LSAT Logical Reasoning Must Be True

Mastering Must Be True (MBT) questions is non-negotiable for a high LSAT Logical Reasoning score. These questions test the purest form of logical reasoning: your ability to extract, without error, what is guaranteed by a set of statements. Unlike other question types that ask you to evaluate an argument, MBT questions demand that you become a precise logician, identifying the single answer choice that is irrefutably proven by the information given.

Identifying the Must Be True Question

Your first strategic move is accurate identification. MBT questions are phrased in several predictable ways: "Which one of the following must be true?", "The statements above, if true, most strongly support which one of the following?", or "Which one of the following can be properly inferred from the passage?" The critical verb is "must." If the question asks what "must be" or is "properly inferred," you are in MBT territory. This is distinct from questions asking what "could be true" (which are much broader) or what would "strengthen" or "weaken" an argument (which introduce new information). Recognizing this language immediately triggers a specific mindset: you are working within the airtight confines of the stimulus. Nothing from the outside world, no reasonable assumptions, and no personal knowledge are allowed. Your only tools are the words on the page.

The Doctrine of Strict Logical Necessity

The core skill here is understanding logical necessity. An answer "must be true" if it is 100% guaranteed by the facts presented. There is no room for probability, plausibility, or "likely" conclusions. To test an answer, apply the Negation Test: if you can imagine any scenario, however unlikely, where the stimulus's facts are true but the answer choice is false, then that choice is incorrect. A valid MBT answer choice will be something the author is logically committed to, often a combination of two or more statements from the stimulus. For example, if the stimulus states, "All philosophers are thinkers," and "Socrates is a philosopher," then the necessary conclusion, "Socrates is a thinker," must be true. The incorrect answers will often be attractive because they are probably true or are reasonable extrapolations, but the LSAT requires airtight proof.

Mastering Conditional Reasoning

Conditional reasoning is the engine behind a significant percentage of MBT questions. You must be fluent in recognizing and diagramming conditional statements. A conditional statement has the form "If A, then B," symbolized as $A\to B$. The LSAT frequently tests the valid inference from this chain: if you have $A\to B$ and $B\to C$, you can conclude $A\to C$. Crucially, you must avoid the common fallacies. From $A\to B$, you cannot conclude that if B is true, A is true (the converse fallacy). You also cannot conclude that if A is false, B is false (the inverse fallacy). The only logically guaranteed inference from $A\to B$ is that if B is false, then A must be false (the contrapositive: $\neg B\to \neg A$). Many MBT answers are simply the contrapositive of a statement hidden in the stimulus.

Navigating Quantifiers and Formal Logic

Closely related to conditional reasoning is logic involving quantifiers like "all," "most," "some," and "none." These questions often present multiple statements about groups and require you to deduce a relationship. A statement like "All A are B" is diagrammed as $A\to B$. "No A are B" means if you are A, you are not B: $A\to \neg B$. The quantifier "some" (at least one) is critical. From "Some A are B," you can validly infer "Some B are A," but you cannot infer anything about "all" or "most." A frequent MBT pattern involves combining a universal statement ("All scientists are researchers") with a particular statement ("Some physicists are scientists") to validly conclude "Some physicists are researchers." Understanding the limited but powerful inferences allowed with "some" is key to avoiding trap answers that improperly broaden the conclusion.

Common Pitfalls

1. The "Could Be True" Trap: This is the most frequent error. An answer choice might be plausible, reasonable, or even highly likely given the stimulus, but if it is not necessarily true, it is wrong. Always ask: "Am I forced to accept this, or is there a possible world where the stimulus is true but this answer is false?"
2. Adding Outside Knowledge: You must resist the urge to use real-world facts to justify an answer. The LSAT creates a self-contained logical universe for each stimulus. If the stimulus defines a "widget" as something that flies, then for the purpose of this question, widgets fly—even if that contradicts everything you know about manufacturing.
3. Misreading Quantifiers: Confusing "some" for "all" or "most" will lead you directly to a wrong answer. If the stimulus says, "Some city planners oppose the project," you cannot support an answer that says, "Most city planners oppose the project" or "All city planners oppose the project." The leap from "at least one" to a majority or totality is unjustified.
4. Overlooking the Contrapositive: When you see conditional language, immediately consider the contrapositive. The correct answer is often the contrapositive of a stated premise, or a conclusion drawn by chaining premises and then taking the contrapositive of the chain. Failing to diagram can cause you to miss this.

Summary

• Must Be True questions require you to identify the conclusion that is necessarily and inarguably proven by the stimulus, with no assumptions or external knowledge.
• The Negation Test is a powerful tool: if negating an answer choice makes it inconsistent with the stimulus, that choice must be true.
• Conditional reasoning ($A\to B$) and its valid inference, the contrapositive ($\neg B\to \neg A$), are fundamental; avoid the fallacies of affirming the consequent or denying the antecedent.
• Quantifier logic ("all," "some," "none") must be handled with precision; you cannot infer "all" from "some" or "most."
• The most attractive wrong answers are often those that could be true or are reasonable but not necessary extensions of the text. Your job is to find the answer that must follow.