Chapter 31:
Fallacies of Weak Induction
Appeal to Authority (pp. 359-360)
• The fallacy of appeal to authority occurs
when someone is taken to be an authority
on a particular topic when he or she is not.
As we acknowledged in Chapter 11, there
are criteria for evaluating a person’s
testimony.
• A. Questions of background regarding the
area
– If a person is testifying outside of his or her
area of expertise, there are grounds for
claiming an illegitimate appeal to authority.
Appeal to Authority (pp. 359-360)
– If an actor or sports personality claims that a certain
brand of pain reliever is particularly effective, you
have little reason to believe the claim: pain relief is
outside the area of expertise of an actor or sports
personality.
– Commercial endorsements by famous personalities
always involve a potential conflict of interest.
• A person has a conflict of interest if he or she appears to
have mixed motives in making a claim.
• When a person famous for his or her sports prowess, acting
prowess, etc. appears in a commercial for cereal, or cars, or
hardware, or anything else that is outside his or her clear
area of expertise, there is a conflict of interest: you don’t
know whether the testimony rests upon their sincere beliefs
or the money they’re paid to be in the commercial. The
question arises even if you have a person known for his or
her prowess in sports who is advertising sporting goods.
Appeal to Ignorance (p. 361)
• The general form of this argument is “There is
not evidence that x, so not x,” or “There is no
evidence that not x, so x.”
• A. Fallacious cases
– Fallacious cases arise when the evidence on both
sides of a question is open:
• “There is no evidence that extraterrestrial creatures do no
live on earth, so they do.”
– You have to be careful, since there are cases in which
the lack of positive evidence is good negative
evidence. If there is no reason to believe that your
roommate smokes, there is good reason to believe
that your roommate does not smoke, for example.
Appeal to Ignorance (p. 361)
• B. Ties to testimonial evidence
– If an authority in a field, or all authorities in a field,
contend that there is no reason to believe some
statement p, you have good reason to believe that not
p. The fact that virtually all scientists agreed that
Fleishmann and Pons did not produce cold fusion in
1989 is good reason to believe that they did not.
Hasty Generalization (pp. 361-362)
• A hasty generalization is a generalization
drawn from either too few cases, or
atypical cases, or both.
– “My Pontiac is unreliable. So, all Pontiacs are
unreliable.”
– This is drawn from one case. It is also a
problematic case: My Pontiac is nearly a
decade old, and it is one of the less expensive
models.
False Cause (pp. 362-365)
• The fallacy of false cause occurs when a false
causal claim is taken as a premise.
• Most superstitious beliefs commit this fallacy.
• A. Taking something to be a cause when it is not
(non causa pro causa)
– I carry a rabbit’s foot. That’s why I’ve been blessed
with good fortune.
– There is no causal connection between carrying a
rabbit’s foot and good fortune. If you have any doubts,
just ask the rabbit.
False Cause (pp. 362-365)
• B. One event occurred before another, so it was
the cause (post hoc ergo propter hoc)
– I walked under a ladder on the way to work. The
whole day was terrible. It’s all because I walked
under a ladder.
• The walking under a ladder occurred before the terrible,
awful, horrid day, but given that alone, there is no reason to
believe there was any connection.
• Of course, if someone dropped paint on you when you
walked under the ladder, there could be a causal connection
between the first action and the resulting day, but walking
under the ladder was not itself the cause of the state of the
day.
Slippery Slope (Wedge)
(pp. 365-366)
• There is a chain of causes, and at least one
causal claim is false.
• Typically, in a slippery slope argument things
begin bad and grow progressively worse.
• A. Causal chains
– In its fallacious form, there is a hypothetical syllogism
with a false premise.
– “If you smoke cigarettes, you’ll try marijuana. If you
try marijuana, you’ll try heroin. If you try heroin, you’ll
become addicted to cocaine. So, you don’t want to
smoke cigarettes.”
– Some people who smoke cigarettes try marijuana, but
they do not all do so. The argument is fallacious.
Slippery Slope (Wedge)
(pp. 365-366)
• B. Nonfallacious slippery slopes
– There are causal chains. Sometimes things begin
bad and grow progressively worse. So, you need to
check to see whether all the premises of the relevant
hypothetical syllogism are true.
Weak Analogy (pp. 366-367)
• You commit the fallacy of weak analogy if (see
the criteria in Chapter 25):
– There are claims of similarities where the similarities
are lacking.
– There are significant disanalogies.
• “Critical thinking tests are like mathematics tests insofar
as they are both written on paper. Calculators are
helpful when taking mathematics tests. So calculators
are helpful when taking critical thinking tests.”
• There are significant disanalogies between
mathematics tests and critical thinking tests, but if you
think a calculator will help, you’re welcome to use one!