An assumption is a belief an argument needs in order to work but does not state. Finding it is a key skill in claims, reasons and evidence, because the gap between a reason and a claim is usually filled by something unwritten.
The example below is invented and non-political, so you can focus on how the argument is built.
How do you find an assumption?
Look at the reason and the claim, and ask what must be true for the first to lead to the second. That missing step is the assumption.
The collapse test checks your answer: suppose the assumption is false. If the argument now fails, you have found a real assumption. If the argument still stands, you have found something else.
Step by step
- Sort the argument into claim and reason, as in the first lesson.
- Put them side by side and read “reason, therefore claim”.
- Ask what bridge is needed between the two and write it as a full sentence.
- Apply the collapse test: if that sentence were false, would the argument fail?
- Judge the support: does the source give any evidence that the assumption holds?
Worked example
An invented letter to a fictional town newsletter says: “The cycle lane on Jalan Contoh is empty at 7 am on a weekday, so the town should remove it.”
Step 1: claim: the town should remove the lane. Reason: it is empty at 7 am on a weekday.
Step 2: “The lane is empty at 7 am on a weekday, therefore the town should remove it.”
Step 3: what bridges these? Two candidates:
- The 7 am count represents how the lane is used at other times.
- A lane should only be kept if it is already being used.
Step 4, collapse test: suppose the 7 am count is not typical, because most riders pass at 5 pm. Then the reason says very little about the lane’s use, and the argument fails. So the first candidate is a real assumption.
Step 5: the letter gives no counts at any other time, so nothing supports it. That is the weakness to report.
The second candidate is a second, separate assumption. A good answer names one clearly rather than listing every possibility.
The mistake to watch for
A common slip is to restate the claim or the evidence and call it an assumption.
Mistaken answer: “The assumption is that the cycle lane is empty.”
That is the author’s stated reason, not a hidden step.
The correction is to look for what is not written.
The letter says the lane is empty at 7 am. The unstated step is that 7 am is representative. Use the collapse test every time, because a restated reason will fail it: the argument does not collapse if you simply repeat what it already says.
Check yourself
1. “Aina scored 90% on the quiz, so she must have studied hard.” Name one assumption.
Show answer
One assumption is that high marks come from hard studying, and not from an easy quiz or prior knowledge. If the quiz was easy for everyone, the argument fails.
2. “The new café is always crowded, so its food must be good.” Name one assumption and apply the collapse test.
Show answer
Assumption: people go to a café because of the food. Collapse test: if people come for the location, the low prices or the air conditioning, the crowd tells us nothing about the food, and the argument fails.
3. “Our class raised RM200 at the bake sale, so a bake sale is a better way to raise money than a car wash.” What does this assume?
Show answer
It assumes a car wash would have raised less than RM200 with similar effort. The class never tried a car wash, so the comparison has no support. If a car wash would have raised RM300, the conclusion fails.
Where this leads next
Spotting an assumption sets you up to correct an overreaching claim in revising an overgeneralised conclusion. If you want to revisit the evidence check first, see explaining whether cited evidence supports a statement.
An assumption is easier to see when someone asks “what if that is not true?” The teachers in our online one-to-one Global Perspectives tuition do this with your own reading, step by step.