead of addressing the
premises of the argument commits this fallacy.
– "You wouldn't believe someone with his political views would you?"
– One type of the personal ridicule fallacy is the 'straw man.' When
someone uses this fallacy, he applies a characterization or
stereotype to his opponents to make them easy to refute.
– For example, saying that "a person who advocates reduced military
spending is . . . in favor of giving in to the Russians," is a straw man
fallacy.
Fallacies of Relevance
• Appeal to the People
– Using the feelings, actions, and/or prejudices of the general populous
as a support of an argument may invalidate it.
– "Everyone's doing it!"
• Appeal to Authority
– Using the opinion of an expert in a field other than the one being
discussed may invalidate the argument.
– "Coke is the favorite soda of 9 out of 10 actors, therefore we should
have Coke at our picnic."
• Appeal to Ignorance
– Assuming that a premise is correct because it can't be disproved
displays the fallacy of ignorance. This is the "guilty until proven
innocent" fallacy.
– "A classic example is this statement by Senator Joseph McCarthy,
when asked to back up his accusation that a certain person was a
communist: 'I do not have much information on this except the
general statement of the agency that there is nothing in the files to
disprove his Communist connections."
Fallacies of Relevance
• Appeal to Pity
– An argument that uses this fallacy may be invalid because it
depends on the idea that one will be more likely to accept the
conclusion if one feels sorry for someone or something associated
with it. This is the "victim" mentality.
– "I know I flunked every exam, but if I don't pass this course, I'll have
to retake it in summer school. You have to let me pass!”
• Appeal to Force
– Threats and intimidations used to force someone to accept an
argument constitute an appeal to force.
– "If you don't do what I tell you, you'll lose your job!"
• Appeal to Money
– Advertisers frequently appeal to the desire to save money or get
more money to induce people to make purchases. Despite their
success, their appeals are fallacious.
– "Buy our products and save up to $50 every year!"
Fallacies of Relevance
• Emotive Language
– Using a word, phrase, or argument only to stimulate emotions invalidates
ones argument.
– "President Clinton's best allies are the Clinton Haters." The author of this
quote used the term "Clinton Haters" to stimulate emotions. He
described the "Clinton Haters" as willing to "say anything and charge him
[Clinton] with anything that comes out of their heads when they get out of
bed in the morning." He gives no evidence that anyone that impulsive
actually exists. In this case, this fallacy is similar to the personal ridicule
fallacy.
• Tu Quoque
– This fallacy is used as a defense, where the person being criticized
accuses his critic of doing the same thing himself. ("Tu quoque" means
"you too.")
– "Son, it is your bedtime. Go to bed." "But dad, you are staying up!"
• Genetic Error
– When someone disregards a premise or an argument only because of
where it came from, they commit a genetic error. "The source of an
argument is irrelevant so far as logical proof is concerned.“
– "[Clinton's] lieutenants . . . dismiss even legitimate questions as
products of 'the attack machine.'"
Fallacies of Relevance
• Anthropomorphism
– When someone projects human feelings and
qualities to animals and inanimate objects, he
commits a fallacy of anthropomorphism.
– "After millions of years of work, Nature had
created many diverse species of plants and
animals."
• Non Sequitur
– When the premises of an argument are not
logically connected to the conclusions, the
argument contains a non sequitur.
– "Trees are green; therefore human beings enjoy
spinach."
Verbal Fallacies
• Verbal fallacies deal principally with the misusage of words. An
argument which contains "improper or ambiguous use of words" is
invalid. Here are some descriptions and examples of verbal fallacies.
• Ambiguity
– Using undefined words or words whose meaning is vague constitutes an
ambiguity. For example, in 1997 the Commonwealth of Virginia proposed
buying "probe kits" for every student to help in math. Regarding this
ambiguity, C. R. Taft said,
– "To be sure, there is the matter of 6,000 'probe kits,' or data-collection
devices. What data these devices collect and how they do so was never
defined clearly."
• Equivocation
– Someone who uses a word in more than one sense, but gives the
impression that only one meaning was meant, is using an equivocation.
Anyone who presents an argument needs to use only one definition for
each of his terms. When more than one definition is used for a certain
word, it can cause confusion and be misleading.
– "Death is a subject of utmost gravity. Gravity is what keeps us from
falling off the Earth. Thus, death is primarily what keeps us from falling
off the Earth."
Verbal Fallacies
• Composition
– Assuming that a group will have the same qualities as the individuals in it
is the fallacy of composition. This fallacy and the next one are types of
equivocation. "In the fallacy of composition, the individual terms that
comprise a group . . . are equivocally confused with the collective term."
– "A spider is a beneficial member of an ecosystem. Therefore, introducing
millions of spiders into an ecosystem would be advantageous."
• Division
– When one assumes that the individuals in a group will have the same
qualities as the group they are in, one commits the fallacy of division.
– "That orchestra is the best in the world, therefore it is made up of the
best musicians in the world." However, the best orchestra in the world
may not have the world's best solo violinist.
• Amphibology
– A sentence that is structured in such a way as to make more than one
interpretation possible is an amphibology.
– "Wanted to sell: A highchair for a baby with a broken leg."
Verbal Fallacies
• Abstraction
– Taking a quote out of context is known as abstraction. Using this
fallacy can totally change what was originally meant. Francis Bacon
purportedly said,
– "Philosophy inclineth a man's mind to atheism." But what he
actually said was, "A little philosophy inclineth man's mind to
atheism, but depth in philosophy bringeth men's minds about to
religion."
• Identifying logical fallacies is an important skill for everyone to
have.
• It not only helps one to avoid accepting false conclusions, but it
also helps one to learn better reasoning and debating skills.
• The process of looking for logical fallacies can help one to better
understand the subject one is reading about or discussing.
• Knowing how to identify fallacies and how to avoid using them,
can make one better prepared to refute false ideas and present
the truth.