Assumptions: The Hidden Premises

Every argument relies on assumptions — unstated beliefs that must be true for the argument to work. Identifying assumptions is one of the most commonly tested skills on the LNAT, and one of the most challenging. This lesson teaches you what assumptions are, how to find them, and the critical distinction between necessary and sufficient assumptions.

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What is an Assumption?

An assumption is an unstated premise that the argument depends upon. It is a belief the author takes for granted without explicitly stating it.

Think of an argument as a bridge between premises and conclusion. The stated premises are the visible supports. Assumptions are the hidden supports — if you remove them, the bridge collapses.

Formal Definition: An assumption is a statement that is not explicitly made in the argument but must be true for the conclusion to follow from the premises.

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A Simple Example

"James has lived in France for ten years, so he must speak French fluently."

Stated premise: James has lived in France for ten years. Conclusion: James speaks French fluently. Assumption: Living in a country for a long time leads to fluency in the local language.

This assumption is not stated but must be true for the conclusion to follow. If the assumption is false (perhaps James lived in an English-speaking expatriate community and never learned French), the conclusion does not follow.

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How to Identify Assumptions

Step 1: Identify the conclusion and premises

You cannot find the gap in an argument without first knowing what the argument is.

Step 2: Look for the gap

Ask yourself: "What must be true — but is not stated — for the premises to support the conclusion?"

Step 3: Apply the Negation Test

The negation test is the most reliable technique for identifying assumptions:

1. Take the proposed assumption
2. Negate it (state the opposite)
3. If negating it destroys the argument, it is a genuine assumption
4. If negating it has no effect on the argument, it is not an assumption

Worked Example of the Negation Test

Argument: "The new bypass will reduce traffic in the town centre, so local shops will benefit from increased footfall."

Proposed assumption: People avoid the town centre because of traffic congestion.

Negation test: "People do NOT avoid the town centre because of traffic congestion."

If this negation is true, then reducing traffic would not increase footfall, and the argument collapses. Therefore, this is a genuine assumption.

Alternative proposed assumption: The bypass will be built on time.

Negation test: "The bypass will NOT be built on time."

If the bypass is merely delayed, the argument still holds — it just applies later. This might affect the timing but does not destroy the logical connection. So this is not a necessary assumption of the argument (though it may be a practical concern).

LNAT Tip: The negation test is your most powerful tool for assumption questions. Practise it until it becomes second nature.

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Necessary vs Sufficient Assumptions

This distinction is crucial for the LNAT.

Necessary Assumption

A necessary assumption is one that must be true for the argument to work. Without it, the argument fails. However, even if it is true, the conclusion might still not follow — other things might also need to be true.

Sufficient Assumption

A sufficient assumption is one that, if true, would make the conclusion follow with certainty from the premises. It is enough on its own to bridge the gap.