Common Mistakes in Thinking
• Informal fallacy – An error in reasoning resulting
from carelessness, inattention, or insensitivity to
language or context in which the argument is
presented. In many instances of informal
fallacies, we are tempted to accept the argument
because we are not attentive enough to recognize
that the evidential structure is unacceptable as
• Informal fallacies are the most common kind of
mistake in arguments. They can be hard to avoid
or to detect precisely because they are so common.
They are the main traps to avoid in arguments and
scrupulously avoid in our own.
Ad hominem
• The first informal fallacy is the argument to the
person. The Latin name, argumentum ad
hominem, is probably the most commonly used of
the Latin words in logic, and sometimes it appears
simply as ad hominem. In this fallacy an attack is
made on the person presenting the argument rather
than on the argument itself. The character,
credentials, reputation, position, or office of the
individual is called into question instead of the
soundness of his o her claim. The person is made
to seem ridiculous or suspicious, and this is meant
to undermine the argument that is presented.
Ad hominem II
• The ad hominem fallacy actually has two versions,
personal and circumstantial. In the personal form
(which is the most common one), the character or
behavior of the person is discredited. Ad hominem
almost always results from saying, in effect,
– “So – and so’s claim should be rejected because so-andso is ____________,” where nearly any term with
presumably negative impact is placed in the blank (for
example, “ignorant,” “a liar,” “a Republican,” “a
Democrat, “ “just saying that to get rich,” and so on).
• Rare exceptions can occur in which a person’s
shortcomings do automatically transfer to the
person’s claim, but such a transaction takes a
peculiar kind of circumstance. For example, “He
was paid to lie about this matter.”
Ad hominem III
• In the 19th century the fight for Irish home rule
was criticized because its leader, Charles Parnell,
was an adulterer. Vincent Van Gogh suffered from
severe depression, but that does not have any
bearing on the aesthetic value of his paintings.
• The point is that in judging ideas one mustn’t
judge the person presenting the ideas. Insane
people can write profound things, the sane can
produce a lot of nonsense; awful people can say
good things, and saints may not be right in
everything they argue.
Ad hominem IV
• In the circumstantial form of ad hominem, the
person’s position is cited as the reason to disregard
what he or she claims. Example:
– John says that we should reject what Father Hennesy
says about the dangers of abortion because, “After all,
he’s a Catholic priest, and priests are required to hold
such views.”
• It may be true that Father Hennesy is a priest and
that is views on abortion represent those of the
Catholic church. That does not make his views
false, however. The specific mistake here, of
course, is to presuppose that people always act for
their own advantage and because of their position,
and that does not seem fair.
Ad hominem V
• “Ad hominem” is Latin for “to the man”
indicating that it is the person and not the subject
matter that’s being addressed.
• Rather than arguments ad hominem, we must use
arguments ad rem, that is, “to the thing itself.”
• A variety of the fallacy of ad hominem
circumstantial is called tu quoque or “you
yourself.” Here an argument is discredited
because the person does not practice what she
preaches. For example, “You tell me not to smoke
but you do.” Of course, if someone is seriously
against smoking than they should not smoke.
However, their judgment is not invalidated by the
personal inconsistency.
Ad hominem VI
• This does not mean, of course, that questions
about a person’s behavior or position are always
beside the point. If we want to know if candidates
are fit for office, their character traits are certainly
relevant. Such virtues as sincerity, fairness,
integrity, and so forth are important
considerations. However, we call it “smear
tactics” when irrelevant facts about a person’s
political position or qualifications for public
office. The fact that a woman may be divorced or
a lesbian, for example, does not seem relevant to
whether she would make a good legislator.
Ad hominem VII
• The argument to the person has its counterpart in the
biographical fallacy in literature. Knowing that Virginia
Woolf suffered from depression may help us interpret some
of her work, but knowing her life has no bearing on the
literary value of her novels.
• In other academic fields the same mistake is termed the
genetic fallacy, and it refers to the attempt to explain away
a claim by referring to the source.
Ad hominem VIII
• In religion, William James refers to the
fallacy as “medical materialism”:
– “Perhaps the commonest expression of this
assumption…is seen in those comments in which
unsentimental people so often pass on their more
sentimental acquaintances. Alfred believes in
immortality so strongly because his temperament is so
emotional. Fanny’s extraordinary conscientiousness is
merely a matter of overinstigated nerves. William’s
melancholy about the universe is due to bad digestionprobably his liver is torpid. Eliza’s delight in her
church is a symptom of her hysterical constitution.
Peter would be less troubled with his soul if he would
take more exercise in the open air, etc.” William James,
Varieties of Religious Experience
– The source of an idea is irrelevant to its truth.
Argument from authority
• The argument from authority fallacy is committed
whenever we argue for some point, not because it is well
grounded in fact or logic but because of the authority of the
person who presented it. The standing or prestige of a
recognized authority is said to guarantee the truth of a
claim, and anyone who doubts it is made to feel
presumptuous or egotistical. The thrust of the argument is
“Who are you to challenge the judgment of this authority
or the experience of that expert?”
Argument from authority II
• In some ways this fallacy is the mirror image of
the argument to the person. Rather than
dismissing a position because of the person who
advocates it, in the argument from authority one
accepts a position because of the person
advocating it. Here too the messenger is confused
with the message. Example:
– Of course different people have different strengths. But
Aristotle’s definition of personhood emphasized
rationality, so the ability to reason is the only personal
characteristic that really matters.
– Cindy Crawford always drinks Pepsi. If I drink a lot of
Pepsi, I might look like Cindy Crawford.
Argument from authority III
• No one can check the evidence for everything that
is claimed, so we must depend on the information
provided by authorities. However, and idea does
not become true simply because an authority says
so; the person must have a good reason to say so.
• If we accept people as authorities it is because we
have confidence that they support their insights
with good thinking and good evidence.
Furthermore, the evidence should be publicly
verifiable, whether in the form of reproducible
experiments or rational reasons that anyone can
Argument from authority III
• A wider problem with the argument from authority
is it suggests that an authority in one field is also
an in another. We make this mistake very often in
the fields of sports and entertainment. For
instance, outstanding baseball players might be
trusted to recommend athletic equipment but they
are authority not qualified to endorse camcorders
or razors in TV commercials. In the same way,
actors are knowledgeable about acting but not
necessarily about long distance telephone service,
cruise lines, refrigerators, or cars. An ethical
problem with such endorsements, of course is that
the celebrities may not actually use the product
they recommend but are mainly interested in the
large endorsement fees.
Argument from Force
• Another informal fallacy is called the argument
from force. This fallacy substitutes an appeal to
motive in place of evidence in the grounds
supporting a claim. In the appeal to force,
someone attempts to get you to accept her claim
because she somehow will hurt you if you do not
agree. Example:
– Your supervisor at work asks you to support her request
for a merit raise. By all accounts, including your
personal observation of her performance, she has done
a thoroughly miserable job and does not deserve merit
recognition in her salary. However, she makes it clear
that your job is on the line if you do not support her. So
you write the letter supporting her request.
Argument from Force II
• Arguments from force amounts to holding a gun to
someone’s head and saying, “I trust you will see
the force of my argument.” This device can be
convincing, and it may ultimately prevail, but not
because it has satisfied logical standards. Al
Capone once remarked, “You can get more with a
kind word and a gun than you can with a kind
word.” Of course, it depends on what you are
trying to “get”. “You have not converted a man
because you have silenced him.” – John Morley
One may get temporary obedience through appeal to
force but lose trust, respect, love, honest loyalty,
and so forth.
Argument from Force II
• In the argument from force the intimidation
sometimes will be overt and physical, as when
citizens are compelled to vote for a dictator upon
pain of death. However, usually the intimidation
is more subtle and indirect. For example, if the
boss strongly suggests that an employee agree that
the safety standards are adequate, or that an
accounting report is accurate, the boss is probably
making an argument from force. The employee
will feel that he or she had better go along, not
blow the whistle, or else the job will go to
someone who can “rise above principles.”
Argument from Force II
• In religious discussions people will sometimes say
“There are no atheists in foxholes,” implying that we
will always turn to religion in times of crisis. Even if
that is true, fear is not an argument for God’s
existence. The most famous example of the argument
from force within religion is “Pascal’s Wager.”
– Pascal was a seventeenth –century scientist and
philosopher. He argued that if we believe in God and
we are right, then we will go to heaven; and if we
believe in God and we are wrong; then at least we will
have a pleasant life on earth thinking that God exists
and we will ultimately receive our heavenly reward.
On the other hand, if we do not believe in God and we
are wrong, then we will surely burn in hell; and if we
do not believe in God and we are right, then we will
have a miserable life on earth, feeling that no divine
mercy, justice or love exists.
Argument from Force II
• Since these are the only options, Pascal argued, we
should certainly bet on belief in God because,
right or wrong, we are far better off. If we wager
on not believing then, right or wrong, we are
bound to suffer. In essence, Pascal is arguing that
if we know what’s good for us we had better
believe in God. Clearly the argument is meant to
intimidate us into accepting God’s existence; in
this respect it functions as an argument from force.
No proof is offered that there is a God, but only
that we ought to believe in God to be on the safe
side. However, to believe that something is real
we need good reasons, not just the fear that we
might be sorry if we didn’t.
Appeal to pity
• Appeal to pity is a fallacy that is committed
whenever the evidential appeal is only to the
altruistic ideals of the hearer. Example:
– Helen is running for a seat on the city council. Though
you like her, you have doubts about her qualifications
and in fact believe that an opposing candidate would
make a better member of the council. When you
communicate your concerns about her qualifications to
a mutual friend, the friend counters by saying that
Helen would be terribly hurt if she were to lose the
election. After thinking this over, you conclude that
maybe Helen’s qualifications are not so bad after all.
Appeal to pity II
• The mutual friend has evoked compassion in you for
Helen, but she has not given you a reason for changing
your opinion of Helen’s qualifications. Does your
compassion for Helen enter the picture at all? Certainly,
but only in this way: You now have to weigh Helen’s hurt
at losing the election against the consequences of having
her as a council member instead of the BETTER
QUALIFIED candidate. Which, you have to decide, is
more important?
Appeal to pity II
• As a student you may have used the argument
from pity at one time or another. You may have
asked for a higher grade on an exam, arguing that
you had studied hard but were emotionally upset
or swamped with work. You might have said that
you have a twenty hour a week job, your car broke
down, you were sick with the flu, or that you are
having trouble with your parents. Under such
circumstances, a sympathetic professor might
allow you additional time to do the work or even
let you retake the exam, but if the professor were
to raise your grade on those grounds he or she
would not be acting in a professional way.
Appeal to pity III
• A higher grade would indicate that you had
mastered the material to a greater degree than you
actually did, thereby giving a false impression to
anyone reading your transcript. Your grade should
reflect your actual level of achievement, not how
much sympathy the professor felt for you.
• Imagine being operated on by a doctor who never
really mastered the skills of surgery but was given
a passing grade by the professor how thought he
or she deserved a break. Imagine driving over
bridges that were designed by engineers who were
allowed to graduate, not because they had met the
qualifications, but because they had worked hard
and came from poor families.
Appeal to pity IV
• Let’s look at another example:
– ROOFER: I’m positive that my work will meet your
requirements. I really need the money, what with my
wife being sick an all.
• The roofer seems to be giving a reason for
thinking that his work will meet your
requirements. But of course he is not; that issue is
unaffected by the fact that he really needs the
money. This fact is not a reason, not even a bad
reason, for concluding anything at all about the
quality of his work.
Appeal to pity V
• Notice, though, that actions performed out of concern for
others are often rationally and ethically justified. Indeed,
in some instances they are among the noblest of human
deeds. If the roofer is qualified and needs the money for
his wife’s illness, and you are willing to take a chance on
him then by all means hire him! Just don’t think that he
has given you a reason for thinking that his work will meet
your specifications; whether it will or not is something you
must establish on other grounds.
Straw Person
• Another fallacy is called straw person, the
mistake of attributing to your opponents a
ridiculous position that they do not hold and that is
easily knocked down like a person made of straw.
By exaggerating, oversimplifying, or distorting the
other person’s views, you set up an easy target.
This absurd position, not the person’s actual one,
is then refuted by showing how ludicrous it is.
• The straw person device is frequently used in
political rhetoric, functioning as a cheap way of
winning points while evading the main issue.
Straw Person II
• For example, a politician might argue,
– “My liberal, environmentalist friend believes in
preserving species, that the spotted owl and the snail
darter are more important than people. I’m sorry, but I
cannot share that view. He would not sacrifice a bird or
a fish to safeguard a human being, but I would rather let
an animal die than a person.”
• Quite obviously, few if any environmentalists would
claim that the life of a bird or fish has a higher value
than that of a human being, but the environmentalists
position is cast in that light to make it easy to refute.
The environmental debate is not over whether we
should kill people rather than animals but the extent to
which we should endanger wildlife for the sake of
economic development.
Straw Person III
• That, of course, is a genuine question that deserves
to be discussed and debated. Should the
protection of an endangered species prevent the
development of our natural resources, especially if
that development means increased employment,
energy, and material products to enhance our
lives? Are we stewards and caretakers of the
earth, or do we have the right, perhaps the
obligation, to exploit the earth for human wellbeing? In cases where the two are incompatible
(as often happens in impoverished third world
countries) which should take priority? These are
real issues , but to frame the debate as a choice
between killing animals or people is an unfair
Straw Person IV
• Also, can’t environmentalism be compatible with
the welfare of human beings? The world can
survive without people, but people cannot survive
without the world.
Poisoning the well
• A sister fallacy to straw person is poisoning the
well. Here one side in an argument is placed in a
position where it cannot refute the other without
discrediting itself. This is done by making the
position an undesirable one for any rational or
decent person to hold.
– For example, a moralist might argue, “Women who
support abortion are selfish and godless people. They
put their own needs above everything and everyone
else, and reject the divine gift of a child’s life.” Since
most women would not want to be regarded as selfish
and godless, they might abandon the argument for
abortion on demand.
Poisoning the well II
• Taking the opposite line, a feminist might argue, “The right
of a woman to have an abortion, to do with her body as she
pleases , is opposed only by reactionary men who want to
keep women in their traditional roles. Every right-thinking
person knows this to be true.” At this point any man who
believes otherwise is reluctant to oppose abortion for fear
of being regarded as a reactionary male and not a rightthinking person. Instances of poisoning the well happen
all of the time in daily life. If a couple is quarreling the
husband might say, “I find you so defensive and your
constant denials that you are defensive only prove my
point.” It is impossible then for the women to defend
herself, since that would only confirm the defensiveness.
Poisoning the well III
• We know we are in the presence of the fallacy whenever
we come across statements such as “Any caring or
concerned person will agree that…,” or “Those who reject
my solution are part of the problem,” “Only an idiot would
• In arguments, if we come across the ploy of poisoning the
well we should expose it for what it is: an attempt to place
anyone who disagrees in an impossible position.
Slippery Slope
• In the slippery slope fallacy we make the mistake
of thinking that if we take one step along a certain
path then nothing can stop us from sliding
inevitably to our ruin. That is, once we set foot on
the slippery slope we are bound to end up in
disaster. The moral, or course, is that we had
better avoid taking that first step.
• Sometimes the fallacy of the slippery slope is
described as the “domino effect”: a slight push on
one domino will topple the others and set a whole
train of events in motion.
Slippery Slope II
• At other times it is referred to as “opening the
floodgates” so that we are inundated and
overwhelmed by the forces that we have released.
The English often refer to “the thin edge of the
wedge,” which suggests that once we allow an
opening this will be pushed to some greater,
undesirable result. Still other metaphors are “we
mustn’t let the genii our of the bottle,” “there is a
ripple effect,” “give them an inch and they’ll take
a mile,” “events can begin to snowball”
(producing an avalanche), and if the camel gets his
nose in the tent, he’ll soon have his entire body
Slippery Slope III
• Bureaucrats commit a common example of the
slippery slope fallacy when they say, “I would like
to help you but if I make an exception for you I
will have to make an exception for everyone else.
Maybe in your situation there are extenuating
circumstances but it is better to stick to the rules,
otherwise everybody will claim they are a special
case.” The fallacy, of course, is to assume that
once you initiate a course of action it is
unstoppable. In fact, that is rarely true. That does
not mean that rules are made to be broken but that
it is possible to break rules without destroying
Slippery Slope IV
• Very few rules are without legitimate exceptions, so if
bureaucrats insist on absolute rules, they are bound to
commit injustices. Of course, any one who claims an
exception has the burden of proof to show why the
exception should be made.
• Sometimes slippery slopes do exist. Some developments
in science and technology can be dangerous. When we
split the atomic nucleus we used the energy for bombs.
However, we cannot assume that whenever we take a first
step we invariable lose control of events, that actions once
unleashed must run their course and produce disaster.
Usually we are faced with a flight of stairs, not a long
slippery slope, and we can rest on any one of them.
Gambler’s Fallacy
• The gambler’s fallacy is another important
mistake to notice because it can trap people in a
cycle of hope and desperation. Gambling has
become an enormous business in the United
States, exceeding the revenues of movies, books,
and recorded music combined. In 1993 alone
gambling revenues exceeded $30 billion. As a
result of gaming, Las Vegas went from a small
desert town of three thousand people to a city of
one million, and states are now competing with
Indian tribes in the race to build casinos.
Gambler’s Fallacy II
• The gambler’s fallacy is one of the factors that
keep people gambling, and it is usually described
in terms of “the maturity of the chances.” A
person playing roulette, for example, will wait to
see which number has not come up in one hundred
or two hundred spins of the wheel. Then she will
bet on that number on the assumption that,
according to statistical probability, the roulette ball
is due to fall in that numbered compartment.
However, at each turn of the wheel, each number
has an equal chance of being chosen.
Gambler’s Fallacy III
• You flip a coin ten times, and each time it comes
up heads. But on the 11th flip, will it come up
heads again, or tails? If you ask a random group of
people, you will likely get two different answers.
• The first will be that the coin will surely come up
heads again - it's already done so 10 times in a
row, so the odds must be in favor of it happening
again. The second answer will be that is must
come up tails on the 11th flip, because it came up
heads 10 times now and the coin is "due" to go the
other way by now.
Gambler’s Fallacy IV
• But both answers are wrong, and both are the
different sides of the gambler's fallacy. Even if we
come up with 10,000 heads in a row, the odds of
number 10,001 being tails is still only 50/50
(assuming that the coin is "fair", of course). The
law of averages does not change and a random
system does not develop a "memory." The
gambler's fallacy represents the very real human
desire to find some sort meaning and pattern in
what appears to be a random system.
– As the French mathematician Joseph Bertrand
remarked, the coin has neither memory or

Common Mistakes in Thinking