Flaws in a Line of Reasoning: Study Guide (English Language 3.5)

When the path breaks

A line of reasoning is a path from evidence to thesis. Topic 3.3 showed how to build one. This topic shows what happens when the path is broken. The course framework says that flaws in a line of reasoning may render an argument specious or illogical. Specious means that the argument seems sound at first but is actually misleading. Illogical means that the conclusion does not follow from the premises.

Reading for flaws is not about catching writers out. It is a way of testing whether an argument deserves to be accepted, and it helps you write better arguments, because you learn what a careful reader will question. This guide follows the course’s own words: you need to recognise that flaws exist and explain what is wrong, using plain descriptions. Learning the traditional names of fallacies is useful shorthand, but you will earn credit for explaining the flaw, not just naming it.

Reasoning flaws: what goes wrong

Flaw	What the writer does	Why it fails	Marlbridge example
Hasty generalisation	Draws a broad conclusion from too few or unrepresentative cases.	A small sample cannot support a sweeping claim.	“My two cousins hate maths, so school ruins maths for everyone.”
False cause (post hoc)	Assumes that because B followed A, A caused B.	Sequence is not cause.	“Since the new principal arrived, test scores have fallen. She must be responsible.”
False dilemma (either/or)	Presents two options as if there are no others.	Real situations usually offer more choices.	“Either we ban all cars or we accept filthy air for ever.”
Slippery slope	Claims that one small step will inevitably lead to extreme consequences.	The chain of consequences is asserted, not shown.	“If pupils may use phones at break, soon they will use them in exams.”
Straw man	Distorts an opponent’s position into one that is easier to attack.	The attack does not address what the opponent actually claims.	“Those who want choice in uniform want pupils to wear whatever they like and for rules to vanish.”
Ad hominem	Attacks the person rather than the argument.	A speaker’s character or circumstances do not decide whether a claim is true.	“She does not even own a bike, so ignore her cycle-lane plan.”
Circular reasoning	Uses the claim as its own support.	It assumes what it needs to prove.	“Uniforms are necessary because pupils need to wear them.”
Red herring	Introduces an irrelevant point that draws attention away.	The point does not bear on the claim.	“Why worry about litter when the council wastes money on its newsletter?”
Faulty analogy	Compares two things that differ in a relevant way.	The comparison’s similarity is superficial.	“Running a school is like running a business, so teachers should be paid by results.”
Non sequitur	The conclusion does not follow from the premises.	A step in the reasoning is missing.	“The football team won, so we should lower the fees.”
Appeal to authority (irrelevant)	Cites someone famous for an issue outside their field.	Expertise does not transfer.	“A famous actor says this vitamin works, so it does.”
Bandwagon	Argues that something is right because many people do it.	Popularity is not proof.	“Everyone is switching to this app, so it must be best.”
Overstating	Claims more than the evidence shows.	The conclusion outruns the evidence.	“One survey of 20 pupils proves the whole school hates the timetable.”

You do not need to memorise every name. You do need to know what is wrong with each pattern.

Specious and illogical: two kinds of failure

Type	Description	Example
Illogical	The conclusion does not follow from the premises even if the premises are true.	“All the students in the choir are good at music; Ravi is good at music; so Ravi is in the choir.”
Specious	It looks plausible but depends on a trick, a gap or an emotional shortcut.	“Can you really say no to saving a child’s life? Then you must support the new tax.” (It hides the question of whether the tax achieves that.)

Both undermine the writer’s credibility and the force of the argument. A reader who spots a flaw may doubt other parts of the argument too.

Questions that expose flaws

When reading an argument, ask:

1. Is the evidence enough for the claim? (hasty generalisation, overstating)
2. Does the conclusion actually follow? (non sequitur, circular reasoning)
3. Is there a rival explanation? (false cause)
4. Are only two options given? (false dilemma)
5. Is the opponent’s view stated fairly? (straw man)
6. Is the attack on the argument or the person? (ad hominem)
7. Is each point relevant? (red herring, irrelevant authority)
8. Is a series of consequences shown or only asserted? (slippery slope)
9. Is the comparison really parallel? (faulty analogy)

Fixing a flaw in your own writing

When you find a flaw in your own draft, the repair usually takes one of these forms:

Flaw	Typical repair
Hasty generalisation	Narrow the claim or add representative evidence.
False cause	Add evidence of causal mechanism or rule out other causes; or say “may have contributed”.
False dilemma	Acknowledge other options and explain why they fall short.
Slippery slope	Show evidence for each step, or limit the claim to the likely next step.
Straw man	Restate the opposing view fairly, then answer it.
Ad hominem	Address the argument, not the person.
Circular reasoning	Provide independent reasons or evidence.

Satire and deliberately flawed reasoning: Swift (1729)

Writers sometimes use flawed reasoning on purpose. In “A Modest Proposal” Jonathan Swift’s narrator calmly proposes that the poor of Ireland sell their children as food. The narrator reasons with the confident tone of an economist, but the line of reasoning depends on a monstrous premise, that poor people’s children are a commodity. Near the end the narrator shuts down alternatives: “Therefore I repeat, let no man talk to me of other expedients”, and then lists real remedies, such as taxing absentee landlords, only to dismiss them. This is a false dilemma in action: the narrator presents his horrifying plan as the only option and refuses to consider the rest.

The flaw is the point. By making the reasoning visibly specious, Swift invites readers to look at the real premises behind policies of his day and to reject them. When you read satire, ask not only “what is wrong with this argument?” but “why has the writer made it wrong, and what does that show?”

Worked example 1: diagnosing flaws in a paragraph

Task. Identify and explain at least three flaws in this paragraph, written by Marlbridge.

Since the new head arrived in September, attendance has fallen. So she is clearly the cause. Either we remove her or attendance will go on falling. Councillor Hale says the head is doing a good job, but he has never taught a day in his life, so we can ignore him.

Step 1: “Since the new head arrived… attendance has fallen. So she is clearly the cause.” False cause. The sentence shows that one event followed another and concludes a cause. Other explanations are possible (a flu outbreak, a change to the bus timetable).

Step 2: “Either we remove her or attendance will go on falling.” False dilemma. Only two options are presented. The school could investigate causes, change the bus schedule or support families.

Step 3: “Councillor Hale… has never taught a day in his life, so we can ignore him.” Ad hominem. It attacks the councillor’s experience instead of answering his claim that the head is doing well.

Step 4: State what is wrong, not just the label. In each case the writer treats an assumption as proven: that sequence shows cause, that only two choices exist, that a speaker’s background decides the truth of his claim.

Check. The answer names three flaws and explains why each fails. A bare list of names would earn little.

Worked example 2: repairing a flawed argument

Task. Rewrite the first two sentences of the paragraph in Worked example 1 so that the reasoning is sound.

Step 1: Keep the evidence, change the claim. The evidence is that attendance fell after September.

Step 2: Acknowledge other possible causes. Name them.

Step 3: Propose a way to test. Suggest a comparison or a source of further evidence.

Step 4: Draft.

Attendance has fallen since September, a change that coincides with the new head’s arrival but also with a flu outbreak and a reduction in bus services. Before attributing the fall to any one cause, the governors should compare attendance on routes that kept their buses with those that did not, and check absence reasons against the dates of the outbreak.

Step 5: Check. The revision still addresses the problem, avoids the leap from sequence to cause, offers a way to test competing explanations, and treats the head fairly. The tone is more credible, which helps the writer’s ethos.

How the exam asks about flaws in reasoning

• Reading: which statement identifies a weakness in the argument’s reasoning; why a conclusion does not follow; what is wrong with an analogy or a causal claim.
• Writing: which revision removes an unsupported leap; which sentence best qualifies an overstated claim.
• Essays: in the argument and synthesis essays, avoid these flaws in your own writing, and use qualification and concession where appropriate. In the rhetorical analysis essay, you may comment on a writer’s reasoning if it helps explain their effect on an audience, but the prompt will focus mainly on strategies and purpose.

Common misconceptions

• “Naming a fallacy is enough.” The exam rewards explaining what is wrong in the specific argument.
• “Any emotional appeal is a fallacy.” Emotion is a legitimate tool. It becomes a flaw when it replaces reasoning or distorts evidence.
• “If the conclusion is true, the reasoning must be sound.” A true conclusion can rest on a bad argument.
• “A flaw means the argument is useless.” Sometimes a flaw can be repaired and the argument saved.
• “Writers never use flawed reasoning on purpose.” Satirists do, to make a point.
• “Slippery slope arguments are always wrong.” They fail when the steps are only asserted. If each step is shown to be likely, the argument can be sound.

Where this leads

Topic 3.6, Methods of development, turns to the common ways writers organise and develop their reasoning. Practise with the practice questions and the checklist.