What is the difference between a premise and an assumption?
GMAC includes analyzing arguments and their assumptions among Critical Reasoning tasks. A premise is provided in the passage; a necessary assumption is unstated but supports the link to the conclusion. Your job is not to write the best possible argument, but to analyze the one you are given.
First identify the exact conclusion. “Revenue will increase” does not mean “profit will increase” or “all customers will be satisfied.” Changing the target can make some answers seem appealing when they are not. To prepare your reading, summarize the reasoning in one line: because a new source of revenue exists, the total will increase. The missing question is what could offset that increase.
What condition is missing from the bookstore’s plan?
Fictional case. Keep revenue and costs separate: the passage is about revenue only.
A bookshop plans to introduce a paid subscription that gives members discounts on books. The owner expects the subscription fees to generate additional revenue. Since all other sources of revenue will remain unchanged except ordinary book purchases, she concludes that the shop’s total revenue will increase. Which assumption is required?
Choose an answer.
The solution, step by step
C.
The method in action
- The reasoning compares a total before and after. Subscription fees add revenue, but discounts or changes in purchasing may reduce ordinary sales revenue. For the total to increase, the loss cannot offset the additional revenue.
- Negating C means that the loss will be equal to or greater than the subscription fees collected. With other revenue sources unchanged, the total no longer increases, so the conclusion fails. C is therefore necessary. In this specific case, C is also sufficient along with the premises: the net gain equals subscription fees minus lost sales revenue, so it is positive.
- A is too strong: a few subscribers may be enough. B is about costs and therefore profit, not total revenue. D and E may describe interesting effects, but they are not necessary to determine the total.
How can you negate an answer without changing it?
The negation test can help evaluate a necessary assumption, provided you negate the statement precisely. “All” becomes “at least one is not”; “some” becomes “none.” Adding an extreme opposite distorts the test.
In the previous exercise, the relevant negation includes the case of equality. Replacing C with “the loss will be enormous” tests only an extreme case and obscures the logical boundary. Write a straightforward negation, then put it back into the argument.
Negate the statement: Some employees can use the proposed shuttle service.
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The solution, step by step
No employee can use the proposed shuttle service.
The method in action
- “Some” asserts that at least one employee can use the service. Its negation states that no employee can use it.
- “Some employees cannot use it” is not the negation: some can use it while others cannot.
- “All employees refuse to use it” adds a choice or refusal that is absent from the statement. Do not replace ability with willingness.
The pitfall
The negation test is not a formula for every question
A statement that strengthens an argument is not necessarily required. Its negation may weaken the conclusion somewhat without destroying the logical link. Before negating all five choices, check what the question asks: finding a necessary assumption, strengthening an argument, weakening an argument, and identifying a supported conclusion are different tasks.
The test can also become confusing if you have misidentified the conclusion or given the words a scope they do not have. In that case, return to the passage. A sense that “this seems less convincing” is not enough to establish necessity.
What should you record in your error log?
For each eliminated choice, name the mismatch: a different quantity, a broader population, an excessive requirement, or information that is merely useful. This classification teaches you to read distractors instead of memorizing the correct letter.
Finally, restate the assumption in your own words and check that negating it breaks the move from the premises to the conclusion. A necessary condition is not necessarily sufficient, though it can be sufficient together with the premises, as in the bookstore case. Do not rank answers by how strong they sound; check their role in this specific argument.
Key takeaways
- Keep the conclusion’s scope precise.
- A necessary condition does not have to prove everything.
- Negate precisely; do not confuse an assumption with a strengthening statement.