An Introduction to Logic &
Logical Fallacies
Beginning Composition
Logic
Logic is concerned with
the principles of correct
reasoning.
People who learn logical
thinking are better able
to tell whether or not a
conclusion follows from
a set of statements or
assumptions.
An Introduction to Logic:
from Monty Python and the Holy Grail
What logic do the
peasants and Sir
Bedevere use to
decide whether or
not to burn the
suspected witch?
Evaluate their
reasoning--should
they burn this
woman or not?
Why?
Syllogism
Examples:
A syllogism is
All good students should get
made up of a
financial aid. (MAJOR)
MAJOR PREMISE,
Sarah is a good student.
a MINOR
(MINOR)
Therefore, Sarah should get
PREMISE, and a
financial aid. (CONCLUSION)
CONCLUSION.
If both premises
Some North Americans are tall.
are true, the
All Canadians are North
conclusion is
Americans.
Some Canadians are tall.
true.
Practice with Syllogisms
Label the MAJOR and MINOR
premises in these syllogisms, as
well as their CONCLUSIONS.
All men are mortal. Socrates is a
man. Therefore, Socrates is
mortal.
All jewelry is expensive. Some
rings are jewelry. Some rings
are expensive.
All zebras have stripes. Some
animals are zebras. Some
animals have stripes.
Complete these syllogisms with
the CONCLUSION that
necessarily follows from these
premises.
All mammals have hair. No
reptiles have hair.
Conclusion:
Some Americans are
Republicans. All
Republicans are
conservatives. Conclusion:
All bananas are fruit. All fruit
is good. Conclusion:
Logical Fallacies
Sometimes you will encounter arguments in which
writers attack the opposition’s intelligence or
patriotism and base their arguments on questionable
(or even false) assumptions. As convincing as these
arguments can sometimes seem, they are not valid
because they contain fallacies--inaccurate or
intentionally misleading arguments.
Learning about some of these fallacies should help you
recognize them in others’ writing and avoid them in
your own writing.
Ad Hominem (attack the person)
An ad hominem
argument attack
the character of a
person rather
than the claim
s/he makes.
Examples:
“Of course you would say marketing
serves a valuable purpose; you work
in marketing.”
“You drank alcohol while underage, so
we won’t accept your technical
recommendations.”
“I just don't trust people who dress like
that; they should take pride in their
appearance and not go around
ungroomed if they want to be taken
seriously!”
Non Sequitur (It Does Not Follow)
The non sequitur
fallacy occurs
when a conclusion
does not follow
from the premises.
It is often
supported by weak
or irrelevant
evidence--or no
evidence at all.
Examples:
Megyn drives an expensive car, so
she must be earning a lot of
money.
Gangs, drugs, and extreme violence
plague today’s prisons. The only
way to address this issue is to
release all non-violent offenders
as soon as possible.
Recently, the polar ice caps have
thickened, and the temperature of
the oceans has stabilized.
Obviously, the earth is healing
itself. We don’t need to do more
to control global warming.
Circular Reasoning/Begging the Question
Begging the Question (or circular
logic) happens when the writer
presents an arguable point as a fact
that supports the argument. This
error leads to an argument that goes
around and around, with evidence
making the same claim as the
proposition. Because it is much
easier to make a claim than to
support it, many writers fall into this
trap.
Example: "These movies are
popular because they make so
much money. They make a lot of
money because people like them.
People like them because they are
so popular."
The argument continues around in
the logical circle because the
support assumes that the claim is
true rather than proving its truth.
Hasty Generalization
A hasty
Examples:
generalization
Pauline Kael, a film critic, said in 1972,
about Richard Nixon being elected:
occurs when
“How can that be? No one I know
someone reaches
voted for Nixon!”
a conclusion based
My Honda broke down. Hondas are
on too little
junk.
evidence.
Three Congressional representatives
Also called
have had affairs. Therefore,
jumping to a
politicians are adulterers.
conclusion.
Slippery Slope
Slippery Slopes suggest that one step
will inevitably lead to more, eventually
negative steps. While sometimes the
results may be negative, the slippery
slope argues that the descent is inevitable
and unalterable. Stirring up emotions
against the downward slipping, this fallacy
can be avoided by providing solid
evidence of the eventuality rather than
speculation.
Example: "If we force public
elementary school pupils to
wear uniforms, eventually we
will require middle school
students to wear uniforms. If
we require middle school
students to wear uniforms,
high school requirements
aren’t far off. Eventually even
college students who attend
state-funded, public
universities will be forced to
wear uniforms."
Bandwagon Appeals (ad populum)
Bandwagon Appeals (ad
populum) try to get everyone on
board. Writers who use this
approach try to convince readers
that everyone else believes
something, so the reader should
also. The fact that a lot of people
believe it does not make it so.
Example: "Fifty
million Elvis fans
can’t be wrong!"
Of course they can.
The merit of Elvis is
not related to how
many people do or
do not like him or
his music.
False Authority
False Authority is a tactic
used by many writers,
especially in advertising. An
authority in one field may
know nothing of another
field. Being knowledgeable
in one area doesn’t
constitute knowledge in
other areas.
Example: A popular
sports star may know a
lot about football, but
very little about shaving
cream. His expertise on
the playing field does
not qualify him to
intelligently discuss the
benefits of aloe. (false
ethos)
Faulty Analogy
Faulty Analogies lead to faulty
conclusions. Writers often use similar
situations to explain a relationship.
Sometimes, though, these extended
comparisons and metaphors attempt
to relate ideas or situations that upon
closer inspection aren’t really that
similar. Be sure that the ideas you’re
comparing are really related. Also
remember that even though
analogies can offer support and
insight, they can’t prove anything.
Example: "Forcing students to
attend cultural events is like
herding cattle to slaughter.
The students stampede in to
the event where they are
systematically ‘put to sleep’
by the program." While the
analogy is vivid, the
difference between cultural
events and cattle slaughter is
so vast that the analogy
becomes a fallacy.
Straw Man
Straw Man is a tactic used by a lot of
writers because they find it easier to
refute an oversimplified opposition.
Writers may also pick only the
opposition’s weakest or most insignificant
point to refute. Doing so diverts attention
from the real issues and rarely, if ever,
leads to resolution or truth.
Example: The debate
over drink machines
centers around cost and
choice. Opponents of the
new drink machines bring
up their location as an
important issue. This
insignificant point has
little relevance to the
actual issues.
Post-Hoc Ergo Propter Hoc
(After This, Therefore Because of This)
A post hoc ergo
Examples:
propter hoc (or post
I wore my purple sweater and my team
hoc) fallacy occurs
won against all the odds. I am going
when someone
to wear that sweater to every game.
Roosters crow just before the sun rises.
assumes that a
Crowing roosters cause the sun to
certain event is the
rise.
cause of an
Bill purchases a new laptop and it
unrelated effect
works fine for months. After
installing a new piece of software, he
simply because the
starts up the computer and it freezes.
event happened
The software must be the cause of
first.
the freeze.
Either-Or
An either-or
Examples:
You either support public
fallacy occurs
prayer in schools or you
when a complex
are an atheist.
issue, which might
Either you support
have many
everything that the
arguable
president proposes or you
are not a patriot.
solutions, is
There are two clear choices:
reduced to only
cutting funding to
two possible
education, or increasing
outcomes.
our national debt.