Presentation: Fallacies - Presumption vs. Relevance
Homework
• Reading: pp. 145-152
The Study
Guide is
available
online
• Recommended Exercises
(do at least the starred problems)
– 4.3.III (of relevance & weak induction)
– 4.4.I
(of presumption & ambiguity)
– 4.4.III: 1-50
Model exercise for the exam
Remember
How does each specific argument commit a fallacy of that type?
• Where precisely is the offending element in the argument?
Kinds of Informal Fallacies
Fallacies of:
1.
2.
3.
4.
Relevance
Weak Induction
Presumption
Ambiguity or Whole/Part
a.
b.
Ambiguity
Whole/Part
• Where premises do
support the conclusion
– Insufficient evidence to
warrant conclusion
•
•
Conclusion unlikely
Fallacies of induction
Kinds of Informal Fallacies
Fallacies of:
1.
2.
3.
4.
• Premises are logically
immaterial to conclusion
Relevance
Weak Induction
• Attention drawn away from
supporting evidence
Presumption
– appeal to some irrelevant
Ambiguity or Whole/Part
a.
b.
Ambiguity
Whole/Part
concern
– often intentionally deceptive
• Premises appear relevant
Kinds of Informal Fallacies
Fallacies of:
1.
2.
3.
4.
Relevance
Weak Induction
Presumption
Ambiguity or Whole/Part
a.
b.
Ambiguity
Whole/Part
• Premises reiterate or
presuppose conclusion
– Simple or deceptive
– May also presume what
needs proving
Fallacies of Presumption
1. Begging the Question
2. Complex Question
Formally, identical
to valid arguments
3. Either … or … (false dichotomy)
4. Suppressed Evidence
Fallacies of Presumption
• Begging the Question
1. Conclusion is restatement of some premise
•
Simple: In same words
•
Complex: In different words of same meaning
•
Deceptive: Key premise missing but asserted as
premise
2. Circular argument
•
Premises presuppose conclusion which, itself,
presupposes premises
Fallacies of Presumption
P⊢P
1 (1) P
A
Fallacies of Presumption
• BQ (restatement)
– We have a brain in our skulls but no mind. The
mind is a myth, since the mind is just a fairy tale
we tell ourselves to believe in life after death.
A myth and a fairy tale
are one and the same thing
Shaky premise
Fallacies of Presumption
• Circular Reasoning
To cast abortion as a solely private moral question…is to
lose touch with common sense: How human beings treat
one another is practically the definition of a public moral
matter. Of course, there are many private aspects of
human relations, but the question whether one human
being should be allowed fatally to harm another is not
one of them. Abortion is an inescapably public matter.
Source: Helen M. Alvaré, The Abortion Controversy, Greenhaven, 1995, p. 23.
(http://www.fallacyfiles.org/begquest.html)
Fallacies of Presumption
• Circular Reasoning
To cast abortion as a solely private moral question…is to
lose touch with common sense: How human beings treat
one another is practically the definition of a public moral
matter. Of course, there are many private aspects of
human relations, but the question whether one human
being should be allowed fatally to harm another is not
one of them. Abortion is an inescapably public matter.
Source: Helen M. Alvaré, The Abortion Controversy, Greenhaven, 1995, p. 23.
(http://www.fallacyfiles.org/begquest.html)
Fallacies of Presumption
• Circular Reasoning
To cast abortion as a solely private moral question…is to
lose touch with common sense: How human beings treat
one another is practically the definition of a public moral
matter. Of course, there are many private aspects of
human relations, but the question whether one human
being should be allowed fatally to harm another is not
one of them. Abortion is an inescapably public matter.
Source: Helen M. Alvaré, The Abortion Controversy, Greenhaven, 1995, p. 23.
(http://www.fallacyfiles.org/begquest.html)
Fallacies of Presumption
• Circular Reasoning
To cast abortion as a solely private moral question…is to
is a public
matter.
lose touchAbortion
with common
sense: How
human beings treat
one another is practically
the definition of a public moral
Why?
matter. Of course, there are many private aspects of
human relations, but the question whether one human
Abortion
is a notfatally
a private
matter.
being should
be allowed
to harm
another is not
one of them. Abortion is an inescapably public matter.
Source: Helen M. Alvaré, The Abortion Controversy, Greenhaven, 1995, p. 23.
(http://www.fallacyfiles.org/begquest.html)
Fallacies of Presumption
• “Either … or …” fallacy
– You should just drop out of school. Either take a
full course load or drop out. And with everything
going on right now you can’t take a full course
load.
PvQ, ~P ⊢ Q
1 (1) P v Q
A
2 (2) ~P
A
Standard Features:
1,2 (3) Q
1,2 vE
1. Express disjunction
2. Reasonable alternatives ignored or suppressed
Fallacies of Presumption
• Complex Question
– After J. Gordon Liddy served time in prison for his role
in the Watergate scandal that brought down President
Nixon, he made an appearance on the Dick Cavett
show. On the show, Cavett mentioned the high rate of
homosexual activity in US prisons.
Cavett: “Did you have any trouble adjusting to
homosexuality in prison?”
Liddy: “No.”
Cavett: “There you have it folks. Mr. Liddy had no
trouble adjusting to homosexuality in prison.”
The two questions in one:
1. Did you adjust to a homosexual lifestyle in prison?
2. Was it difficult?
Fallacies of Presumption
• Suppressed Evidence
– Cough syrup is an effective remedy for adults
suffering from common respiratory distress. Kids
will benefit as equally.
Recognition of suppressed evidence can be challenging?
1. Obvious instances are, of course, simple to identify
2. More difficult cases: background knowledge often
necessary
Fallacies of Presumption
"The Second Amendment to the Constitution states that the right of
the people to keep and bear arms shall not be infringed. But a law
controlling handguns would infringe the right to keep and bear
arms. Therefore, a law controlling handguns would be
unconstitutional.
"In fact, the Second Amendment reads, "A well regulated militia being
necessary to the security of a free state, the right of the people to
keep and bear arms shall not be infringed." In other words, the
amendment states that the right to bear arms shall not be infringed
when the arms are necessary for the preservation of a wellregulated militia. Because a law controlling handguns (pistols)
would have little effect on the preservation of a well-regulated
militia, it is unlikely that such a law would be unconstitutional. (p.
146 of our text)
Fallacies of Presumption
"The Second Amendment to the Constitution states that the right of
the people to keep and bear arms shall not be infringed. But a law
controlling handguns would infringe the right to keep and bear
arms. Therefore, a law controlling handguns would be
unconstitutional.
"In fact, the Second Amendment reads, "A well regulated militia being
necessary to the security of a free state, the right of the people to
keep and bear arms shall not be infringed." In other words, the
amendment states that the right to bear arms shall not be infringed
when the arms are necessary for the preservation of a wellregulated militia. Because a law controlling handguns (pistols)
would have little effect on the preservation of a well-regulated
militia, it is unlikely that such a law would be unconstitutional. (p.
144 of our text)
Fallacies of Presumption
"The Second Amendment to the Constitution states that the right of
the people to keep and bear arms shall not be infringed. But a law
"The Supreme Court, splitting along
controlling handguns would infringe the right to keep and bear
ideological
lines, onhandguns
June 26would
<2008>
arms. Therefore,
a law controlling
be
unconstitutional.
declared the District of Columbia's
ban
on handgun
"In fact, the Second
Amendment
reads, ownership
"A well regulated militia being
necessary
to the security of a free(Washington
state, the right of
the people to
unconstitutional."
Post)
http://www.washingtonpost.com/wp-dyn/content/story/2008/06/23/ST2008062300649.html?hpid=topnews
keep
and bear arms shall not be infringed." In other words, the
amendment states that the right to bear arms shall not be infringed
when the arms are necessary for the preservation of a wellregulated militia. Because a law controlling handguns (pistols)
would have little effect on the preservation of a well-regulated
militia, it is unlikely that such a law would be unconstitutional. (p.
144 of our text)
Homework
• Reading: pp. 145-152
The Study
Guide is
available
online
• Recommended Exercises
(do at least the starred problems)
– 4.3.III (of relevance & weak induction)
– 4.4.I
(of presumption & ambiguity)
– 4.4.III: 1-50
Model exercise for the exam
Remember
How does each specific argument commit a fallacy of that type?
• Where precisely is the offending element in the argument?