Flaws in Reasoning

A flaw in reasoning (also called a logical fallacy) is an error in the logic of an argument — a point where the premises do not properly support the conclusion, even though the author presents them as if they do. Identifying flaws is one of the most frequently tested skills on the LNAT. This lesson covers the most common fallacies you need to recognise.

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Why Flaws Matter on the LNAT

LNAT passages are drawn from opinion pieces, editorials, and argumentative writing. These texts frequently contain reasoning errors — sometimes subtle, sometimes glaring. Questions may ask you:

• "Which of the following is a flaw in the author's reasoning?"
• "The argument is vulnerable to which of the following criticisms?"
• "Which of the following most accurately describes a weakness in the argument?"

You need to be able to name and describe these flaws accurately.

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The Major Logical Fallacies

1. Ad Hominem (Attack the Person)

What it is: Attacking the person making the argument rather than the argument itself.

"Professor Williams argues that climate change requires urgent action, but she drives a petrol car, so her argument is hypocritical and should be dismissed."

Why it is a flaw: The professor's personal behaviour has no bearing on whether her argument about climate change is logically sound. Even a hypocrite can make a valid argument.

2. Straw Man

What it is: Misrepresenting an opponent's argument to make it easier to attack.

"Those who support immigration reform want completely open borders with no controls whatsoever."

Why it is a flaw: Supporting immigration reform does not necessarily mean supporting open borders. The arguer has distorted the opponent's position to create an easier target.

LNAT Tip: Straw man fallacies are common in LNAT passages because the test draws from opinion writing, where authors sometimes exaggerate opposing views.

3. False Dichotomy (Either/Or Fallacy)

What it is: Presenting only two options when more exist.

"Either we ban all cars from city centres or we accept that air pollution will continue to worsen. There is no middle ground."

Why it is a flaw: There are many options between a total ban and doing nothing — congestion charges, low emission zones, improved public transport, electric vehicle incentives, and so on.

4. Slippery Slope

What it is: Arguing that one action will inevitably lead to a chain of increasingly extreme consequences, without evidence for each step.

"If we allow students to use calculators in exams, they will never learn mental arithmetic. They will become completely dependent on technology, and within a generation, basic numeracy will disappear entirely."

Why it is a flaw: Each step in the chain is presented as inevitable, but no evidence is offered to support the progression from one step to the next.

5. Circular Reasoning (Begging the Question)

What it is: The conclusion is assumed in one of the premises — the argument goes in a circle.

"Freedom of speech is important because people should be allowed to say what they think."

Why it is a flaw: "People should be allowed to say what they think" is simply a restatement of "freedom of speech is important". The argument provides no independent reason for its conclusion.

6. Appeal to Authority

What it is: Claiming something is true solely because an authority figure says so, particularly when the authority is not an expert in the relevant field.

"A famous footballer says this diet pill works, so it must be effective."

Why it is a flaw: Being famous or being an expert in one field does not make someone an authority on nutrition or medicine. (Note: appealing to a genuine expert in the relevant field is not inherently fallacious, but it can still be questioned.)

7. Hasty Generalisation