1. Critical Thinking
Faulty Reasoning: Fallacies
Bernard Ho
HonBSc, BEd, MSc
Humber College Institute of Technology and Advanced Learning
205 Humber College Boulevard
Toronto, ON M9W 5L7
bernard.ho@humber.ca
(416)675-6622 ext. TBA

2. Outline
• Fallacies in general
– Weaknesses in arguments
• Fallacies of relevance
– Arguments with irrelevant premises
• Fallacies of insufficient evidence
– Arguments with unacceptable premises

3. Arguments Try to Prove a Point
• If an argument is good, it is good no matter who
makes it.
• Arguments are good or bad because of their own
intrinsic strengths or weaknesses, not because of
who offers them up.
• An argument can fail because:
– The reasoning is faulty (invalid or weak);
– The premises are false (unsound or uncogent);
– Or both.

4. Arguments
Strict Necessity Test:
Is it the arguer’s intention to make the
conclusion follow necessarily from the premises?
YES NO
Deductive Inductive
If the premises are If the premises are
hypothetically true, do they hypothetically true, do they
guarantee the conclusion? make the conclusion probable?
YES NO YES NO
Valid Invalid Strong Weak
Are the premises actually true? Are the premises actually true?
YES NO YES NO
Sound Unsound Cogent Uncogent

5. Fallacies
• Fallacies can seem plausible and persuasive,
but really make no logical sense.
• You need to study fallacies to:
– Avoid committing them;
– Detecting when others do it.

6. Two Categories of Fallacies
• Fallacies of relevance
– Arguments that use premises that have nothing to
do with the conclusion.
– Premises are irrelevant.
• Fallacies of insufficient evidence
– Arguments with premises that are relevant to the
conclusion but still dubious.
– Premises fail to provide enough support for the
conclusion.

7. Fallacies of Relevance
• These fallacies have premises that are irrelevant to the
conclusion.
– Genetic fallacy
– Appeal to the person (Personal attack)
– Attacking the motive
– Tu quoique (Look who’s talking)
– Two wrongs make a right
– Appeal to popularity (Bandwagon)
– Appeal to ignorance
– Appeal to emotion (Appeal to pity)
– Red herring
– Straw man

8. Fallacies of Insufficient Evidence
• These fallacies have unacceptable premises:
– Begging the question
– False dilemma
– Slippery slope
– Hasty generalization
– Faulty analogy
– Questionable cause

9. Fallacy of Relevance: Genetic Fallacy
• Arguing that a claim is true or false solely because
of its origin.
Taylor’s argument regarding the existence of God
can’t be right because she’s an atheist.
We should reject that proposal for solving the
current welfare mess. It comes straight from the
Conservative Party.
Russell’s idea about tax hikes came to him in a
dream, so it must be a stupid idea.

10. Fallacy of Relevance: Genetic Fallacy
• These arguments fail because they reject a
claim based solely on where it comes from,
not on its merits.
• Judging a claim only by its source is a recipe
for error. Think of it this way:
– A good argument presented by a moron is still a
good argument.
– A bad argument presented by a genius is still a
bad argument.

11. Fallacy of Relevance:
Appeal to the Person
• Rejecting a claim by criticizing or discrediting
the person who makes it rather than the claim
itself.
• Also called an ad hominem or personal attack.
X is a bad or disreputable person.
Therefore, X’s argument must be faulty.

12. Fallacies of Relevance:
Appeal to the Person
• Common in criminal court (watch LAW & ORDER)
• Prosecutors and defence attorneys often try to
weaken their opponent’s case by discrediting
their witnesses.
Dr. Raza testified that Dr. Auster’s alcoholism led
Auster to incorrectly prescribe medication for
Suzanne Morton, thus causing her death. But Dr.
Raza earned his medical degree from the
University of Peshawar and has only practiced
medicine for a few years. His argument,
therefore, is worthless.

13. Fallacies of Relevance:
Appeal to the Person
• These arguments fail because they attempt to
discredit a claim by appealing to something
that is almost always irrelevant to it: a
person’s character, motives, or personal
circumstances.
• They say nothing about the quality of the
argument.

14. Fallacies of Relevance:
Appeal to the Person
Dr. Raza testified that Dr. Auster’s alcoholism led
Auster to incorrectly prescribe medication for
Suzanne Morton, thus causing her death. But Dr.
Raza is only testifying so that he avoids being
charged with falsifying medical documents.
Therefore, his argument should be rejected.
• Sometimes, it is reasonable to doubt a person’s
premises because of who they are.
– When you have reason to expect bias.
– When they seem to lack relevant expertise.

15. Fallacy of Relevance: Tu Quoique
• Also called Look Who’s Talking
• Rejecting a person’s argument or claim
because that person fails to “practice what
they preach”
X fails to follow his/her own advice.
Therefore, don’t believe his advice.

16. Tu Quoique?
I won’t stop smoking just because my doctor
tells me to. He won’t stop smoking either!
– This is a fallacy.
I should stop smoking like my doctor told me;
but so should my doctor!
– This is not a fallacy because no argument is
being rejected on the basis of the arguer
being a hypocrite.

17. Fallacy of Relevance:
Two Wrongs Make a Right
• Trying to make a wrong action look right, by
comparing it to another wrong (perhaps
worse) action.
X is as bad or worse than Y. Therefore Y is not
wrong.

18. Fallacy of Relevance:
Two Wrongs Make a Right
I don’t feel guilty about cheating on Dr.
Boyer’s test. Half the class cheats on his tests.
Why pick on me, Officer? Nobody comes to a
complete stop at that sign.

19. Two Wrongs Make a Right?
• Sometimes actions can be justified by the fact
that other actions have taken place.
I killed the man because he was about to kill me.
It was an act of self-defense.
I jumped into the pool when it was closed and off-
limits because my friend jumped in and was
drowning.

20. Fallacy of Relevance:
Appeal to Popularity
• Arguing that a claim must be true merely
because a substantial number of people
believe it.
• Also called bandwagon argument.
Everyone (or almost everyone, most people,
many people) believes X.
So X is true.

21. Fallacy of Relevance:
Appeal to Popularity
Most people agree that owning an SUV is safer
than owning a car. So I guess it must be true.
Of course the war is justified. Everyone believed
that it’s justified.
The vast majority of Canadians believe that
there’s a supreme being, so how could you doubt
it?

22. Fallacy of Relevance:
Appeal to Popularity
• These arguments fail because they assume
that a proposition is true merely because a
great number of people believe it.
• But as far as the truth of a claim is concerned,
what many people believe is irrelevant.

23. Appeal to Popularity?
Of course smoking causes cancer! Everybody
says so!
• A fallacy is a mistake of reasoning.
• An argument can use faulty reasoning, but it
can still have a true conclusion.

24. Appeal to Popularity?
• Not all appeals to popular beliefs or practices
are fallacious.
• If the premises are relevant to the conclusion,
these arguments are not fallacious.
All the villagers say that the water is safe to
drink. Therefore, the water is probably safe to
drink.

25. Fallacy of Relevance:
Appeal to Ignorance
• Arguing that a lack of evidence proves
something.
• The problem arises by thinking that a claim
must be true because it hasn’t been shown to
be false.
No one has shown that ghosts aren’t real, so
they must be real.

26. Fallacy of Relevance:
Appeal to Ignorance
It’s clear that God exists, because science
hasn’t proved that he doesn’t exist.
You can’t disprove my theory that Bigfoot lives
in the forests of B.C. Therefore, my theory
stands.

27. Fallacy of Relevance:
Appeal to Ignorance
• Lack of evidence alone can neither prove nor
disprove a proposition.
• Lack of evidence simply reveals our ignorance
about something.

28. Burden of Proof
• Appeals to ignorance involve the notion of
burden of proof.
– Burden of proof is the weight of evidence of
argument required by one side in a debate or
disagreement.
– Problems arise when the burden of proof is placed
on the wrong side.

29. Burden of Proof
• Usually rests on the side that makes a positive
claim.
• If you think that psychics exist, you bear the
burden of proof.
• If you think X causes cancer, you usually bear
the burden of proof.

30. Fallacy of Relevance:
Appeal to Emotion
• Using emotions as premises in an argument.
• Trying to persuade someone of a conclusion
primarily by arousing his or her feelings,
rather than presenting relevant reasons.
• These arguments fail because emotions are
irrelevant to the conclusion.

31. Fallacy of Relevance:
Appeal to Emotion
You should hire me for this network analyst
position. I’m the best person for the job. And if I
don’t get a job soon my wife will leave me, and I
won’t have enough money to pay for my
mother’s heart operation.
Officer, there’s no reason to give me a traffic
ticket for going too fast because I was just on my
way to the hospital to see my wife who is in a
serious condition to tell her I just lost my job and
the car will be repossessed.

32. Fallacy of Relevance: Scare Tactic
• Threatening to harm those who may not
accept the argument's conclusion.
• The threat is irrelevant to the conclusion.
If you don’t accept what I say something bad
will happen. Therefore, what I say is true.
This gun control bill is wrong for America, and any
politician who supports it will discover how wrong s/he is
at the next election

33. Scare Tactic?
• Not all threats are fallacies.
• If the threat is a natural consequence of an act
or belief, then the threat is relevant.
You should not pass that law because it will
hurt the public welfare.

34. Fallacy of Relevance: Red Herring
• Raising an irrelevant issue and then claiming
that the original issue has effectively been
settled.
• The irrelevant issue is just a distraction.
• All fallacies of relevance are red herrings, but
reserve this to describe fallacies that do not fit
into the other categories.

35. Fallacy of Relevance: Red Herring
Every woman should have the right to an
abortion on demand. There’s no question
about it. These anti-abortion activists block
the entrances to abortion clinics, threaten
abortion doctors, and intimidate anyone who
wants to terminate a pregnancy.

36. Fallacy of Relevance: Red Herring
• The last part of the argument may be true, and it
may be bad, but it’s not relevant.
• The issue is whether women should have the
right to abortion on demand.
• The arguer shifts the subject to the behaviour of
anti-abortion activists, as though their behaviour
has some bearing on the original issue.
• Their behaviour, of course, has nothing to do with
the main issue.

37. Fallacy of Relevance: Straw Man
• Distorting, weakening, or oversimplifying
someone’s position so that it can be more
easily attacked or refuted.
Distort a claim.
Refute the distorted claim.

38. Straw Man?
The Opposition is opposed to the new military
spending bill, saying that it’s too costly. Why
does the NDP always want to slash everything
to the bone? They want a pint-sized military
that couldn’t fight off a crazed band of
terrorists, let alone a rogue nation!

39. Straw Man?
The B.C. Civil Liberties Union has criticized a
new anti-porn law because they say it
constitutes unreasonable censorship. As usual,
they are defending the porn industry! They
want to make it easier for sickos to distribute
kiddy porn. Don’t let them do it. Don’t let
them win yet another battle in defence of
perversion.

40. Fallacies of Insufficient Evidence
• These fallacies have unacceptable premises:
– Begging the question
– False dilemma
– Slippery slope
– Hasty generalization
– Faulty analogy
– Questionable cause

41. Fallacy of Insufficient Evidence:
False Dilemma
• Asserting that there are only two alternatives
to consider in some issue when there are
actually more than two.

42. Fallacy of Insufficient Evidence:
False Dilemma
Look, either you support the war, or you are a
traitor to your country. You don’t support the
war. So you’re a traitor.
– This argument only works if there really are only
two alternatives.
– Because this argument does not allow other
possibilities, it is fallacious.
– If you can think up more possibilities, then you
may be looking at a false dilemma.

43. Fallacy of Insufficient Evidence:
Slippery Slope
• Arguing, without good reasons, that taking a
particular step will inevitably lead to a further,
undesirable step (or steps).
• If you take the first step on a slippery slope,
you will have to take others because the slope
is slippery.

44. Fallacy of Insufficient Evidence:
Slippery Slope
• Arguing, without good reasons, that taking a
particular step will inevitably lead to a further,
undesirable step (or steps).
• If you take the first step on a slippery slope,
you will have to take others because the slope
is slippery.
– But not all slopes are necessarily slippery.
– Some consequences are not inevitable.

45. Fallacy of Insufficient Evidence:
Slippery Slope
If the Federal Government’s “Gun Registry”
goes ahead, law-abiding citizens will have to
register their hunting rifles. Next thing you
know, the government will go further and
rifles will be banned altogether. And ultimately
all guns will be banned, and then before long,
anything that could be used as a weapon will
be illegal. So if the Gun Registry goes ahead,
we might as well turn in our pen-knives and
baseball bats now!

46. Fallacy of Insufficient Evidence:
Slippery Slope
Oppose increases in tuition! Or next thing you know,
tuition will be $20,000 per year!
We should not ban child pornography. After all, next
they’ll ban all pornography, then all erotica, and then
all romance novels!
If I give you an extension on your essay just because
you had the flu, next thing you know people will want
extensions because they have a hangover!