Common Errors of Reasoning Explained

The following classic errors of reasoning appear with some frequency. The
review is given in layman’s, not philosophical, terms:
Uncertain Use of a Term or Concept
As an argument progresses, the author must use each term in a constant,
coherent fashion. Using a term in different ways is inherently confusing and
undermines the integrity of the argument. Here is an example:
“Some people claim that the values that this country was built on are
now being ignored by modern-day corporations. But this is incorrect.
Corporations are purely profit-driven enterprises, beholden only to their
shareholders, and as such they can only assess objects based on their
value.”
The term “value” is used in the example above in two different senses: first in a
moral or ethical sense and then in a monetary sense. This shift in meaning
undermines the author’s position.
This type of answer choice appears more frequently as an incorrect answer than
any other type. Here are examples of how this error of reasoning is described in
LSAT answer choices:
“depending on the ambiguous use of a key term”
“it confuses two different meanings of the word ‘solve’ ”
“relies on interpreting a key term in two different ways”
“equivocates with respect to a central concept”
“allows a key term to shift in meaning from one use to the next”
“fails to define the term”
Source Argument
Also known as an ad hominem, this type of flawed argument attacks the person
(or source) instead of the argument they advance. Because the LSAT is
concerned solely with argument forms, a speaker can never validly attack the
character or motives of a person; instead, a speaker must always attack the
argument advanced by a person. Here is an example:
“The anti-smoking views expressed by Senator Smith should be
ignored. After all, Smith himself is a smoker!”
A source argument can take different forms, including the following:
1. Focusing on the motives of the source.
2. Focusing on the actions of the source (as in the above example).
In the real world, you will often hear source arguments used by children and
politicians (the two being alike in a number of ways, of course).
Here are examples of how this error of reasoning is described in LSAT answer
choices:
“makes an attack on the character of opponents”
“it is directed against the proponent of a claim rather than against the
claim itself”
“he directs his criticism against the person making the argument rather
than directing it against the argument itself”
“it draws conclusions about the merit of a position and about the content
of that position from evidence about the position’s source”
“assuming that a claim is false on the grounds that the person defending
it is of questionable character”
Circular Reasoning
In circular reasoning the author assumes as true what is supposed to be proved.
Consider the following example:
“This essay is the best because it is better than all the others.”
In this example the premise and the conclusion are identical in meaning. As we
know, the conclusion should always follow from the premise. In the example
above, the premise supports the conclusion, but the conclusion equally supports
the premise, creating a “circular” situation where you can move from premise to
conclusion, and then back again to the premise, and so on. Here is another
example: “I must be telling the truth because I’m not lying.”
Here are examples of how this error of reasoning is described in LSAT answer
choices:
“it assumes what it seeks to establish”
“argues circularly by assuming the conclusion is true in stating the
premises”
“presupposes the truth of what it sets out to prove”
“the argument assumes what it is attempting to demonstrate”
“it takes for granted the very claim that it sets out to establish”
“it offers, in place of support for its conclusion, a mere restatement of
that conclusion”

Errors of Conditional Reasoning

In Chapter Six we discussed several mistakes LSAT authors make when using
conditional reasoning, including Mistaken Negation and Mistaken Reversal.
While you should now be comfortable recognizing those errors, Flaw in the
Reasoning questions will ask you to describe those mistakes in logical terms.
This often proves to be a more difficult task.
When describing a Mistaken Negation or a Mistaken Reversal, the test makers
must focus on the error common to both: confusing the sufficient condition with
the necessary condition. As such, here are examples of how these errors of
reasoning are described in LSAT answer choices:
“taking the nonexistence of something as evidence that a necessary
precondition for that thing also did not exist” (Mistaken Negation)
“mistakes being sufficient to justify punishment for being required to
justify it” (Mistaken Reversal)
Note that the authors can either mistake a necessary condition for a sufficient
condition, or mistake a sufficient condition for a necessary condition:
Confuses a necessary condition for a sufficient condition
“it treats something that is necessary for bringing about a state of affairs
as something that is sufficient to bring about a state of affairs”
“from the assertion that something is necessary to a moral order, the
argument concludes that that thing is sufficient for an element of the
moral order to be realized”
Confuses a sufficient condition for a necessary condition
“confuses a sufficient condition with a required condition”
It is interesting to note the frequency with which the words “sufficient” (or its
synonym “assured”) or “necessary” (or its synonym “required”) are used when
analyzing the answer choices used to describe conditional reasoning. This
occurs because those words perfectly capture the idea and it is difficult to avoid
using at least one of those words when describing conditionality. This is a huge
advantage for you: if you identify a stimulus with conditional reasoning and are
asked a Flaw question, you can quickly scan the answers for the one answer
that contains “sufficient,” “necessary,” or both.


Straw Man
This error occurs when an author attempts to attack an opponent’s position by
ignoring the actual statements made by the opposing speaker and instead
distorts and refashions the argument, making it weaker in the process. In
figurative terms, a “straw” argument is built up which is then easier for the
author to knock down.
Often this error is accompanied by the phrase “what you’re saying is” or “if I
understand you correctly,” which are used to preface the refashioned and
weakened argument. Here is an example:
Politician A: “The platform proposed by my party calls for a moderate
increase in taxes on those individuals making over $20,000
per year, and then taking that money and using it to rebuild
the educational system.”
Politician B: “But what you’re saying is that everyone should pay higher
taxes, and so your proposal is unfair.”
In the example above, Politician B recasts Politician A’s argument unfairly.
Politician A indicated the tax increase would apply to those with incomes over
$20,000 where Politician B distorts that to “everyone should pay higher taxes.”
Here are examples of how this error of reasoning is described in LSAT answer
choices:
“refutes a distorted version of an opposing position”
“misdescribing the student representative’s position, thereby making it
easier to challenge”
“portrays opponents’ views as more extreme than they really are”
“distorts the proposal advocated by opponents”
General Lack of Relevant Evidence for the Conclusion
Some LSAT authors misuse information to such a degree that they fail to
provide any information to support their conclusion or they provide information
that is irrelevant to their conclusion. Here is an example:
“Some critics claim that scientific progress has increased the polarization
of society and alienated large segments of the population. But these
critics are wrong because even a cursory glance at the past shows that
society is always somewhat polarized and some groups are inevitably
alienated.”
The author provides irrelevant evidence in an attempt to refute the claim that
“scientific progress has increased the polarization of society and alienated large
segments of the population.” Citing facts that such a situation has always existed
does not help disprove that scientific progress has increased the severity of the
situation.
Here are examples of how this error of reasoning is described in LSAT answer
choices:
“The author cites irrelevant data.”
“draws a conclusion that is broader in scope than is warranted by the
evidence advanced”
“It uses irrelevant facts to justify a claim about the quality of the
disputed product.”
“It fails to give any reason for the judgment it reaches.”
“It introduces information unrelated to its conclusion as evidence in
support of that conclusion.”
Internal Contradiction
As discussed in the answer key to the previous chapter, an internal contradiction
(also known as a self-contradiction) occurs when an author makes conflicting
statements. The example used was:
“Everyone should join our country club. After all, it’s an exclusive
group that links many of the influential members of the community.”
The self-contradiction occurs when the speaker says “Everyone should join”
and then follows that by saying that it is “an exclusive group.” Exclusive, by
definition, means that some people are excluded.
The following show how this error of reasoning is described in LSAT answer
choices:
“bases a conclusion on claims that are inconsistent with each other”
“the author makes incompatible assumptions”
“introduce information that actually contradicts the conclusion”
“offers in support of its conclusion pieces of evidence that are mutually
contradictory”
“some of the evidence presented in support of the conclusion is
inconsistent with other evidence provided”
“assumes something that it later denies, resulting in a contradiction”
Appeal Fallacies
While there are a number of “appeal” fallacies that appear in traditional logic
(Appeal to Fear, Appeal to Force, Appeal to Tradition, etc.), the following three
are the most applicable to the LSAT:
1. Appeal to Authority
An Appeal to Authority uses the opinion of an authority in an attempt to
persuade the reader. The flaw in this form of reasoning is that the
authority may not have relevant knowledge or all the information
regarding a situation, or there may a difference of opinion among
experts as to what is true in the case. Here is an example:
“World-renowned neurologist Dr. Samuel Langhorne says that
EZBrite Tooth Strips are the best for whitening your teeth. So,
you know if you buy EZBrite you will soon have the whitest
teeth possible!”
The primary defect in this argument is its use of a neurologist as an
authority figure in an area of dentistry. While Dr. Langhorne can
reasonably be appealed to in matters of the brain, dental care would be
considered outside the scope of his expertise.
Here are examples of how this error of reasoning is described in LSAT
answer choices:
“the judgement of experts is applied to a matter in which their
expertise is irrelevant”
“the argument inappropriately appeals to the authority of the
mayor”
“it relies on the judgment of experts in a matter to which their
expertise is irrelevant”
“accepts a claim on mere authority, without requiring sufficient
justification”
2. Appeal to Popular Opinion/Appeal to Numbers
This error states that a position is true because the majority believes it to
be true. As you know, arguments are created by providing premises that
support a conclusion. An appeal to popular opinion does not present a
logical reason for accepting a position, just an appeal based on numbers.
Here is an example:
“A recent poll states that 75% of Americans believe that
Microsoft is a monopoly. Antitrust law states that monopolies
have a deleterious effect on the marketplace (with the exception
of utilities), and therefore Microsoft should be controlled or
broken into smaller pieces.”
The author uses the results of a poll that indicate many people think
Microsoft is a monopoly to conclude that Microsoft is in fact a
monopoly. This type of persuasion is often used in the arguments made
by advertisements (“All the trend setters use EZBrite Tooth Strips”),
politicians (“Everyone loves the environment. Vote for the Green
Party!), and children (“C’mon, try this. Everyone does it.”).
This type of reasoning most often appears as an incorrect answer. Here
are examples of how this error of reasoning is described in LSAT
answer choices:
“it treats popular opinion as if it constituted conclusive evidence
for a claim”
“attempts to discredit legislation by appealing to public
sentiment”
“a claim is inferred to be false merely because a majority of
people believe it to be false”
“the argument, instead of providing adequate reasons in support
of its conclusion, makes an appeal to popular opinion”
3. Appeal to Emotion
An Appeal to Emotion occurs when emotions or emotionally-charged
language is used in an attempt to persuade the reader. Here is an
example:
“Officer, please do not give me a ticket for speeding. In the last
month I’ve been fired from my job, kicked out of my apartment,
and my car broke down. I don’t deserve this!
Here are examples of how this error of reasoning is described in LSAT
answer choices:
“attempts to persuade by making an emotional appeal”
“uses emotive language in labeling the proposals”
“the argument appeals to emotion rather than reason”
Survey Errors
The makers of the LSAT believe that surveys, when conducted properly,
produce reliable results. However, surveys can be invalidated when either of the
following three scenarios arise:
1. The survey uses a biased sample.
Perhaps the most famous example of a biased survey occurred in 1936.
The Literary Digest weekly magazine sent out ballots to some 10
million voters (2.3 million were returned), and returns indicated that a
solid majority would vote for Republican candidate Alf Landon in the
upcoming presidential election. On the basis of these results (and the
size of the sample), the Literary Digest predicted that Landon would
win easily. Of course, when the election was held Franklin Roosevelt
won in a landslide. The Literary Digest erred by sending the ballots to
groups such as telephone owners and automobile owners, groups that in
that era (late Depression) tended to be among the wealthiest individuals
and overwhelmingly Republican. The Literary Digest ended up polling
a large number of Republicans and on that basis declared that the
Republican candidate would win.
Note that a secondary error with the polling done by the Literary Digest
is that the sample is self-selected; that is, the individuals being polled
decided whether or not to respond. That opportunity introduces bias into
the survey process because certain types of individuals tend to respond
to surveys whereas others do not.
2. The survey questions are improperly constructed.
If a survey question is confusing or misleading, the results of the poll
can be inaccurate.
Questions can be confusing, such as “Do you feel it is possible that
none of the candidates would not vote to increase taxes?” (The question
actually asks, “Do you feel it is possible that all of the candidates would
vote to increase taxes?”). If a respondent cannot understand the
question, how can they accurately answer the question?
Questions can also be misleading, such as “How soon should the U.S.
government withdraw from the United Nations?” The question
presumes that the United States should withdraw from the United
Nations—a course of action that the respondent may not agree with.
3. Respondents to the survey give inaccurate responses.
People do not always tell the truth when responding to surveys. Two
classic questions that often elicit false answers are “What is your age”
and “how much money do you make each year?”
If respondents give false answers to survey questions, the results of the
survey are skewed and inaccurate.
Here are examples of how the errors of reasoning above are described in LSAT
answer choices:
“uses evidence drawn from a small sample that may well be
unrepresentative”
“generalizes from an unrepresentative sample”
“states a generalization based on a selection that is not representative of
the group about which the generalization is supposed to hold true”
Exceptional Case/Overgeneralization
This error takes a small number of instances and treats those instances as if they
support a broad, sweeping conclusion. Here is an example:
“Two of my friends were shortchanged at that store. Therefore,
everyone gets shortchanged at that store. ”
This answer appears most frequently as an incorrect answer in Flaw questions,
but as with any of the errors described in this chapter, occasionally it appears as
a correct answer. Here are examples of how this error of reasoning is described
in LSAT answer choices:
“supports a universal claim on the basis of a single example”
“The argument generalizes from too small a sample of cases”
“Too general a conclusion is made about investing on the basis of a
single experiment”
“bases a general claim on a few exceptional instances”
Errors of Composition and Division
Composition and division errors involve judgments made about groups and
parts of a group.
An error of composition occurs when the author attributes a characteristic of
part of the group to the group as a whole or to each member of the group. Here
is an example:
“Every party I attend is fun and exciting. Therefore, my life is fun and
exciting.”
Here are examples of how this error of reasoning is described in LSAT answer
choices:
“assuming that because something is true of each of the parts of a whole
it is true of the whole itself”
“improperly infers that each and every scientist has a certain
characteristic from the premise that most scientists have that
characteristic”
“takes the view of one lawyer to represent the views of all lawyers”
An error of division occurs when the author attributes a characteristic of the
whole (or each member of the whole) to a part of the group. Here is an
example:
“The United States is the wealthiest country in the world. Thus, every
American is wealthy.”
Here is an example of how this error of reasoning is described in LSAT answer
choices:
“presumes, without providing justification, that what is true of a whole
must also be true of its constituent parts”
False Analogy
As discussed in the answer key to the problem set in the previous chapter, an
analogy is a comparison between two items. A False Analogy occurs when the
author uses an analogy that too dissimilar to the original situation to be
applicable. Here is an example:
“Just as a heavy rainfall can be cleansing, the best approach to maintain
a healthy relationship is to store up all your petty grievances and then
unload them all at one time on your partner.”
The comparison in the example fails to consider that a heavy rainfall and an
emotionally charged situation are fundamentally different.
Here are two examples of how a False analogy is described in LSAT answer
choices:
“treats as similar two cases that are different in a critical respect”
“treats two kinds of things that differ in important respects as if they do
not differ”
False Dilemma
A False Dilemma assumes that only two courses of action are available when
there may be others. Here is an example:
“Recent accidents within the oil industry have made safety of operation
a critical public safety issue. Because the industry cannot be expected to
police itself, the government must step in and take action.”
The argument above falsely assumes that only two courses of action exist:
industry self-policing or government action. But this ignores other courses of
action, such as consumer watchdog groups.
Do not confuse a False Dilemma with a situation where the author legitimately
establishes that only two possibilities exist. Phrases such as “either A or B will
occur, but not both” can establish a limited set of possibilities, and certain realworld
situations yield only two possibilities, such as “you are either dead or
alive.”
Here is an example of how a False Dilemma is described in LSAT answer
choices:
“fails to consider that some students may be neither fascinated by nor
completely indifferent to the subject being taught”
Errors in the Use of Evidence
Mis-assessing the force of evidence is a frequent error committed by LSAT
authors. Each of the following describes an error of reasoning involving the
force of evidence:
1. Lack of evidence for a position is taken to prove that position is false.
Just because no evidence proving a position has been introduced does
not mean that the position is false. Here is an example:
“The White House has failed to offer any evidence that they
have reached a trade agreement with China. Therefore, no such
agreement has been reached.”
In the example above the White House may have valid reasons for
withholding information about the trade agreement. The lack of
confirming evidence does not undeniably prove that a trade agreement
has not been reached.
Here are two examples of how this error of reasoning is described in
LSAT answer choices:
“treats failure to prove a claim as constituting denial of that
claim”
“taking a lack of evidence for a claim as evidence undermining
that claim”
2. Lack of evidence against a position is taken to prove that position is
true.
This error is the opposite of the previous error. Just because no evidence
disproving a position has been introduced does not mean that the
position is true. Here is a famous example:
“There has been no evidence given against the existence of God,
so God must exist.”
The lack of evidence against a position does not undeniably prove a
position. Here is an example of how this error of reasoning is described
in LSAT answer choices:
“treating the failure to establish that a certain claim is false as
equivalent to a demonstration that the claim is true”
3. Some evidence against a position is taken to prove that position is false.
The introduction of evidence against a position only weakens the
position; it does not necessarily prove the position false. Here is an
example:
“Some historians claim that a lengthy drought preceded the fall
of the Aztec empire. But we know from Aztec writings that in at
least one year during the supposed drought there was minor
flooding. Thus, the claim that there was a lengthy drought prior
to the fall of the Aztec empire is false.”
The evidence offered in the example above weakens the claim that there
was a lengthy drought, but it does not disprove it. A drought by
definition is a prolonged period of unusually low rainfall, and thus it
would be possible for flooding to occur on occasion, but not enough
flooding to overcome the general drought conditions.
Here is an example of how this error of reasoning is described in an
LSAT answer choice:
“it confuses undermining an argument in support of a given
conclusion with showing that the conclusion itself is false”
4. Some evidence for a position is taken to prove that position is true.
The introduction of evidence for a position only provides support for the
position; it does not prove the position to be undeniably true. Here is an
example:
“We know that the defendant was in the vicinity of the robbery
when the robbery occurred. Therefore, the defendant is guilty of
the robbery.”
As the above example proves, partial support for a position does not
make the position invincible (especially in LSAT arguments, which are
relatively short). As you might expect, partial evidence for a position can
be outweighed by evidence against that position.
Here is an example of how this error of reasoning is described in an
LSAT answer choice:
“the argument takes evidence showing merely that its conclusion
could be true to constitute evidence showing that the conclusion
is in fact true”
Time Shift Errors
Although this error has a rather futuristic name, the mistake involves assuming
that conditions will remain constant over time, and that what was the case in the
past will be the case in the present or future.
“The company has always reimbursed me for meals when I’m on a
business trip, so they will certainly reimburse me for meals on this
business trip.”
Clearly, what has occurred in the past is no guarantee that the future will be the
same. Yet, many LSAT authors make this assumption, especially when
hundreds or thousands of years are involved. Here are examples of how this
error of reasoning is described in LSAT answer choices:
“treats a claim about what is currently the case as if it were a claim about
what has been the case for an extended period”
“uncritically draws an inference from what has been true in the past to
what will be true in the future”
Numbers and Percentage Errors
In Chapter Fifteen we will discuss numbers and percentages problems in detail.
Meanwhile, consider that many errors in this category are committed when an
author improperly equates a percentage with a definite quantity, or when an
author uses quantity information to make a judgment about the percentage
represented by that quantity.
Here is an example of how this error of reasoning is described in an LSAT
answer choice:
“the argument confuses the percentage of the budget spent on a program
with the overall amount spent on that program”