Sufficient and Necessary Assumptions Made Easy

Students often confuse Sufficient and Necessary Assumption questions.

On first glance, that’s reasonable. Both question types ask about assumptions. Correct answers can be both Necessary and Sufficient Assumptions. 

But these questions are actually testing fundamentally different concepts.

Necessary Assumptions Ask What Must Be True

Necessary Assumption questions ask you to find something the argument depends on.

You’re looking for something the author absolutely has to agree with. If the correct answer were false, the argument would fall apart. That’s why it’s “necessarily assumed”.

The correct answer usually feels modest or restrained because it only needs to keep the argument alive, not prove it.

Sufficient Assumptions Ask What Would Prove the Conclusion

Sufficient Assumption questions work differently.

You’re looking for a new fact that fixes the argument and proves the conclusion. If the answer choice is true, the argument wins.

That’s why these answers are usually strong. The correct answer has to do enough work to remove every problem with the reasoning.

The Difference Is Easier Than Students Think

Nathan and Josh use a simple example in this episode.

Josh wants to hit a bucket of golf balls at the driving range. The bucket costs $10. Therefore, Josh can hit balls this afternoon.

A Necessary Assumption would be that Josh has at least some ability to pay. If he has no money at all, the argument falls apart.

A Sufficient Assumption would be that Josh has $10 or more. That proves he can buy the bucket.

Harder Questions Test the Difference

On easier questions, the same answer can satisfy both question types. If an answer says Josh has exactly $10, it is both necessary and sufficient. Josh needs the money, and having it proves he can pay. Harder LSAT questions separate those ideas.

A Necessary Assumption question might ask:

“The conclusion relies on which one of the following assumptions?”

A Sufficient Assumption question might ask:

“Which one of the following, if assumed, enables the argument’s conclusion to be properly drawn?”

Differences in the answer choices can make the distinction more subtle, but different nonetheless. 

If an answer says Josh has $12, that would be sufficient because $12 is enough to buy the bucket. But it would not be necessary because Josh does not specifically need $12. He needs at least $10.

An answer saying Josh has at least $8 is necessary because anyone with $10 also has $8. But it is not sufficient because $8 alone would not be enough to buy the bucket.

Don’t Group Them Together

It’s a mistake to treat Necessary and Sufficient Assumption questions as being in the same family of questions. They’re not.

Necessary Assumption questions belong to the closed family of questions because they ask what must be true based upon the argument. You’re not asked to consider any additional information. Sufficient Assumption questions belong to the open family because they ask what new information would prove the conclusion. They ask you to go beyond the argument and add to it in some way.

Thinking about them separately makes both easier.

Focus on the Job of the Answer

For Necessary Assumption questions, ask:

Does this answer have to be true for the argument to work?

For Sufficient Assumption questions, ask:

If this answer were true, would it prove the conclusion?

That’s the difference. One question asks what the argument depends on. The other asks what would make the argument airtight. Create a free account and drill both necessary and sufficient assumption questions.