A “sufficient assumption” question is one that asks you to identify a statement that, if added to an argument, would cause the argument’s conclusion to be logically inferred from the premises. (These questions are also called “assumption questions” and “justify questions.”) The question stem for a sufficient assumption question might read:

  • The conclusion follows logically if which of the following is assumed?
  • The conclusion above can be properly drawn if which of the following is assumed?
  • Which of the following, if true, enables the conclusion to be properly drawn?

We solve these questions by following five steps:

  1. Identify the argument’s conclusion.
  2. Identify how the conclusion is supported.
  3. Identify the gap in the argument’s reasoning
  4. Choose the answer that bridges the gap in reasoning.

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


Understanding Sufficient Assumptions

Sufficient assumptions are statements that, if added to an argument, would make that argument logically valid. Consider the following example:

Socrates is a man. Thus, Socrates is mortal.

This is not a valid argument because the conclusion does not follow logically from the premises. Even if we assume that the premise (“Socrates is a man.”) is true, it’s still possible for our conclusion (Socrates is mortal.) to be false.

But if we add in one more statement, we get a valid argument. What if we add “All men are mortal” as a second premise to the argument? Now we have a valid argument. If both premises are true, the conclusion must also be true.

The statement “All men are mortal” is a sufficient assumption for the argument above. When added to the argument, it creates logical validity. It fills in the logical gap.

Note how this is different than a necessary assumption.  A necessary assumption is required by the argument. (In the argument above, “Some men are mortal” would be a necessary assumption.) A sufficient assumption, on the other hand, allows us to logically infer the argument’s conclusion. When looking for a sufficient assumption, our focus is not on what is needed by the argument; rather, our focus is on what would complete the argument.

Let’s examine each step for solving sufficient 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.

Identify the Gap in the Argument’s Reasoning

Sufficient 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. Identifying these gaps is what allows people to consistently get sufficient assumption questions correct.

Choose the Answer

The correct answer will completely fill in the logical gap. Choose than answer that, if you were to add it to the original argument, would create a valid argument. Incorrect answer choices will never create a valid argument; correct answer choices always will.

Sufficient Assumption Question Example

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


All students who go to law school are smart. Therefore, Harry is smart.
The conclusion above is properly drawn if which of the following is assumed?

 A. Harry thinks he is smart.
 B. Harry scored exceptionally well on the LSAT.
 C. Harry teaches at a law school.
 D. Harry is a student who goes to a law school.
 E. Law school students are smarter than other students.

Let’s go through each of the steps for solving sufficient assumption questions.

Identify the Conclusion

The conclusion is “Harry is smart.” We know this because it is supported by the only other statement in the argument.

Identify How that Conclusion is Supported

The only support for the conclusion in this argument is “All students who go to law school are smart.” The author of this argument apparently hold law students in very high regard.

Identify the Gap in the Argument’s Reasoning

 

The gap here obviously must exist between the only premise and the conclusion. We don’t yet know why all law students being smart supports the claim that Harry is smart. Is Harry a law student? Is Harry smarter than all law students? We don’t know. But something needs to connect the premise to the conclusion in order for this argument to be valid.

Choose the Answer

Let’s take a look at each answer choice.

A. Harry thinks he is smart.

This does not bridge the gap. Even if Harry does think he’s smart, how does that connect the premise to the conclusion? The argument is still not valid.

B. Harry scored exceptionally well on the LSAT.

Good for Harry! But we still don’t have a valid argument. We need to connect the claim that all law students are smart to the claim that Harry is smart.

C. Harry teaches at a law school.

This is closer, but we still don’t have a valid argument. The argument never asserts anything about those who teach at law school; it only talks about law students (and Harry, of course). So we still are not bridging the gap between the premise and the conclusion.

D. Harry is a student who goes to a law school.

Correct! This fills in the logical gap and completes the argument. Consider what happens if we add this statement to the original argument:

All students who go to law school are smart. Harry is a student who goes to a law school. Therefore, Harry is smart.

Notice how the conclusion must be true if the premises are true. That’s logically validity. And that means we have found the correct answer.

E. Law school students are smarter than other students.

This tells us nothing about Harry, and thus, it does not bridge the gap between the premise and the conclusion.

 

Sufficient Assumption Questions

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