Must Be True (aka “Inference”) questions are foundational to both the Logical Reasoning and Reading Comprehension section 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…”), and 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 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
Now, let’s look at an entirely different class of questions: Assumptions. Classified under the Help Family of Logical Reasoning questions, they ask you to identify a statement that the argument assumes or presupposes:
“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, again, 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
Herein lies the interesting bit: 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, and 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.