Answer
Syllogism: All good test scores mean good course
grades. Samsa’s test scores are all good. Samsa gets good
course grades.
Conditional: If you get good test scores, then you get
good course grades. Samsa gets good test scores. There-
fore, he gets good course grades.
How Deduction Can Be
Misused
In the next lesson, you will learn about specific ways in
which deductive arguments are used incorrectly,
whether negligently or deliberately. The better you
become at spotting these “logical fallacies,” the less likely
you will be to accept one as truth.
Simply, a deductive argument is invalid for one of
two possible reasons: either or both of the premises are
invalid, or the wrong conclusion was reached even
though the premises are valid. This example contains
a premise that is not true:
All Americans wear sneakers. (Major premise)
Harold is an American. (Minor premise)
Therefore, Harold wears sneakers. (Conclusion)
Since all Americans do not wear sneakers, the
major premise is not true. That makes the conclusion,
and therefore the deductive argument itself, invalid.
In this case, the wrong conclusion is reached:
Many Americans wear sneakers.
Harold is an American.
Therefore, Harold wears sneakers.
Note that by restating the invalid premise to make
it valid, you have not made the conclusion true. Harold
may or may not be in the group of “many” who wear
sneakers. The conclusion makes an assumption that
goes beyond the information contained in the premises.
In Short
Deductive reasoning takes two premises, which may be
rules, laws, principles, or generalizations, and forms a
conclusion based upon them. In order to be valid, a
deductive argument must have premises that are true
and a conclusion that logically follows from those
premises, without trying to go beyond them.When you
understand how these arguments work, you will know
how to construct your own strong arguments.You will
also avoid being influenced or persuaded by faulty
deductive reasoning when you recognize it and see its
flaws.
–DEDUCTIVE REASONING–
98
■Find a deductive argument in print. Put it in the form of a diagram, listing the major premise, minor
premise, and conclusion. Is it valid? If not, why?
■The next time you need to persuade someone to do something, such as eat at your favorite restau-
rant instead of theirs or see the movie you prefer, argue for your choice using deductive reasoning.
Skill Building Until Next Time
LESSON 12 EXPLORED the characteristics of a valid deductive argument. You know that you
need two premises which are true, and a conclusion that logically follows from them without
assuming or inferring any information not contained in the premises.An invalid argument con-
tains one or more errors. It might have a factual error, such as a premise that is not true, or a conclusion
that is not supported by the premises. Or, it may contain an error in logic. This type of error is known as a
fallacy.
There are a number of logical fallacies that can occur in deductive arguments. There are four major
logical fallacies:
1. Slippery Slope
2. False Dilemma
3. Circular Reasoning
4. Equivocation
Each of these will be explained in detail in the next sections.
LESSON Misusing
Deductive
Reasoning—
Logical Fallacies
LESSON SUMMARY
In this lesson you will see how the relationship between deductive rea-
soning and logic works, or does not work. This lesson explores four
of the most common logical fallacies that make deductive reasoning
fall apart.
13
99
The argument might have two true premises, and
a conclusion that takes them to an extreme. This is
known as the slippery slope fallacy. Or, it might be a
false dilemma fallacy, which presents in its major
premise just two options (“either-or”) when in reality
there are others. In circular reasoning, also known as
begging the question, there is just one premise, and the
conclusion simply restates it in a slightly different form.
And finally, equivocation uses a word twice, each time
implying a different meaning of that word, or uses one
word that could mean at least two different things.
Arguments intended to convince or persuade may
be believable to many listeners despite containing such
fallacies, but they are still invalid. Recognizing these fal-
lacies is sometimes difficult. But it is important to be
able to do so to prevent being mislead, or persuaded by
faulty logic.
Slippery Slope
In Lesson 12, we discussed conditionals, which are one
of the ways in which a deductive argument may be
framed. Conditionals use an “if-then” premise to lead
to a conclusion (example: if you do not pay your elec-
tric bill, then your power will be turned off). When a
conditional contains a logical fallacy, it is called a slip-
pery slope.
In this type of fallacy, it is asserted that one event
will or might happen, and then, inevitably, another,
more serious or drastic, event will occur. The slippery
slope does not explain how the first event leads to the
other. Often, it leaves out a number of steps between
the two events, without saying why they will simply be
bypassed. The argument takes the following form:
1. Event A has/will/might occur.
2. Therefore, event B will inevitably occur.
The slippery slope argument makes an oppo-
nent’s argument seem more extreme. It says that event
A will eventually lead to an extreme, unwanted event
B. The argument infers that the only way to avoid event
B is to not do event A, or even anything at all. The gun
lobby uses the slippery slope all the time to argue
against any type of gun control. They say that any small
measure, such as registration or waiting periods to pur-
chase firearms, will lead to drastic control, or even con-
fiscation of their weapons.
Here is another example:
“We have to stop the tuition increase!
Today, it’s $5,000; tomorrow, they will be
charging $40,000 a semester!”
Note that there are many possible steps between
event A, the tuition increase, and event B, the charging
of $40,000 a semester. An increase could occur every
year for ten years or more before there was a jump from
five to forty thousand dollars. In addition, tuition might
never reach $40,000. This is a slippery slope because
one tuition hike to $5,000 does not inevitably lead to
a charge of $40,000.
Other examples are listed below. Keep in mind the
possible intermediate steps between event A and event
B in each, and the likelihood, or unlikelihood, that B
will ever be a result of A.
■Don’t let him help you with that. The next
thing you know, he will be running your life.
■You can never give anyone a break. If you do,
they will walk all over you.
■This week, you want to stay out past your cur-
few. If I let you stay out, next week you’ll be
gone all night!
–MISUSING DEDUCTIVE REASONING—LOGICAL FALLACIES–
100
Practice
Rewrite the following argument to remove the slippery
slope fallacy:
We shouldn’t give military aid to other
countries. The next thing you know, we
will have thousands of troops overseas
dying for no good reason.
Answer
Answers will vary, but all should give realistic, possible
reasons why we should not give military aid to other
countries. There should be a logical step from event A
(giving military aid) and event B (the answer).
Responses might include: it’s too dangerous; the next
thing you know, they will be asking for more; we
shouldn’t let our military get spread out too thinly, etc.
False Dilemma
A false dilemma is an argument which presents a lim-
ited number of options (usually two), while in reality
there are more options. In other words, it gives a choice
between one or another (“either-or”) even though there
are other choices which could be made. The false
dilemma is commonly seen in black or white terms; it
sets up one thing as all good and the other as all bad.
When one option (typically the “all bad”one) is argued
against, the false dilemma concludes that the other
must be true.
Example
Stop wasting my time in this store! Either
decide you can afford the stereo, or go
without music in your room!
This argument contains a logical fallacy because
it fails to recognize that there are many other possibil-
ities than just buying one particular (expensive) stereo
and going without music. You could, for instance, buy
a less expensive stereo or even a radio. Or, you could
borrow a stereo and have music in your room without
making a purchase. There are many options beside the
two presented as “either-or” in the argument.
Other common false dilemmas include:
Love it or leave it.
Either you’re with us, or you’re against us.
Get better grades or you will never go to
college.
False dilemmas are also common in politics.
Many politicians would like you to believe that they,
and their party, have all the right answers, and their
opponents are not only wrong, but they are ruining the
country. They set up a choice between all good and all
bad. Political speeches often include rhetorical ques-
tions that contain false dilemmas. For instance: “Price
supports on agricultural production are part of the
socialist agenda. My opponent in this race consistently
votes for price supports on dairy and tobacco products.
It is time to stop electing socialists to Congress. Should
you vote for my opponent, who wants to lead our coun-
try on the path toward socialism, or should you vote for
me, and restore democracy?
Practice
Which of the following is NOT a false dilemma?
a. Your grades are lousy. Either study more, or drop
out of school.
b. We have a big game tonight. Either we will win
and be eligible for the tournament, or we will lose
and our season will be over.
c. Stop driving like a maniac! Either slow down, or
take the bus.
d. I can’t believe you didn’t vote to raise the mini-
mum wage. Either you missed the vote, or you
just don’t care about the working poor!
–MISUSING DEDUCTIVE REASONING—LOGICAL FALLACIES–
101
Answer
Choice bis not a false dilemma. It is a statement of fact
that there are only two possible outcomes, a win or a
loss. All the other choices present only two options,
when in fact there are others to consider.
Circular Reasoning
A valid deductive argument has a conclusion that fol-
lows logically from the premises. It does not infer or
assume anything from the premises, but relies only on
the information contained within them. In the fallacy
of circular reasoning, often called begging the question,
you assume as truth the premise you are supposed to
be proving. In all valid deductions, the conclusion
(what you are trying to prove) follows two premises. In
an invalid argument using circular reasoning, the con-
clusion follows a single premise. In other words, the
premise that is supposed to prove the truth of the con-
clusion is simply the conclusion restated with a slight
variation. Circular reasoning looks like this: A is B,
therefore A is B.
When a premise is left out, there is no argument.
The person making the claim is simply telling to you
believe that what he is telling you is true.
Examples
1. “I told you to clean your room!”“Why?”
“Because I said so!”
2. “Why do you think the Yankees are the best
team in baseball?”“Because they are.”
How could these examples go from being invalid
to valid, logical arguments? They need to add a second
premise that supports, or gives reason for, the conclu-
sion. Example 1 might add: “Your room is so messy that
you can’t find anything in it,” or, “All of your laundry
is on the floor, and it won’t get washed until you clean
it up and put it in the washer.” Example 2 could add:
“They have won the World Series 26 times in the last
39 appearances,” or, “They are the only team to sweep
the World Series ten times.”
Practice
Which of the following does not beg the question?
a. I like the Brewers because they’re my favorite
team.
b. Ghosts exist because I saw something once that
could only have been a ghost.
c. The Seafood Shack is the best restaurant in town
because it’s so much better than all the others.
d. They signed Bruce Springsteen to headline the
concert because he’s a rock legend and a huge
star.
Answer
Choice ddoes not beg the question. It gives two reasons
why Springsteen was signed. It would have been an
example of circular reasoning if it went: “They signed
Bruce Springsteen to headline the concert because he’s
a concert headliner.”
Equivocation
The fallacy of equivocation can be difficult to spot,
because both of the premises appear to be true, and
sometimes the conclusion seems to follow them. How-
ever, in this fallacy, the meaning of a certain word is
unclear and it causes the meaning of the entire argu-
ment to be invalid. This can occur either by using the
same word twice, each time with a different meaning,
or by using one word that has an ambiguous meaning.
–MISUSING DEDUCTIVE REASONING—LOGICAL FALLACIES–
102