Must Be True Questions
Must Be True (aka “Inference”) questions are foundational to both the Logical Reasoning and Reading Comprehension sections of the test. Most commonly, their question stem indicates that the information in the stimulus should be taken as true (“if the statements above are true…”). It then asks you to identify an answer choice that is proven or supported by it (“…which one of the following must also be true?”). All Must Be True answer choices must pass the Fact Test. The Fact Test states that the correct answer choice can always be proven by referring to the facts stated in the stimulus. In other words, since the correct answer choice must necessarily be true given the information contained in the stimulus, there is an inherently conditional relationship between the two.
Stimulus True → Answer choice True
Assumption Questions
Now, let’s look at an entirely different class of questions: Assumptions. They are part of the Help Family of Logical Reasoning questions, and ask you to identify a statement that the argument assumes:
“Which one of the following is an assumption required by the argument?”
“The argument assumes which one of the following?”
Since the assumption is an unstated a premise that must be true in order for the argument to be true, there is an inherently conditional relationship between the argument contained in the stimulus and the correct answer choice.
Conclusion Valid → Assumption True
This is why the Assumption Negation Technique works 100% of the time. The correct answer choice to an Assumption question, when negated, must weaken the argument contained in the stimulus. In other words, when the assumption is not true, the conclusion is not true:
Assumption NOT True → Conclusion NOT Valid
Assumption Negation Technique vs. Fact Test
Herein lies the interesting bit: the Assumption Negation Technique is ultimately not too dissimilar from the Fact Test. Both Must Be True and Assumption questions require you to identify an answer choice that can either be proven by, or one that depends on, the information contained the stimulus. Try a little trick. If you were to replace a typical Assumption question stem with a Must Be True question stem, you’ll probably come up with the same exact answer choice.
These similarities notwithstanding, Assumption and Must Be True questions must be classified separately. In Assumption questions, the stimulus contains a (usually flawed) argument. So, we use the logical opposite of the correct answer choice to reveal that flaw. While the Fact Test might also work, the Assumption Negation Technique is an easier and more intuitive approach for that type of question. By contrast, the correct answer choice to a Must Be True question is directly supported by the stimulus, which need not contain an argument and is always logically valid. This is why you should never use the Assumption Negation Technique to solve Must Be True questions. The technique presupposes argumentation, and many Must Be True questions do not contain arguments.
While these differences are important enough to warrant classifying Must Be True and Assumption questions differently, it is worth pointing out the subtle conceptual links that bind even the most dissimilar types of LR questions. Noticing such similarities should help you develop a slightly more holistic approach to your LSAT prep.
Sal says
Okay John! Thank you for the response!
“Could many Assumptions simply be repurposed as Must instead, with the same argument and answer choices and just a new question stem? Yes!”
I was referring to this part of your earlier comment. May be I put it incorrectly.
I just wanted to re-confirm that whether some Assumption questions can be repurposed and presented as Must be True questions with the same answer choices and same correct answer?
Thanks!
Jon Denning says
They can, yes. There are fundamental differences in how Must and Assumption stimuli are typically constructed—facts sets in Must vs full arguments in Assumption—so swapping one for the other isn’t something the test makers would do, but in principle Necessary Assumption answers are always proven by the author’s conclusion (“If the author is correct, what would that guarantee?”) and thus, from that perspective, are definitionally the same as strict Must Be True.
Sal says
Can an answer choice be correct on both Assumption and Must be True questions?
Jon Denning says
Hi Sal,
Thank you for your comment! In short, because of the nature of what an Assumption question is asking——what do we know an author believes in order to make their argument?——the correct answer to it must, by definition, “be true” if we take the conclusion as true. Or put slightly differently: the argument’s conclusion, if valid, guarantees the correct answer choice is true in an Assumption question.
So from that perspective the Assumption answer works quite similarly (often identically) to a Must Be True question answer, since in Must we’re told to believe everything in the stimulus, which would include the conclusion in Assumption.
It’s generally not quite as clear/straightforward going from Must to Assumption however, as most Must Be True stimuli don’t contain an argument…and of course an argument is at the heart of Assumption questions.
Could many Assumptions simply be repurposed as Must instead, with the same argument and answer choices and just a new question stem? Yes! Granted, the test makers don’t seem terribly inclined to do that (i.e. make the questions/stimuli the same), but in principle Assumption –> Must is a viable transformation in most cases. Could many Must be altered to Assumption with just a swapped question stem? Typically no, because you need an argument for Assumption and those are rare in Must, as I mentioned above.
What I encourage then is to treat this discussion as largely a conceptual exercise, where thinking of Assumption answers as proven by the stimulus/conclusion can be a huge help to people struggling with that question type. But less so as a universal, practical tactic where Assumption and Must become perfectly interchangeable. [Not that that’s what you’re thinking, necessarily, but we’ve seen it enough times that it’s worth a word of caution 🙂 ]
I hope that helps!
Sal says
Hi Jon,
Thank you so much for a detailed response. I had tried to reason it out from different angles only to create more cobwebs in my head. But your explanation has definitely sorted it out for me 🙂
So, in a non-lsat universe if we see the assumption of the argument in the answer choice in a Must be True question, it will be the correct answer?
Thanks.
Jon Denning says
Happy to help!
To your question, the scenario you suggest—an assumption in an argument in a Must Be True question/answer choice—isn’t really one that occurs, since Must stimuli very, very rarely contain arguments in the first place, and assumptions don’t exist without an opinion involved. That is, facts (the basis of Must) don’t contain assumptions; assumptions arise from the *interpretation* of those facts to form a conclusion. That means right answers in Must won’t contain any assumptions, but instead will just be reasonable conclusions based on the information given in the stimulus.
In fact, I’ll go one further and point out that if you *do* see an answer in Must that you feel contains an assumption or logical leap, where you’d have to assume/take things for granted in order to accept it, that answer is wrong! Assumptions are vulnerabilities in arguments, where the author believes they’re true but you as a reader don’t have reason to accept them yet…so in Assumption questions the answer fills them in explicitly, and in Must any answer that contains an assumption is wrong by default because that’s information you can’t be sure of.