LSAT Logical Reasoning: Necessary Assumption Questions

A “necessary assumption” question is one that asks you to identify an assumption required by an argument’s reasoning. (These questions are also called “assumption questions” and “required assumption questions.”) The question stem for a necessary assumption question might read:

  • Which of the following does the above argument rely on?
  • Which of the following assumptions is required by the argument?
  • The argument depends on which of the following?

We solve these questions by following four steps:

  1. Identify the argument’s conclusion.
  2. Identify how the conclusion is supported.
  3. If possible, identify a gap in the argument’s reasoning.
  4. Use the negation test to identify the best answer choice.

We’ll look at each step. But first, let’s talk about what a necessary assumption is.

Understanding Necessary Assumptions

Necessary assumptions are presumptions required an argument’s reasoning. Put more simply, they are statements that the argument needs to be true. For example, consider the following argument:

Helen is a terrible CEO. It might be true that every year that she has been CEO of Turnip Co, the company has made a significant profit. But Turnip Co made even greater profits before Helen joined the company. And the overwhelming majority of stockholders would be overjoyed if Helen stepped down.

This argument requires a number of unstated claims to be true in order to work. One is that it’s possible for a terrible CEO to be in charge of a consistently profitable company. Notice how the argument needs that assumption to be true. Without it, our argument could not support it’s conclusion. Thus, we have identified a necessary assumption. Another necessary assumption is that the majority stockholder opinion can sometimes accurately reflect the correct judgement of a CEO. The argument assumes that the majority stockholders are correct about Turnip Co’s CEO. Thus, the argument necessarily assumes that the majority of stockholders could be correct about a CEO.

Think of necessary assumptions as argument keystones. They are unstated premises that, if contradicted, would cause the entire argument to collapse.

Let’s examine each step for solving necessary assumption questions, and then we’ll take a look at another example.

Identify the Conclusion

The main conclusion of the argument is supported by everything else in the argument; it’s what the rest of the argument is attempting to convince you is true. If you struggle with identifying conclusions, take a look at the Identify the Conclusion Questions lesson.

Identify How that Conclusion is Supported

Make a note of the premises. What supports the conclusion? Why does the author think we should believe the conclusion? Silently paraphrase the support to yourself, trying to understand why the author finds the premises to be so convincing.

If Possible, Identify a Gap in the Argument’s Reasoning

Necessary assumption questions never provide us with valid arguments. The premises do not necessarily lead to the conclusion; something is missing from the argument. We call these missing statements “assumptions.” An assumption is a claim that the reasoning in the argument takes for granted. Assumptions are not explicitly stated in an argument; rather, they are implied by the reasoning.

If you can pinpoint where an assumption has been made by the argument, then you have identified a gap in the reasoning. Often the correct answer to a necessary assumption question will partially (or completely) correct that gap in reasoning. So by making a mental note of what the gap is, you make the correct answer easier to identify.

Use the Negation Test to Identify the Best Answer Choice

Remember that a necessary assumption is required by an argument’s reasoning. Thus, the negation of a necessary assumption will contradict the argument’s reasoning. This provides us with a valuable tool: the negation test.

The negation test can be performed on each answer choice for a necessary assumption question. The step of the negation test are:

  1. Negate the answer choice (assume that the answer choice is not true).
  2. Determine if the negated answer choice contradicts the original argument. In other words, if the negated answer choice was true, would the argument still work? If not, you’ve found a contradiction.
  3. Choose the answer choice that, when negated, contradicts the argument.

When properly executed, the negation test works every time.

Of course, the negation test can be time consuming. This is why you should note any gaps in the arguments and negate the answer choices first that attempt to fill a gap in reasoning. You’ll likely move more efficiently through necessary assumption questions this way.

Necessary Assumption Question Example

Let’s look an example question and work through it together.

George arrived at the bus stop at 6:07, and the last bus going downtown that day arrived at 6:08. If George had arrived at the bus stop only a few minutes later, he would not have been able to go downtown that day.

The argument requires the assumption that

A. George needed to go downtown.
 B. The bus did not run out of gas when it took George downtown.
 C. George did not own a car.
 D. George had the appropriate amount of bus fare.
 E. The bus was the only way George could go downtown.

Let’s work through each of the steps.

Identify the argument’s conclusion.

The conclusion of the argument is: “If George had arrived at the bus stop only a few minutes later, he would not have been able to go downtown.” If this is not clear to you, refer back to the lesson on Identify the Conclusion Questions.

Identify how the conclusion is supported.

The conclusion is supported by the fact that George arrived at the bus stop only one minute before the bus arrived. The conclusion is also supported by the fact that this was the last bus going downtown. These statements support the conclusion because they demonstrate that if George had arrived a few minutes later, he would have missed the last bus to go downtown. This apparently would have meant that George could not take a bus downtown.

If possible, identify a gap in the argument’s reasoning.

The big gap in reasoning here exists between George missing the bus and not having any means to go downtown. We know that George could not catch any other bus at that bus stop, but could he potentially find another way to go downtown? Perhaps he could drive a car, walk, or bicycle to downtown. We are assuming in the argument that the bus is George’s only means to go downtown.

Use the negation test to identify the best answer choice.

The negation of each answer choice is:

A. George did not need to go downtown.
B. The bus ran out of gas when it took George downtown.
C. George did own a car.
D. George did not have the appropriate amount of bus fare.
E. The bus was not the only way George could go downtown.

Notice how negation only required that we negate the main verb of each answer choice. It’s that simple. By negating the main verb, we get a look at the logical opposite of each answer choice.

Let’s take a look at each negated answer:

A. George did not need to go downtown.

No contradiction. George’s need to go downtown has no impact on his ability to go downtown.

B. The bus ran out of gas when it took George downtown.

No contradiction. The bus running out of gas does not contradict George not being able to go downtown if he missed the bus. In fact, the two have nothing to do with each other at all.

C. George did own a car.

No contradiction. But it looks like one at first! George owning a car does seem to contradict the argument’s conclusion. But does it really? Perhaps the car is broken, loaned out to a friend, out of gas, hooked up to a tow truck, stuck in a ditch, or unusable for some other reason. There is no real contradiction here.

D. George did not have the appropriate amount of bus fare.

No contradiction. George not having bus fare in no way contradicts not being able to go downtown.

E. The bus was not the only way George could go downtown.

Contradiction! If the bus is not the only way George can go downtown, then he is still able to go downtown even if he misses the bus! The conclusion is no longer true. Thus, we have our answer.

Hopefully you noticed immediately that Answer E was promising just because it appeared to fill the big gap in the argument. If so, great! You could have started by negating Answer E, noticing that the negation contradicted the argument, and correctly answer the question without ever having to waste time negating bad answer choices.

 

Word of Caution of the Negation Test

The negation method works well for necessary assumption questions, but you have to be careful to do it correctly. Negate using a logical opposite, not a polar opposite. For instance, the negation of “All dogs go to heaven” is not “No dogs go to heaven.” This is the polar opposite, but it isn’t the logical opposite. The correct negation is “Not all dogs go to heaven.” (Notice how we’re only claiming that the original statement is not true!) This statement can also be worded: “Some dogs do not go to heaven.” The logical opposite of “All do” is “Some do not.” If not all dogs go to heaven, then some dogs do not go to heaven.

Similarly, the negation of “Some dogs go to heaven” is “No dogs go to heaven.” You might be tempted to think the negation should be “Some dogs do not go to heaven.” But this does not really contradict the original statement; both can be true. Therefore, the logical negation of “some” is “none.”

Keep in mind that you can work backwards with negations. The negation of a negation is simply the original statement. So if you read “Some dogs do not go to heaven,” you can negate by thinking “All dogs go to heaven.”

Below is a negation chart for you to use. You can memorize this chart, but a better approach would be to think through each negation. Think about why the original statement and the negation cannot be true at the same time.

Easy Negation Chart

All → Not some
Not some → All
None (Not all) → Some
Some → None