Fallacies of
Weak Induction
1
Introduction
The key characteristic of these fallacies is
that the connection between the
premises and conclusion is not strong
enough to support the conclusion
2
Appeal to Unqualified Authority:
Definition
This fallacy occurs when the appeal is
made to an authority or witness that is
not trustworthy or is not qualified in that
particular area.
3
Appeal to Unqualified Authority:
Example

We should all accept the claim that the
moon is made of green cheese. After
all, Professor McNellis says it is, and
he’s a recognized authority in ethics.

Comment: McNellis is not qualified in
the area of astronomy!
4
Appeal to Unqualified Authority:
Key Question

Does the arguer appeal to an authority
who is actually not qualified in that area
for support of his/her conclusion ?
5
Appeal to Ignorance
Occurs when the premises of an
argument state that nothing has been
proved one way or the other about
something, and the conclusion then
makes a definite assertion about that
thing
6
Appeal to Ignorance:
Example

Example: People have been trying for
centuries to disprove the claims of
astrology, and no one has ever
succeeded. Therefore, we must
conclude that the claims of astrology
are true.

Comment: The only justified conclusion
is that we don’t know.
7
Appeal to Ignorance:
Key Question

Does the arguer claim something is true
because no one has disproved it or that
something is false because no one has
proved it to be true?
8
Hasty Generalization
Definition
This fallacy occurs in inductive
generalizations where there is a
reasonable likelihood that the sample is
not representative of the group
9
Hasty Generalization:
Example


Four Arab fundamentalists were
convicted of bombing the World Trade
Center in New York City. The message
is clear: Arabs are nothing but a pack of
religious fanatics prone to violence.
Comment: This draws a conclusion
about a whole group on the basis of one
incident.
10
Hasty Generalization:
Key Question

Does the speaker/author draw an
inductive conclusion based on a sample
that is too small or too
unrepresentative?
11
False Cause:
Definition

Occurs whenever the link between
premises and conclusion depends on
some imagined causal connection that
probably does not exist
12
False Cause: Example

During the past two months, every time that
the cheerleaders have worn blue ribbons in
their hair, the basketball team has been
defeated. Therefore, to prevent future
defeats, the cheerleaders should get rid of
those blue ribbons.

Comment: There’s no reason to believe
wearing the ribbons caused the defeats.
13
False Cause:
Key Question

Does the conclusion maintain that one
thing caused another when it probably
did not?
14
Slippery Slope:
Definition

This fallacy occurs when the conclusion
rests on an alleged chain reaction and
there is not sufficient evidence to
believe that the chair reaction will
actually occur.
15
Slippery Slope:
Example

“Immediate steps should be taken to outlaw pornography
once and for all. The continued manufacture and sale of
pornographic material will almost certainly lead to an
increase in sex-related crimes such as rape and incest.
This in turn will gradually erode the moral fabric of society
and result in an increase in crimes of all sorts. Eventually
a complete disintegration of law and order will occur,
leading the the total collapse of civilization”

Comment: It’s not proved that legal pornography leads to
these consequences.
16
Slippery Slope:
Key Question

Does the speaker/author claim that a
single step will eventually lead to
disastrous consequences when there is
little evidence that this will actually
occur?
17
Weak Analogy:
Definition

This fallacy occurs in some arguments
from analogy in cases where the arguer
ignores important differences between
the type types of cases
18
Weak Analogy
Example
“Political dissent is like a cancer in the body
politic. And we all know what the best treatment
for cancer is: radical surgery. So when we see
dissidents, we should cut them out of society as
fast as possible before they spread!”
Comment: This ignores important differences
between cancer and political dissent, especially:
people have rights, cancer cells don’t.
19
Weak Analogy:
Key Question

Does the arguer introduce an analogy
but ignore important differences
between the two classes of things being
compared?
20