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IMAT Logical Reasoning · Critical Thinking

Hidden Assumptions and the Negation Test

An assumption is an unstated premise the argument needs; deny it and the conclusion no longer follows.

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Why this matters

Assumption questions appeared in every Cambridge paper from 2011 to 2022, 20 times in all, usually as 'which one of the following is an assumption on which the argument depends'. The wrong options are mostly things that would help the argument but are not needed by it.

Concept 1 of 2: Finding the hidden assumption by locating the gap

An assumption is a missing premise: something the author needs to be true but never says. Arguments leave a gap between what the reasons show and what the conclusion claims. A new idea that appears in the conclusion but nowhere in the premises points straight at the assumption that bridges it.

Definition

An assumption is an unstated premise that the argument depends on.

  • It is not stated in the passage. A premise the passage gives is never the assumption.
  • Method: find the conclusion and the premises; find the gap (a word or idea in the conclusion that the premises never reach); state the bridge that closes it.
  • Common gaps: that two cases are comparable; that a plan will work; that nothing else explains the evidence; that a sample is typical; that the people involved will behave as expected.
  • The assumption is usually modest: just enough to close the gap, no more.

Worked example

Find the assumption. 'When the city museum stopped charging for entry last year, its visitor numbers rose by a third. Its shop and café now earn more than the entry fees ever did. The art gallery next door charges a fee and has few visitors. So if the gallery also stopped charging, it would earn more money than it does now.'
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A stated premise is not an assumption

IMAT often puts one of the passage's own reasons, slightly reworded, among the options. Because it is stated, it is not assumed. An assumption is what the author takes for granted without saying.

Concept 2 of 2: The negation test for assumptions

If an argument truly depends on a statement, then denying that statement should make the argument fall apart. A statement that only helps the argument does not pass: deny it and the argument still stands. This test separates what is needed from what is merely nice to have.

Definition

Negation test: write the opposite of the candidate. If the conclusion no longer follows, the candidate is an assumption. If the argument survives, it is not.

  • Negate with care. Not 'all' is 'at least one is not'; not 'some' is 'none'; not 'will' is 'will not'; not 'the main cause' is 'not the main cause'.
  • Strong candidates (all, only, every, never) often fail the test, because the argument needs only a weaker version.
  • An assumption is required; a strengthener only makes the conclusion more likely. An option can be a strengthener without being an assumption.

Worked example

'A footbridge across the river would let people on the east bank walk to the new health centre in ten minutes. At present they must drive or take a bus round by the main road. So building the footbridge would reduce car journeys to the health centre.' Test three candidates: (a) some east bank residents who now drive to the centre would be willing to walk there; (b) most east bank residents are in good health; (c) the footbridge would be cheap to build.
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Negating 'all' gives 'not all', not 'none'

The opposite of 'all the customers will change' is 'at least one will not'. Negating it as 'no customer will change' makes the test far too harsh and leads you to accept strong options that the argument does not need.

Helpful is not the same as required

An option can make the conclusion more likely without the argument depending on it. In an assumption question, if denying the option leaves the argument standing, it is a strengthener at most, not the answer.

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