Necessary Assumption Questions
The argument stands on a hidden floor. Find the floorboard it cannot lose.
Two tools pair with this method. The miss-pattern analyzer shows which question types cost you the most points, and the blind review worksheet gives you a second pass at every miss.
What the question asks
The question asks what must be true for the argument to work. The stem says something like "Which one of the following is an assumption required by the argument?" or "The argument depends on assuming which one of the following?"
A necessary assumption is a fact the conclusion depends on. If it is false, the argument collapses. It can be small. "At least some" is often enough.
Step-by-step method
- Find the conclusion and the evidence. As always, separate the claim from the support. The assumption hides in the space between them.
- Find the gap. Ask what the author takes for granted to move from evidence to conclusion. That unspoken step is usually close to the necessary assumption.
- Use the negation test. Negate each candidate choice. Ask: if this were false, would the argument still stand? The choice whose negation kills the argument is the necessary assumption.
- Accept weak answers. The right answer does not have to be strong. Necessary assumptions are often modest: "some," "at least one," "not all." Do not reject a choice for being weak.
- Reject strong answers. A choice that goes further than the argument needs is probably sufficient, not necessary. The argument can survive without the extra strength.
Worked examples
Both examples are original. They are not real LSAT questions.
Example 1
The train arrived on time every day last week, so it will arrive on time today.
The argument requires the assumption that...
The answer: the conditions that kept the train on time have not changed.
Why: Negate it: conditions have changed. Then last week’s record says nothing about today, and the argument falls apart. The argument depends on things staying roughly the same.
Example 2
The new drug is safe because the clinical trial reported no serious side effects.
Which one of the following is an assumption required by the argument?
The answer: the trial would have detected and reported serious side effects if they had occurred.
Why: Negate it: the trial would have missed them. Then "no reports" means nothing, and the safety claim loses its support. The argument needs the trial to be a reliable detector.
The denial test, done right
The denial test is the most reliable tool on this question type. Take each answer choice and state its opposite. The choice whose denial destroys the argument is the necessary assumption. The test works because a necessary assumption, by definition, is something the argument cannot live without.
Most students apply the test sloppily. They deny the choice loosely, or they deny it in a way that leaves the argument a way out. Be precise. If the choice says "the survey was representative," the denial is "the survey was not representative." It is not "the survey was flawed in some unspecified way." Then ask: with that denial in place, does the reasoning still hold? If yes, the choice was not necessary.
Two habits make the test fast. First, run it only on the two or three choices that survived your first pass, not all five. Second, remember that necessary assumptions are usually the weaker-sounding choices. A choice that would guarantee the conclusion is probably sufficient, not necessary. When two choices both feel required, pick the one the argument needs less of.
Common traps
- A choice that is sufficient but not necessary. It would guarantee the conclusion, but the argument does not need that much.
- A choice the argument never uses. If negating it leaves the reasoning intact, it is not required.
- A choice that repeats the conclusion. The argument cannot assume its own ending.
- A tempting strong statement. When two choices both seem needed, pick the weaker one.
Practice the method
The free diagnostic scores every LR question type and shows exactly where you are losing points.