6.1 Must Be True / Strict Inference Questions

Understanding Must Be True (MBT) Questions

Must Be True (MBT) questions represent the purest test of deductive logic on the Law School Admission Test (LSAT). Unlike argument-based question types—such as Weakening, Strengthening, or Flaw questions—Must Be True questions do not ask you to evaluate the validity of an author's reasoning or identify assumptions. Instead, you are given a set of statements that you must accept as 100% true, and your sole task is to identify which statement among the choices must also be true as an inescapable consequence of those premises.

On the LSAT, a statement is considered to 'must be true' only if it is impossible for the statement to be false while all given premises are true. If an answer choice could potentially be false under even one hypothetical scenario consistent with the stimulus, it is incorrect. This strict threshold sets MBT questions apart from inductive or probabilistic reasoning.

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Recognizing Question Stems

Must Be True question stems explicitly instruct you to treat the stimulus information as indisputable truth and select an answer that follows deductively. Common stem phrasings include:

• "If the statements above are true, which one of the following must be true?"
• "Which one of the following logically follows from the statements above?"
• "Which one of the following can be properly inferred from the passage?"
• "If the information above is correct, which one of the following is guaranteed to be true?"

Key Term — Inference: On the LSAT, an 'inference' is defined strictly as a deduction that must be true based on given facts. In everyday language, 'infer' often means to speculate or guess, but on the LSAT, an inference carries 100% logical certainty.

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Anatomy of the Stimulus

Unlike most Logical Reasoning stimuli, MBT passages rarely contain a main argument, conclusion, or author's perspective. Instead, they present a fact set—a collection of observations, scientific reports, historical accounts, or conditional rules. Your primary goal when reading an MBT stimulus is to identify how these facts intersect or chain together.

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Formal Logic Operations in Must Be True Questions

Many MBT stimuli rely on formal conditional logic. To deduce valid inferences, you must apply three fundamental logical rules and avoid two classic logical fallacies.

Valid Deductive Rules

1. Modus Ponens (Affirming the Antecedent): If $A \rightarrow B$ is true, and $A$ occurs, then $B$ must occur.
2. Modus Tollens / Contrapositive (Denying the Consequent): If $A \rightarrow B$ is true, and $B$ does not occur ($\neg B$), then $A$ cannot occur ($\neg A$). The contrapositive is logically equivalent to the original statement.
3. Hypothetical Syllogism (Logic Chaining): If $A \rightarrow B$ and $B \rightarrow C$, then $A \rightarrow C$ (and its contrapositive $\neg C \rightarrow \neg A$).

Invalid Deductive Fallacies (Wrong Answer Traps)

• Fallacy of the Converse (Mistaken Reversal): Assuming that because $A \rightarrow B$ and $B$ occurs, $A$ must have occurred ($B \rightarrow A$).
• Fallacy of the Inverse (Mistaken Negation): Assuming that because $A \rightarrow B$ and $A$ does not occur, $B$ cannot occur ($\neg A \rightarrow \neg B$).

Valid Chain:    A  --->  B  --->  C   (Deduction: A ---> C,  ~C ---> ~A)
Invalid Trap:   C  --->  A            (Mistaken Reversal)
Invalid Trap:  ~A  ---> ~C            (Mistaken Negation)

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Quantifier Rules and Set Overlaps

When stimuli use quantifiers, valid deductions depend on understanding exact mathematical definitions:

• All (100%): Every single member of group $X$ belongs to group $Y$.
• Most (51% to 100%): More than half of group $X$ belongs to group $Y$.
• Some (1% to 100%): At least one member of group $X$ belongs to group $Y$.
• None (0%): Zero members of group $X$ belong to group $Y$.

Critical Quantifier Combination Rules

1. All + All Chain: $A \rightarrow B$ and $B \rightarrow C$ yields $A \rightarrow C$.
2. All + Most Overlap: If All $A$ are $B$, and Most $A$ are $C$, then Some $B$ are $C$.
3. Most + Most Overlap: If Most $A$ are $B$, and Most $A$ are $C$, then at least Some $B$ must be $C$ (because two majorities of the same set must overlap).

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Step-by-Step Problem-Solving Strategy

Step 1: Identify the Question Type

Confirm from the stem that you are solving a strict Must Be True question requiring 100% proof.

Step 2: Analyze and Diagram the Stimulus

Read the stimulus carefully. If it contains conditional statements or quantifiers, translate them into concise symbols. Determine if any statements chain together.

Step 3: Formulate a Prephrase

Combine the premises to deduce the logical outcome before reading the answer choices. For conditional chains, write down the combined rule and its contrapositive.

Step 4: Evaluate Answer Choices Against the Proof Standard

Check each answer choice against your diagram and stimulus facts. Ask: Is this 100% guaranteed, or could it fail under certain conditions?

Step 5: Verify Language Force

Match the force of language. Correct MBT answers frequently use cautious words like can, may, some, at least one, whereas extreme words like always, never, only, primary require explicit backing in the text.

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Worked Example & Step-by-Step Breakdown

Stimulus

Every economic policy that reduces corporate tax rates leads to increased private investment. No policy that leads to increased private investment causes short-term revenue deficits for municipal governments, provided that public spending remains stable. Last year, City X maintained stable public spending, yet its municipal government experienced a short-term revenue deficit.

Step-by-Step Analysis

1. Diagramming Premise 1: $\text{Reduce Tax} \rightarrow \text{Increase Private Investment}$
2. Diagramming Premise 2: $(\text{Increase Private Investment} \text{ AND } \text{Stable Spending}) \rightarrow \text{NO Revenue Deficit}$
3. Contrapositive of Premise 2: $\text{Revenue Deficit} \text{ AND } \text{Stable Spending} \rightarrow \text{NO Increase in Private Investment}$
4. Chaining with Premise 1 Contrapositive: $\text{NO Increase in Private Investment} \rightarrow \text{NO Reduced Tax}$
5. Applying Facts: City X had a Revenue Deficit AND Stable Spending. Therefore, City X did NOT have increased private investment, which means City X did NOT enact an economic policy that reduced corporate tax rates last year.

Fact: Deficit + Stable Spending
  ==> NO Increased Private Investment (via Contrapositive of P2)
  ==> NO Corporate Tax Rate Reduction (via Contrapositive of P1)

Evaluating Choices

• Choice A (Correct): City X did not enact an economic policy that reduced corporate tax rates last year.
• Choice B (Wrong - Inverse Fallacy): If City X cuts taxes next year, it will definitely experience stable spending.
• Choice C (Wrong - Could Be True): Private investment in City X declined by more than 10% last year.
• Choice D (Wrong - Out of Scope): Municipalities that increase spending always experience revenue deficits.

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Must Be True vs. Could Be True vs. Must Be False

| Feature | Must Be True (MBT) | Could Be True (CBT) | Must Be False (MBF) | | :--- | :--- | :--- | | Logical Truth Value | 100% Guaranteed | 1% to 99% Possible | 0% Possible (Impossible) | | LSAT Role | Correct Answer in MBT | Incorrect Trap in MBT | Correct Answer in MBF | | Language Force | Usually Moderate / Weak | Variable | Directly Contradictory | | Proof Required | Fully proven by premises | Compatible with premises | Disproven by premises |

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Common Wrong Answer Traps

1. Could Be True Choices: Statements that are plausible or possible based on the real world, but are not strictly proven by the stimulus facts.
2. Exaggerated / Extreme Scope: Choices that use absolute terms like always, never, guaranteed, best, all when the stimulus only supported qualified claims.
3. Mistaken Reversal & Negation: Answers that flip or negate conditional rules without taking a valid contrapositive.
4. New / Outside Information: Choices introducing concepts, historical facts, or expert consensus not mentioned in the stimulus.