Effective Argumentation
Prof. R. Cherubin
Corrected URLs
Some web sites identifying and explaining logical and argumentative
fallacies
• Logical Fallacies and the Art of Debate (G. Whitman, California State
University at Northridge): http://www.csun.edu/~dgw61315/fallacies.html
•
Logical Fallacies (philosophical society.com):
http://www.philosophicalsociety.com/Logical%20Fallacies.htm
•
Logic (Garth Kemerling, Philosophy Pages):
http://www.philosophypages.com/lg/index.htm
•
Some Logical Fallacies (J.P. Craig, University of Iowa):
http://www.uiowa.edu/~c100298/fallacies.html
•
Logical Fallacies (Logical Fallacies .Info; many links and illustrative
examples): http://www.logicalfallacies.info/index.html
•
Fallacies (Bradley Dowden, Internet Encyclopedia of Philosophy):
http://www.iep.utm.edu/f/fallacy.htm
Evidence is only the beginning
• For an argument to be effective, in the
sense used in this course, you must do
more than present the information that you
take to be evidence for the truth or validity
of your conclusion. You also need to show
how – by what reasoning – that evidence
makes your conclusion true or valid.
Evidence is only the beginning
• Even valid evidence can be used
ineffectively or deceptively to reach
unsupported or incorrect conclusions.
• Therefore correct and adequate reasoning
is also part of effective argument.
How to Argue Well – And How Not To Do It
• Ineffective or incorrect arguments can appear in
your sources. Do not be lulled into trusting
everything a distinguished author says, just
because he or she is distinguished or presents
things in an impressive manner.
• It’s also possible that you may develop an
ineffective or incorrect argument concerning the
data you have gathered.
• This lecture will show you some common
problems, and how to detect and avoid them.
The Logical Fallacy “Post Hoc Ergo Propter Hoc”
(“After this, therefore because of this”)
• A “post hoc ergo propter hoc” argument asserts that
since event A happened (or always happens) before
event B, A must cause B.
• While it may be true that A is the cause of B, it also may
not be true. In any case, the fact that A happened (or
always happens) before B does not BY ITSELF show
that A is a cause of B. More information and reasoning
are needed to show whether A causes B.
The Logical Fallacy “Post Hoc Ergo Propter Hoc”
(“After this, therefore because of this”)
• Example:
• “Global temperatures have risen since the founding of
McDonald’s® restaurants. Therefore McDonald’s is a
cause of temperature change.”
• Is it true that McDonald’s causes temperature change? -Perhaps, perhaps not. Is the argument above enough to
show that McDonald’s causes temperature change? –
No. – What other evidence and reasoning are needed?
The Logical Fallacy “Cum Hoc Ergo Propter Hoc”
(“With this, therefore because of this”)
• A cum hoc ergo propter hoc argument claims that since
A and B occurred together, A must cause B. (A variant of
this fallacy suggests that if A and B occur together every
time they are observed, then they must occur together.)
• While it may be true that A caused B, it may instead be
true that B caused A. Or it may be the case that neither
of them caused the other. It may be the case that they
occurred together by coincidence, or because of some
third factor that the arguer has not identified.
The Logical Fallacy “Affirming the Consequent”
• This is conceptually related to the previous two, but
covers in addition arguments that do not involve
temporal relationships.
• An argument that “affirms the consequent” begins with a
statement of the form “If A, then B.” It then produces
evidence that B is the case. From that, it concludes that
A is the case.
• As before, it may in fact be true that A is the case. But
more is needed to show that it is true.
The Logical Fallacy “Affirming the Consequent”
Example 1:
“It is raining, therefore the streets are wet.” (If it is raining (A), then the
streets are wet (B).)
“The streets are wet.” (B)
“Therefore it is raining.” (Therefore A)
• What’s wrong with this?
The Logical Fallacy “Affirming the Consequent”
(this image comes from cpgis.gmu.edu)
The Logical Fallacy “Affirming the Consequent”
Example 2:
“If an animal is moving its tail (A), then the animal is alive
(B).”
“The animal is alive.” (B)
“Therefore it is moving its tail.” (Therefore A)
What’s wrong with this chain of reasoning?
The Logical Fallacy “Affirming the
Consequent”
Thus in general, when you have an “if-then” statement and
the consequent (the “then” part) is true, that does not BY
ITSELF make the antecedent (the “if” part) true. If you
want to show that the antecedent is true, you need to
find more evidence and show how that evidence makes
the antecedent true.
Logical Fallacy “Circular Reasoning”
• “Circular” arguments look as though they are proving
something, but on closer inspection one finds that they
are simply repeating something without proving it. The
antecedent (the “if” part) is actually the same as the
consequent (the “then” part, or the “therefore” part).
• One common kind of circular argument is known as
“begging the question” (i.e. begging off from the
question, avoiding a question). In this kind of argument,
something questionable is assumed to be true, and the
conclusion of the argument simply repeats this
assumption without supporting it.
Logical Fallacy “Circular Reasoning”
Example 1:
“The SAT is a good indicator of intelligence. Intelligence
varies directly with test scores.”
“Therefore the students who do well on the SAT are more
intelligent than those who do not.”
What’s wrong with this reasoning?
Logical Fallacy “Circular Reasoning”
Example 2:
“There is conservation of matter and energy.”
“No thing can come from nothing.”
“There has been no thing or process that lacked a predecessor, and
there has been no thing or process that disappeared leaving nothing
behind.”
“Therefore the sum of matter and energy in the universe remains, and
always will remain, the same.”
- What’s wrong with this reasoning? – Note that the second, third, and
fourth steps really echo the first step in one way or another; and the
fourth step is simply a restatement of the first. – Note also that the
poor quality of the argument does not mean that there is NOT
conservation of matter and energy (though if there isn’t...you heard it
here first).
“Fallacy of Composition”
“Fallacy of Division”
• In the fallacy of composition one assumes that if the
parts of a thing have a certain quality or attribute, then
the whole thing (that of which the parts are parts) has
that quality or attribute too.
• In the fallacy of division one assumes that if a thing as a
whole has a certain quality or attribute, then its parts (or
a specific part of it) has that quality or attribute too.
“Fallacy of Composition”
“Fallacy of Division”
Example 1 (“composition”)
“No state, territory, or protectorate in the U.S. can grant
passports to citizens. Therefore the U.S. cannot grant
passports to its citizens.”
Example 2 (“division”)
“A healthy cat can play with string. Therefore its appendix
can play with string.”
“Fallacy of Composition”
“Fallacy of Division”
• There are of course cases where the whole and its parts
do have characteristics in common. For example, if
every square inch of a car’s exterior is red, then the
whole of the car’s exterior is red. And if one part of a
person’s body has a certain DNA sequence, the person’s
whole body has that sequence.
“Fallacy of Composition”
“Fallacy of Division”
• But the difference between committing the fallacy of
composition or the fallacy of division on the one hand,
and making a true statement about the relationship
between a whole and its parts on the other, is this:
• In the fallacies, one assumes that because the whole
has a characteristic, the part must have it, or vice-versa,
without investigating whether the characteristic in
question is one that must be shared by part and whole.
Argumentative Fallacy “False Dichotomy”
• An argument commits the fallacy of false dichotomy
when it suggests falsely that there are only two
possibilities in a given situation, and then claims that if
one of the two is not fulfilled, the other must be.
• Sometimes the assumption that only two possibilities
exist is not made explicit, but it becomes clear from the
way the argument proceeds.
Argumentative Fallacy “False Dichotomy”
• Example 1:
“Everyone in this country is either a Democrat or a
Republican.”
“Senator Jeffords is not a Republican.”
“Therefore he is a Democrat.”
What’s wrong with this?
Argumentative Fallacy “False Dichotomy”
• Example 2:
According to Plato, Socrates was
accused of atheism by someone
who argued in the following way:
“Socrates does not believe in the
gods of Athens, therefore he is an
atheist.”
Leaving aside the question of whether
Socrates actually believed in the
gods of Athens, it’s clear that his
accuser assumed...
Argumentative Fallacy “False Dichotomy”
...that the gods of Athens are the only gods, so that one who does not
believe in them does not believe in any gods.
In assessing others’ arguments, and in constructing your own, take time
to ask whether there are alternative possibilities that have not been
considered.
Logical Fallacy “Undistributed Middle”
• An argument commits the fallacy of undistributed middle
if it claims falsely that because all A’s are B’s and all C’s
are B’s, all A’s are C’s. (Or: because all A’s are B’s and
person C is a B, person C is an A.)
Example 1:
“All wombats are marsupials.”
“All sugar gliders are marsupials.”
“Therefore all wombats are sugar gliders.”
Logical Fallacy “Undistributed Middle”
Example 2:
“All atheists support the separation of church and state.”
“George Mason supported the separation of church and
state.”
“Therefore George Mason was an atheist.”
What’s wrong with this inference?
Logical Fallacy “Undistributed Middle”
“That religion, or the duty which we owe to our Creator and
the manner of discharging it, can be directed by reason
and conviction, not by force or violence; and therefore,
all men are equally entitled to the free exercise of
religion, according to the dictates of conscience; and that
it is the mutual duty of all to practice Christian
forbearance, love, and charity towards each other.”
George Mason, Virginia Declaration of Rights, Article XVI.
Argumentative Fallacy “Argument from Authority”
An argument from authority claims that the reader should
accept something just because a person who is
distinguished or accomplished or experienced (in a
relevant field, or, in the more egregious cases, in any
field) says it is true.
But if the purported authority really is credible in this case,
he or she (or the author citing him or her) will provide the
reasoning and evidence to show why his or her claims
are correct.
Argumentative Fallacy “Argument from Authority”
Remember, the fact that someone has a Ph.D., even from
a great institution, does not mean that everything that he
or she writes reflects the standards of research that
merited the Ph.D.
Or, perhaps there is some research of which he or she is
unaware. Or perhaps the piece you are reading was
written some time ago, and later research showed flaws
in what you are reading.
• Then if a writer commits a fallacy or if for some other
reason we don’t know whether what he or she says is
true, should we conclude that what he or she says is
false?
• For example, if a writer claims that a new sub-species of
capybara has been discovered, but does not show that
the new sub-species is any different from the ones
already known, should we conclude that the new kind of
capybara does not exist?
Known capybaras – from the Phoenix Zoo
www.phoenixzoo.org/images/animals/capybara.jpg
No – that’s the fallacy of Argument from Ignorance.
In an argument from ignorance, a writer falsely assumes
that since we do not know that something is true, or that
something exists, we are justified in concluding that it is
not true.
But in fact, absence of evidence is not evidence of
absence. The fact that we have not seen evidence of the
new kind of capybaras does not mean that they do not
exist – and it does not mean that they do exist. It means
we do not have enough information to tell.
Tips for Arguing Well
• Become aware of your starting assumptions, and the
starting assumptions of the authors you are reading. Ask
yourself whether these assumptions are true; if you don’t
know, try to find out. If you can’t find out, consider that
the author’s arguments will be at best conditionally true:
true if the assumptions are true.
• Examine all arguments and explanations – your own and
the ones you read. Can you see how the conclusions
follow from the starting-points? If not, why not?
Tips for Arguing Well
• Examine all arguments and explanations – your own and
the ones you read. Can you see how the conclusions
follow as inferences from the starting-points? If not, why
not? Is the problem a gap in reasoning? A gap in the
evidence the author gives? Something else?
• Examine the principles that you (and the authors you
read) use to make inferences. Try to articulate them. For
example, does the author seem to reach the conclusion
that kangaroo migrations cause tsunamis because he or
she has observed that tsunamis often follow kangaroo
migrations? If so, he or she is reasoning fallaciously, in
that…
Tips for Arguing Well
• … ‘after’ does not by itself imply ‘because.’
Tips for Arguing Well
• Show all work. Show how and why your data support
your conclusion. Make sure you understand how (or
whether) other authors’ data support their conclusions.
The end of an argument neither justifies nor replaces the
means.
• Take extra care to examine the reasoning of authors
whose ideas or conclusions you agree with. The fact that
you are sympathetic to someone’s position does not
mean that this person has presented a valid argument,
or accurate evidence, for what he or she is saying. It’s
easy to take for granted that someone who agrees with
you has good reasons to do so.
Tips for Arguing Well
• Ask a friend – or better still, an enemy – to read what
you’ve written, and ask if he or she thinks you’ve
supported your conclusions adequately. Then return the
favor.
• “By no means did the gods reveal all things to mortals
from the beginning, but in time, by searching, mortals
discover better.”
- Xenophanes (6th century BCE)