Unit 8 · Detecting Reasoning Errors
Ferdinand's Atlas — Spot the Reasoning Error
Lesson 2 of 34
Ferdinand thinks knowing a city's name is enough to find its country — but his hidden assumption is that every city name is unique. Find the option that disproves exactly that assumption.
Unwarranted assumption questions appear across all difficulty levels of NSW Selective TS — they test whether you can identify the gap between what someone says and what they quietly assume to be true.
The examiner checks whether you can extract a speaker's hidden assumption and match it to the option that directly invalidates it — rather than choosing options that are merely related to the topic.
A speaker claims they can do something (look something up, identify something, predict something) without needing extra information. Their method secretly relies on an assumption (uniqueness, completeness, consistency). One option disproves that assumption; the others are off-topic.
Best approach: Ask: what must be true for the speaker's method to work? That is the hidden assumption. Then find the option that directly shows that assumption might be false. Options about unrelated problems (name changes, countries with no cities) are distractors.
Question
Ferdinand has an atlas which he can use to look up any city in the world.
Ferdinand said: "If you tell me the name of a city, you don't need to tell me which country it's in – I can look it up in my atlas."
Which one of the following sentences shows the mistake Ferdinand has made?
Select an answer to see the explanation.
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