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Arguments by analogy are among the most common forms of reasoning in everyday debate and in LNAT passages. When an author argues that because something is true in one case, it should also be true in a similar case, they are using analogical reasoning. The LNAT tests your ability to evaluate whether such analogies are valid.
An argument by analogy has the following structure:
The argument works by claiming that because two things are similar in known respects, they are likely similar in a further, unknown respect.
"Requiring cyclists to wear helmets has significantly reduced head injuries. Similarly, requiring motorcyclists to wear high-visibility jackets would significantly reduce motorcycle accidents."
The analogy compares cycling helmet laws to a proposed motorcycle high-visibility rule. Both involve mandatory safety equipment for two-wheeled road users. But does the analogy hold?
The strength of an analogy depends on whether the similarities between the two cases are relevant to the conclusion being drawn, and whether there are relevant differences that undermine the comparison.
| Question | Purpose |
|---|---|
| What are the similarities between the two cases? | Identify the basis for the analogy |
| Are these similarities relevant to the conclusion? | A similarity is only useful if it relates to why the conclusion would hold |
| What are the differences between the two cases? | Identify potential weaknesses in the analogy |
| Are these differences relevant to the conclusion? | A difference only undermines the analogy if it affects the conclusion |
| Factor | Cycling helmets | Motorcycle hi-vis jackets |
|---|---|---|
| Type of road user | Two-wheeled, vulnerable | Two-wheeled, vulnerable |
| Type of safety measure | Protective (reduces injury severity) | Preventative (reduces collision likelihood) |
| Mechanism | Physical protection for the head | Increased visibility to other drivers |
| Nature of the problem | Head injuries when crashes occur | Crashes occurring because motorcyclists are not seen |
Assessment: The analogy is weak because the two safety measures work through entirely different mechanisms. Helmets protect against injury after a collision; high-visibility jackets aim to prevent collisions. The relevant factor — how the safety measure reduces harm — is fundamentally different.
"The smoking ban worked in Ireland, so it will work in England."
Evaluate by asking: Are Ireland and England sufficiently similar in the relevant respects (pub culture, enforcement capacity, public attitudes)?
"We regulate alcohol because it causes harm. Cannabis also causes harm. Therefore, we should regulate cannabis in the same way."
Evaluate by asking: Are the harms of alcohol and cannabis sufficiently similar? Is "causing harm" the only relevant factor for regulation, or do other differences matter (addictiveness, social context, existing legal framework)?
"Appeasement failed in the 1930s. Diplomatic engagement with this regime is therefore doomed to fail."
Evaluate by asking: Is the current situation sufficiently similar to the 1930s? Are the regimes, geopolitical contexts, and available options comparable?
Questions may take several forms:
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