Free Courses › Logical Reasoning

The big question types · Lesson 5 of 14

Necessary assumption questions

After this lesson, you can find the assumption a Logical Reasoning argument depends on and confirm it with the negation test.

23 min video, plus notes and 1 practice question below

In this lesson

• Distinguish a necessary assumption, what the argument can't survive without, from a sufficient assumption, what would guarantee the conclusion
• Read a question stem's language (required, depends upon, relies on versus guarantees, enables) to know which assumption type you're facing
• Classify a necessary assumption as a connector that fills a logical gap or a blocker that protects against an outside threat
• Run the 5-step method on any argument: find the conclusion, find the premise, spot the gap, predict the assumption, and negate
• Eliminate wrong answers fast by spotting the 5 common traps: out of scope, opposite, extreme language, restated premise, and reverse logic

What "necessary" actually means

A necessary assumption is not something that would make an argument stronger, it's something the argument can't survive without. Think of an argument as a bridge: the evidence sits on one side, the conclusion sits on the other, and assumptions are the support beams underneath holding the 2 together. Pull a beam out and the bridge collapses. That's the test: pull the assumption out. If the argument turns to nonsense without it, you found a necessary assumption. If the argument still works fine without it, keep looking.

This is different from asking what would help the argument. If a doctor says "you have strep throat, so I'm prescribing antibiotics," the doctor is quietly assuming you're not allergic to antibiotics. That assumption is essential: without it, the recommendation could be dangerous. A necessary assumption is the bare minimum the argument needs, nothing more.

Read the question stem before you read the choices

The question stem tells you exactly what's being tested, if you read it correctly. Stems like "which one of the following is required for the argument to hold," "the argument depends upon," or "the argument relies on" are all coded language for necessary assumption questions. They're asking what must be true for the argument not to collapse. Watch for the vaguer stem "the argument assumes that," which can point to either a necessary or a sufficient assumption question.

The way to tell them apart is strength. Language about what the argument needs, requires, or depends on signals necessary. Language about what would guarantee the conclusion or let it logically follow signals sufficient. Necessary assumptions must be true. Sufficient assumptions make the conclusion 100% certain. Confusing the 2 is one of the fastest ways to get stuck on this question type.

Two shapes every necessary assumption takes

Every necessary assumption answer falls into 1 of 2 categories: a connector or a blocker. A connector fills a logical gap between the evidence and the conclusion, a leap the author made without saying so. Take this argument: all marathon runners have excellent stamina, Jenna has excellent stamina, so Jenna must be a marathon runner. That skips over the fact that swimmers, soccer players, and plenty of other people also have excellent stamina. The unstated assumption, that only marathon runners have this kind of stamina, is the connector that bridges the gap.

A blocker protects the argument from an outside threat instead of connecting 2 ideas. Take this one: buying new software will increase productivity, so it's a good business decision. That argument is vulnerable if the software costs a huge amount of money, since the cost could cancel out the productivity gains. The necessary assumption here is a blocker, something like "the cost of the software doesn't outweigh the productivity benefits." Ask whether the argument has a gap between its terms (look for a connector) or a hidden vulnerability outside the argument (look for a blocker).

The 5-step method

Run every necessary assumption question through the same 5 steps. Step 1, find the conclusion: sum up the author's point in one sentence, since the LSAT never labels it for you. Step 2, find the premise or premises, the reasons given for that conclusion, often flagged by words like "because" or "since." Step 3, spot the gap by asking whether the evidence actually guarantees the conclusion. It never fully does, so something is missing, either a leap in logic (a connector) or a lurking risk (a blocker). Step 4, predict what would need to be true to close that gap. Even a rough guess narrows the choices fast.

Step 5, run the negation test on your top candidate: negate the answer choice and ask whether the argument falls apart if that negation is true. If it does, the choice is necessary. Say a candidate answer reads "the new policy will not reduce employee morale." Negate it: the new policy will reduce employee morale. If the conclusion is that the policy will improve productivity, and morale is now cratering, that probably kills the argument, which confirms the original, un-negated version is necessary.

5 traps that make wrong answers look right

Watch for 5 patterns in wrong answers. Out-of-scope answers bring in a concept the stimulus never mentioned, like citing environmental benefits in an argument that's only about cost. Opposite answers actively weaken the argument instead of supporting it, so they can never be something the argument depends on. Extreme language, words like "always," "never," "only," or "must," usually signals a wrong answer, since necessary assumptions tend to be modest, not sweeping.

Restated-premise answers just repeat something the stimulus already said, which means they add nothing new and can't be the missing assumption. Reverse-logic answers flip a cause-and-effect or if-then relationship the author never claimed, like turning "A causes B" into "B causes A." Spotting these 5 traps turns you from guessing which answer sounds right to filtering out the ones that can't be right.

Watch the 5 steps run on a real argument

Here's a full run on one argument: installing solar panels will lower our electricity bill, therefore installing the panels will reduce our overall expenses. Step 1, the conclusion: installing solar panels will reduce overall expenses. Step 2, the premise: installing solar panels will lower the electricity bill. Step 3, spot the gap: a lower electricity bill doesn't automatically mean lower total expenses, maybe electricity is a small part of the budget, or installation costs eat the savings.

Step 4, predict the assumption, trying both shapes. A connector: electricity costs make up a significant portion of overall expenses, since if electricity is only 2% of spending, saving there doesn't move the needle. A blocker: no other cost, like installation or maintenance, rises enough to cancel out the savings. Either shape could be the necessary assumption, depending on how the answer choices are worded.

Step 5, negate to confirm. Take the connector answer, "the cost of electricity is a significant component of total expenses," and negate it: electricity costs are not significant. If that's true, saving on electricity can't meaningfully cut total expenses, and the conclusion collapses. That confirms it's necessary. Run this same sequence, conclusion, premise, gap, predict, negate, on every necessary assumption question, and the 5 traps from the last section stop being traps. They're just choices you already know how to filter out.

The short version

• Don't pick an answer just because it would help the argument. A necessary assumption is the bare minimum it needs, nothing more, like the antibiotic-allergy assumption a doctor makes without saying it out loud.
• Log which type trips you up more on your next set, connector gaps or blocker gaps. Most students are weak on just one, and that's the one worth drilling on purpose.
• Make the negation test non-negotiable. After you think you've found the assumption, negate it before you lock in your answer, skipping that step is where confident wrong answers happen.

Mark lesson 5 complete