Unit 3 · Topic 3.5

3.5 Spotting flaws in reasoning

Reasoning breaks down when a conclusion doesn't follow from the evidence: generalizing from too few cases, assuming that one thing caused another because it came first, or offering only two choices when there are more. Spot the weak link and explain why it fails, rather than memorizing a long list of fallacy names.

Key terms

flawed reasoning
logical fallacy
hasty generalization
false cause

When reasoning only looks convincing

An argument is specious when it seems sound at first glance but falls apart when you look closely. It's illogical when the conclusion simply doesn't follow from the evidence. Either way, the flaw is a gap between what was shown and what was claimed. Your job is to find the gap and name it in plain words.

Common weak links

These patterns are worth recognizing. The names are handy shorthand, but the exam cares more about whether you can explain the problem. All examples are written for this page.

Flaw	What goes wrong	Example
Hasty generalization	A broad conclusion from too few or unusual cases	Two seniors were rude to me, so seniors are rude.
False cause	Assuming that because B followed A, A caused B	Crime rose after the new mall opened, so the mall caused crime.
Either-or (false dilemma)	Only two options offered when there are more	Either we cut the arts program or we raise taxes.
Slippery slope	Claiming one step will lead to extreme results without showing how	If we allow phones at lunch, soon no one will learn anything.
Attacking the person	Dismissing an argument because of who made it	Ignore her budget plan; she's never run a business.
Circular reasoning	Using the claim as its own proof	This rule is fair because it's the right rule.
Appeal to popularity	Treating something as true or good because many believe it	Everyone's switching to this app, so it must be safer.

Find the hidden assumption

Most flawed reasoning depends on an unstated assumption. "Crime rose after the mall opened, so the mall caused it" assumes nothing else changed at the same time. Once you say the assumption out loud, the weakness is clear. Ask: what would have to be true for this conclusion to follow, and is it?

Finding the assumption also shows you how to rebut the argument: produce evidence that the assumption is false.

Avoiding flaws in your own essays

Don't treat one example as proof of a rule; show a pattern or limit your claim.
When you claim a cause, rule out other explanations or admit they exist.
Avoid "always," "never" and "everyone" unless you can truly defend them.
Respond to opponents' reasoning, not their character.

Worked examples

Try each one yourself first, then open the solution.

Example 1
Diagnosing a flawed paragraph
Practice passage (written for this page): "Since the town opened its skate park last spring, vandalism downtown has gone up. Clearly, the skate park attracts troublemakers. Either we close the park or we accept a ruined downtown. Besides, the people defending the park are mostly skaters, so of course they would say that." Identify the flaws in the reasoning and explain each one.
Show the solutionHide the solution
1. Step 1: Sentence 2 assumes the timing proves the cause. Other things might explain the rise: more people downtown, a new business, fewer police patrols. This is a false cause.
2. Step 2: Sentence 3 offers only two options. The town could add lighting, cameras or supervision, or change park hours. This is an either-or flaw.
3. Step 3: Sentence 4 dismisses the park's defenders because of who they are instead of answering their reasons. This attacks the person.
4. Step 4: Pull it together: each flaw lets the writer skip the evidence that would actually be needed.
Answer: The passage assumes that because vandalism rose after the park opened, the park caused it, without ruling out other explanations (false cause). It then offers only two choices, closing the park or accepting a "ruined downtown," when many other responses exist (either-or). Finally, it dismisses the park's defenders because they are skaters rather than answering their arguments (attacking the person). To be convincing, the writer would need evidence connecting the vandalism to park users and a fair look at other solutions.

Common mistakes

Naming a fallacy without explaining how the reasoning fails in this specific case.
Calling any argument you disagree with a fallacy. Disagreeing with a conclusion isn't the same as finding a flaw in its reasoning.
Assuming emotional language is automatically a flaw. Emotion is fine if the reasoning underneath holds up.

On the exam

Reading questions may ask which statement would most weaken an argument or what assumption a writer makes. Look for the gap between evidence and conclusion.
In the argument essay, pointing out the weak link in the opposing view is a strong way to rebut it, and it shows readers you're thinking carefully.

Connected topics

Videos

Top Logical Fallacies Explained | English Professor Breaks Down Common Argument Mistakes
Dr. Whitney KostersWatch on YouTube (opens in a new tab)
Can you outsmart this logical fallacy? - Alex Gendler
TED-EdWatch on YouTube (opens in a new tab)
How to Argue - Philosophical Reasoning: Crash Course Philosophy #2
CrashCourseWatch on YouTube (opens in a new tab)
Logical Fallacies
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Logical Fallacies Explained
Alexander Lyon Communication CoachWatch on YouTube (opens in a new tab)

Check yourself

5 questions on 3.5 Spotting flaws in reasoning. Pick an answer to see if you got it, and why.

If I understand the idea, the Bazar invites several of us to write upon the above text. It means the change in my life’s course which introduced what must be regarded by me as the most important condition of my career. But it also implies—without intention, perhaps—that that turning-point itself was the creator of the new condition. This gives it too much distinction, too much prominence, too much credit. It is only the last link in a very long chain of turning-points commissioned to produce the cardinal result; it is not any more important than the humblest of its ten thousand predecessors. Each of the ten thousand did its appointed share, on its appointed date, in forwarding the scheme, and they were all necessary; to have left out any one of them would have defeated the scheme and brought about some other result. I know we have a fashion of saying “such and such an event was the turning-point in my life,” but we shouldn’t say it. We should merely grant that its place as LAST link in the chain makes it the most conspicuous link; in real importance it has no advantage over any one of its predecessors.

When I was twelve and a half years old, my father died. It was in the spring. The summer came, and brought with it an epidemic of measles. For a time a child died almost every day. The village was paralyzed with fright, distress, despair. Children that were not smitten with the disease were imprisoned in their homes to save them from the infection. In the homes there were no cheerful faces, there was no music, there was no singing but of solemn hymns, no voice but of prayer, no romping was allowed, no noise, no laughter, the family moved spectrally about on tiptoe, in a ghostly hush. I was a prisoner. My soul was steeped in this awful dreariness—and in fear. At some time or other every day and every night a sudden shiver shook me to the marrow, and I said to myself, “There, I’ve got it! and I shall die.” Life on these miserable terms was not worth living, and at last I made up my mind to get the disease and have it over, one way or the other. I escaped from the house and went to the house of a neighbor where a playmate of mine was very ill with the malady. When the chance offered I crept into his room and got into bed with him. I was discovered by his mother and sent back into captivity. But I had the disease; they could not take that from me. I came near to dying. The whole village was interested, and anxious, and sent for news of me every day; and not only once a day, but several times. Everybody believed I would die; but on the fourteenth day a change came for the worse and they were disappointed.

This was a turning-point of my life. (Link number one.) For when I got well my mother closed my school career and apprenticed me to a printer. She was tired of trying to keep me out of mischief, and the adventure of the measles decided her to put me into more masterful hands than hers.

A young printer wanders around a good deal, seeking and finding work; and seeking again, when necessity commands. N. B. Necessity is a CIRCUMSTANCE; Circumstance is man’s master—and when Circumstance commands, he must obey; he may argue the matter—that is his privilege, just as it is the honorable privilege of a falling body to argue with the attraction of gravitation—but it won’t do any good, he must OBEY.

Mark Twain, "The Turning-Point of My Life," Harper's Bazar (February 1910). The magazine had invited several well-known writers to describe the turning point of their lives. In the omitted section between paragraphs 1 and 2, Twain discusses Julius Caesar's crossing of the Rubicon as a famous "turning-point." A short paragraph is also omitted between paragraphs 3 and 4.

Question 1 of 5

A skeptical reader might find a weakness in the reasoning of paragraph 1 because Twain

Mr. Washington distinctly asks that black people give up, at least for the present, three things,— First, political power, Second, insistence on civil rights, Third, higher education of Negro youth,—and concentrate all their energies on industrial education, and accumulation of wealth, and the conciliation of the South. This policy has been courageously and insistently advocated for over fifteen years, and has been triumphant for perhaps ten years. As a result of this tender of the palm-branch, what has been the return? In these years there have occurred:

1. The disfranchisement of the Negro. 2. The legal creation of a distinct status of civil inferiority for the Negro. 3. The steady withdrawal of aid from institutions for the higher training of the Negro.

These movements are not, to be sure, direct results of Mr. Washington’s teachings; but his propaganda has, without a shadow of doubt, helped their speedier accomplishment. The question then comes: Is it possible, and probable, that nine millions of men can make effective progress in economic lines if they are deprived of political rights, made a servile caste, and allowed only the most meagre chance for developing their exceptional men? If history and reason give any distinct answer to these questions, it is an emphatic No. And Mr. Washington thus faces the triple paradox of his career:

1. He is striving nobly to make Negro artisans business men and property-owners; but it is utterly impossible, under modern competitive methods, for workingmen and property-owners to defend their rights and exist without the right of suffrage. 2. He insists on thrift and self-respect, but at the same time counsels a silent submission to civic inferiority such as is bound to sap the manhood of any race in the long run. 3. He advocates common-school and industrial training, and depreciates institutions of higher learning; but neither the Negro common-schools, nor Tuskegee itself, could remain open a day were it not for teachers trained in Negro colleges, or trained by their graduates.

W. E. B. Du Bois, historian and sociologist, The Souls of Black Folk (1903), from Chapter III, "Of Mr. Booker T. Washington and Others." Du Bois is responding to the program Washington set out in his 1895 Atlanta speech. "Disfranchisement" means taking away the right to vote; "suffrage" is the right to vote.

Question 2 of 5

In paragraph 3, Du Bois says these developments "are not, to be sure, direct results of Mr. Washington's teachings." This qualification primarily serves to

Question 3 of 5

Du Bois claims that Washington's "propaganda has, without a shadow of doubt, helped their speedier accomplishment." A reader who wanted to test this claim would most need evidence showing

(1) Ten years ago, Brookfield Middle School added twenty minutes of free reading to every school day. (2) Students could read anything they liked, from comic books to biographies. (3) Our high school should do the same.

(4) A daily reading period would help students who rarely read for fun. (5) Many students say they are too busy to read outside school. (6) When reading time is built into the day, those students get the practice they would otherwise miss.

(7) Some teachers worry that twenty minutes a day would take time away from their lessons. (8) Brookfield solved this by shortening each class by three minutes. (9) Teachers there say the change was barely noticeable.

(10) Most important, more reading leads to better reading. (11) Students who read more build bigger vocabularies, which makes textbooks easier to understand, which in turn makes every class less frustrating. (12) A daily reading period is a small change that could improve learning in every subject.

Written for this practice set: a student's draft of an argument for the school newspaper. Brookfield and its school are invented.

Question 4 of 5

The writer is considering adding the following sentence after sentence 9. "Since Brookfield added its reading period, the town's high school graduation rate has risen, so the reading period must have caused the increase." Should the writer add this sentence?

Question 5 of 5

Which of the following sentences, if added to paragraph 2, would contain a hasty generalization the writer should avoid?

0 of 5 answered