Informal Fallacies
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Quiz!
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Midterm Study Guide
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Inconsistency Ad Hominem
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Circumstantial Ad Hominem
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Genetic Fallacy
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Post hoc ergo propter hoc
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For next time: Read Chapter 7 pages 216-223
Quiz!
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In one sentence, define the term 'inherent credibility'
Quiz!
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A major car manufacturer has just hired Robert Downey
Jr. to act as their spokesperson. In a recent commercial
RDJ claimed that “diesel technology is no longer a dirty
business.” What do you make of RDJ's claim?
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(a) That RDJ is an interested party
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(b) That RDJ is a legitimate authority in this domain
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(c) That the claim has some inherent credibility based on your
background beliefs and experience
(d) That appealing to RDJ is a fallacious appeal to authority
Quiz!
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“If you eat something for lunch then you will be
eating something during lunch” is an example of
what kind of claim?
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(a) A claim of value
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(b) A claim of irrelevance
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(c) A tautological claim
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(d) A contradiction
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(e) A syntactically ambiguous claim
Quiz!
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(A): I have heard that you have failed to lower the
unemployment rate in the last few years. Is that true?
(B): We have pumped billions into the local economy and we are
doing everything possible to bring in more investments
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This is an example of which fallacy?
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(a) Belief Bias
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(b) ad hominem
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(c) red herring
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(d) blue herring
Quiz!
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When does a personal attack become an ad
hominem?
Midterm Study Guide!
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You now have your hands on the midterm study guide
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The Midterm is next Friday (October 28)
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If a topic or term is not on the study guide then it will
not be on the midterm
Many of the questions will be slightly modified for the
midterm but not significantly (ex- argument diagrams)
There is one exception: all of the IDs on the midterm
are on the study guide
IDs
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The identification questions require at most 2
sentences
1. Define the term
2. Explain the significance of the term (why did we
study it?)
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Let's try an example:
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Semantic Ambiguity
Example
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Semantic Ambiguity:
When a claim has multiple meanings as a result of
the meaning of one the words in it. Semantic
ambiguity is important because it can lead us to
commit the fallacies of composition and division.
Ad hominems
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We have discussed several distinctions having to do
with critical evaluations of a source:
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1. Personal attacks
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2. Looking for interested parties
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3. Ad Hominem attacks
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We said that ad hominem attacks occur only when
we criticize the source of a claim as a way of
responding to the content of a claim
Ad Hominem types
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We have already talked a bit about one of the most
common forms of the ad hominem fallacy
The personal attack ad hominem
But there are other ways in which we can criticize a
source as a way of responding to the content of a
claim that do not include distinctly personal attacks
There is some overlap between fallacies like ad
hominem and other fallacies
An ad hominem attack may also be a red herring
Inconsistency Ad Hominem
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We commit the fallacy of the inconsistency ad hominem
when:
As a way of responding to a someone's (or some
organization's) claim we accuse them of contradicting
themselves
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You claim that you care about the environment but didn't
you fly over here in a big jet? Hypocrite!
An inconsistent person can make cogent claims and the
simple act of having changed one's mind is not indicative of
the soundness or strength of the arguments that they offer
Examples
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(A) We need to make sure that everyone pays their fair
share of taxes and the current tax rates are unfair.
(B) Didn't you cheat on your taxes last year? Why
should I listen to you?
In this case A is being accused of believing that taxes
should be fair but of unfairly evading taxation. This
has little to do with whether A's claim is true
It is also an example of a red herring
Examples (more)
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Images are not, strictly
speaking, arguments in the
technical sense
But this image also
commits the inconsistency
ad hominem fallacy
It does this by implying that
Kerry's claims are not
credible because of an
inconsistency
Circumstantial Ad Hominem
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We can also fallaciously question a claim's source by
faulting a source's circumstances as the true reason for
the claim
(A): The war on drugs is a hypocritical mess that needs
to be ended
(B): You're just saying that because you want to get
cheaper marijuana
Notice that B's response says nothing about A's claim
but implies that A accepts the claim for personal or
selfish reasons
Genetic Fallacy
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A genetic fallacy includes any fallacy that casts doubt on a
claim as a result of its history instead of its content
Ad hominem attacks are a kind of genetic fallacy. All
genetic fallacies ignore the claim and focus on the source
For example:
(A) We need to allow oil and gas exploration in the US
and the only way to do that is by relaxing environmental
laws
(B) Isn't that a Republican policy? No thanks
Genetic Fallacy
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The genetic fallacy, like ad hominem, is extremely common:
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(A) Public universities are a great resource
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(B) No they aren't, publicly owned institutions were first
built by socialist governments!
Whether or not the idea of state ownership of property (or
services: police, fire, healthcare, etc) originated in one kind
of government and economic system says little about the
merits of those things today.
Post Hoc Ergo Propter Hoc
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Post hoc ergo propter hoc is Latin for “after this therefore
because of it”
We commit the fallacy of PHEPH when we assume a causal
relationship exists between things just because one followed
another
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A happens then B happens
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:. A causes B
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The fact that A precedes B is not evidence that A causes B
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In scientific reasoning this has a slogan: correlation does not
imply causation!
Post Hoc Ergo Propter Hoc
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Statistics have shown that days in which ice cream
sales are high are also days with the greatest number
of shark attacks. This implies that high ice cream
sales are responsible for shark attacks.
Every time I have a cold I drink eight red bulls every
day for three days. My cold disappears within a day
or two after this. Therefore my red bull trick must be
curing my colds.
For Monday
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Read: Chapter 7 pages 216-223