Assumption questions come in two species, and confusing them is the single most expensive mistake in LR. Learn the distinction once, and both question types get easier permanently.
Sufficient assumption: a premise that, if true, would guarantee the conclusion. It bridges the gap completely. Think of it as the missing puzzle piece that finishes the picture. Stems sound like: "Which one of the following, if assumed, would allow the conclusion to be properly drawn?" The test: add the answer to the premises — does the conclusion now follow airtight? If yes, it's sufficient. (It might be stronger than needed; that's fine.)
Necessary assumption: a premise the argument cannot live without. If it's false, the argument collapses. Think of it as load-bearing: remove it and the conclusion falls. Stems sound like: "Which one of the following is an assumption required by the argument?" / "The argument depends on assuming which one of the following?"
The negation test — your tool for necessary assumption. Negate each answer choice (turn it into its opposite) and ask: does the argument survive? If negating an answer destroys the argument, that answer is necessary. If the argument shrugs it off, eliminate it. This test is decisive; use it on every necessary-assumption question you're unsure about.
A minimal pair. Argument: "Maya trains six days a week. Therefore she will finish the marathon."
- Sufficient: "Anyone who trains six days a week finishes the marathon." Add it — conclusion follows airtight. (Overkill, but sufficient.)
- Necessary: "Maya will actually start the marathon." Negate it — "Maya will not start" — and the conclusion is dead. The argument needs this, even though it's far weaker than the sufficient version.
Notice the relationship: necessary assumptions are usually much weaker claims than sufficient ones. When stuck between two choices on a necessary question, the weaker, more boring one is usually right — the LSAT punishes students who pick the exciting strong claim that the argument doesn't actually need.
Key takeaways
- Sufficient = guarantees the conclusion (bridge the gap completely). Necessary = the argument dies without it.
- Negation test for necessary: negate the choice; if the argument collapses, that's your answer.
- "Required / depends on" → necessary. "If assumed, allows the conclusion" → sufficient.
- On necessary questions, prefer the weaker, load-bearing claim over the exciting strong one.