Simple Guide on Necessary Assumption Questions

Intro to Necessary Assumption (NA) Questions

This blog is a part of the “Approach Question Type” series and like all articles in this series will focus on step 4 of the “Analyze Stimulus” step. If you need a refresher on how to approach LR questions generally, make sure to check up on our blog "How to Approach the Logical Reasoning Section.”

‍Table of Contents:

• What is a NA Question?
• Approach
• Two Example Walkthroughs

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What is a NA Question

NA questions require you to identify an assumption that must be true for the argument to be valid. If this assumption is not true, the argument falls apart. The method to find the correct answer is to understand how to make the argument completely fall apart.

Question Stems

• "Which of the following is an assumption required by the argument?"
• "The argument depends on which of the following assumptions?"
• "Which of the following is a necessary assumption?"

NA questions are not the same as Sufficient Assumption (SA) questions — think of them as very distant cousins. Besides depending on your ability to understand and parse through assumptions in an argument, NA and SA questions have little in common so be sure not to confuse the two.

In contrast, sufficient assumption questions require you to identify an assumption that, if true, guarantees the conclusion. The assumption is strong enough that it makes the argument valid on its own. To read more about SA questions, check out our guide here.

A Necessary Assumption has a logical flow of two options like this:

1. NA not satisfied → argument’s conclusion cannot be true (falls apart)
2. NA is satisfied → argument may or may not be true.

Take special note that a Necessary Assumption being fulfilled does not prove the validity of the argument. A Necessary Assumption is always a part of the core structure of the argument; without it, the argument collapses. This may make it seem like a strengthener for the argument, which in a way is true. However, Necessary Assumptions are crucial for the argument's validity, acting as a “bare minimum” before we can even attempt to engage with the conclusion. Strengtheners in the LR section enhance the argument's persuasiveness but are not indispensable.

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Approach

There are three crucial concepts of which you should familiarize yourself with to improve your NA skills.

Identify the Argument: Understand the argument's conclusion and the evidence provided to support that conclusion. Accept the statements and the argument as true. 

Prediction Practice Drill: Determine some possible NA’s — what must be true for the argument to work before looking at the answer choices. In practice, you can make infinite amounts of assumptions that must be true, any of which or none of which may appear in the answer choice options. Your goal here is not to actually predict the answer choice option, though it would be nice if your prediction did show up. 

The practice here is to get in the mind of thinking how to absolutely nullify an argument. Necessary requirements to arguments must be satisfied, and in many arguments, these requirements are indeed assumed. There can be necessary assumptions pulled out in every sentence, some are so obvious we don’t need to mention. For example, if we are told that Michael is a strong law school applicant, a reasonable assumption that all of us likely agree on  is that Michael is not a dog (as far as I know, dogs cannot be lawyers!). Sure, maybe you didn’t think of that initially, but it is indeed necessary. 

Use this  prediction practice drill as a starting point; it’s okay to point out even the most obvious, “of course!” assumptions, because sometimes the LSAT will have even these types of assumptions laid out in the answer choices. Eventually you want to be able to parse through more obscure, nuanced NA’s, but NA’s can be very bare minimum, so be sure to not overlook the basic “obvious” ones. 

Negation Test for Answer Choices: One way to confirm you've identified the correct necessary assumption is to use the negation test. The negation test helps determine whether a statement is a necessary assumption by checking if negating (i.e., denying) the statement would invalidate the argument. If negating the statement causes the argument to fall apart, then the statement is a necessary assumption. 

If we didn’t assume that Michael is not a dog earlier, negating this statement would mean Michael is a dog. That would make the argument he is a strong law school applicant impossible!

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Two Example Walkthroughs

Example 1: Dog friendliness

• Premise: All Golden Retrievers are friendly. All Chihuahuas and dogs named “Cow” are mean.
• Conclusion: My dog is friendly.

Which of the following is an assumption required by the argument?

Answer Choices:

1) My dog is a Golden Retriever

2) My dog is a large dog.

3) My dog has never bitten anyone.

4) My dog is not a Chihuahua.

Negation Analysis

1) My dog is a Golden Retriever.

• Negation: My dog is not a golden retriever. This does not make the argument fall apart. Other dog breeds besides Golden Retrievers can be friendly. The conclusion that my dog is friendly is still possible and that’s all that matters.

• Evaluation: Stating my dog is a Golden Retriever breed essentially restates the conclusion and acts as a Sufficient Assumption guaranteeing the conclusion. It does not provide an assumption that is necessary to make the conclusion merely possible.

2) My dog is a large dog.

• Negation: My dog is not a large dog. This really does nothing to the argument as we are told nothing about size and friendliness. So, the affirmative and the negative of this statement is irrelevant to us.

• Evaluation: The size of my dog does not influence whether he is friendly or mean based on the premise provided. The premise only distinguishes between Chihuahuas and other dogs without considering the size of the dog. Therefore, whether my dog is a large dog or not is irrelevant to the conclusion and is not a necessary assumption.

3) My dog has never bitten anyone.

• Negation: My dog has bitten someone. But that does not make the argument fall apart! Maybe it was in self-defense, or by accident, or a light friendly bite — we don’t know, and again we know nothing about behavior nullifying a dog’s ability to be considered friendly. 

• Evaluation: While not biting might suggest that my dog is friendly, it is not directly relevant to the premise which differentiates friendliness based on the type of dog (Golden Retrievers vs. Chihuahuas vs. dogs named “Cow” vs. other dogs). The premise does not provide any information about the behavior of dogs based on whether they have bitten someone, as that is likely based on our outside knowledge. (In this LSAT world, it may be possible that dogs that bite people can be labeled “friendly”, we don’t know!). Hence, this is not a necessary assumption for the conclusion to be valid.

4) My dog is not a Chihuahua and/or named “Cow”

• Negation: My dog is a Chihuahua and/or named Cow. This would make the entire argument fall apart, because that would allow us to conclude: My dog → Chihuahua/Cow → mean. Because negating this statement makes the entire argument fall apart, this verifies that it is indeed an NA.

• Evaluation: This is the correct answer. For the conclusion that my dog is friendly to be even possible, it must be true that my dog is not one of the exceptions (i.e., a Chihuahua or named Cow). If my dog were a Chihuahua, it would be a mean dog and make the conclusion impossible (friendly). Thus, this is a necessary assumption.

Example 2: China and Rubber Ball

When explorer Smith visited China in 1891 A.D., he observed kids playing a dodgeball type game with a rubber ball. China began using rubber around 1700 A.D. Thus, we can be certain that the game must have been invented sometime between the late 18th century and 1891. 

1) Chinese kids played all types of game with the rubber ball. 

2) The making of rubber balls was really good for the Chinese toy economy.

3) The dodgeball game was popular throughout China.

4) The dodgeball game has been played only with a rubber ball.

5) Smith was the first one to record Chinese children playing a game with a rubber ball.

Analysis Answer Choices:

1) Chinese kids played all types of games with the rubber ball.

• Negation: Chinese kids did not play all types of games with the rubber ball (i.e., they played some games with other types of balls or equipment).

• Evaluation: This negation does not affect the argument's conclusion about the invention of the dodgeball game. The timeline of the game’s invention is not dependent on the variety of games played with rubber balls. We don’t know, nor care, what games these kids are (or are not) playing with the rubber ball!

2) The making of rubber balls was really good for the Chinese toy economy.

• Negation: The making of rubber balls was not really good for the Chinese toy economy (i.e., it had little or no impact on the toy economy).

• Evaluation: This negation is unrelated to the argument's conclusion about the invention of the dodgeball game. The economic impact of rubber balls does not influence the timeline of the game's invention.

3) The dodgeball game was popular throughout China.

• Negation: The dodgeball game was not popular throughout China (i.e., it was only popular in certain regions or among certain groups).

• Evaluation: The popularity of the game does not affect the timeline of its invention. The conclusion relies on when the game was first played with a rubber ball, not its popularity. 

4) The dodgeball game has been played only with a rubber ball.

• Negation: The dodgeball game has not been played only with a rubber ball (i.e., it could have been played with other types of balls as well).

• Evaluation: If the dodgeball game could have been played with other types of balls, then the specific observation of a rubber ball by Smith does not definitively establish when the game was invented. The timeline for the invention of the game becomes uncertain if the game was not exclusively played with a rubber ball. Therefore, negating this assumption undermines the entire argument, making it a necessary assumption.

5) Smith was the first one to record Chinese children playing a game with a rubber ball.

• Negation: Smith was not the first one to record Chinese children playing a game with a rubber ball (i.e., others had recorded this before him).

• Evaluation: The timing of Smith’s observation does not impact the timeline of the game’s invention. The argument's conclusion about when the game was invented is not affected by whether Smith was the first to record it. 

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