Must Be
False

Master the art of identifying what cannot be true given a set of statements: a critical skill for LSAT Logical Reasoning.

LSAT Logical Reasoning

What Does "Must Be False" Mean?

On the LSAT, a "Must Be False" question asks you to find the answer choice that cannot possibly be true if the statements in the stimulus are accepted as fact. It is not merely unlikely: it is logically impossible.

The Logical Landscape of Truth Values

Recognizing the Question Stem

"Must Be False" questions appear in several wordings. Recognizing the stem is your first step.

Classic Forms

"If the statements above are true, which of the following must be false?"

Synonym Forms

"Which of the following cannot be true?" or "Which is impossible given the above?"

Negation Forms

"The statements above, if true, most strongly support the denial of which of the following?"

The Core Rule: Contradiction Test

Formal Logical Structure

In formal logic, "Must Be False" is the negation of what the stimulus entails. If the stimulus establishes a conditional chain, the correct answer will violate that chain.

Conditionals

If the stimulus says \( \text{All A are B} \), then \( \text{Some A are not B} \) must be false.

Quantifiers

If \( \text{No X are Y} \), then \( \text{Some X are Y} \) must be false. Watch for quantity shifts.

Must Be False vs. Could Be False

	Must Be False	Could Be False
Certainty	100% impossible	Possibly false
Test	Contradicts the stimulus	Merely not guaranteed true
Trap	Choosing something merely unlikely	Choosing something that must be true
Standard	No scenario makes it true	Some scenario makes it false

How to Apply the Strategy

1

Read the stimulus carefully. Identify every claim, conditional, and quantifier. Treat every statement as true.

2

Identify the question type. Confirm the stem asks for what "must be false" or "cannot be true."

3

Map the logical structure. Diagram conditionals, contrapositives, and any chain of inference.

4

Test each answer. Ask: "Can this be true while the stimulus is also true?" If yes, eliminate.

5

Confirm the contradiction. The remaining choice should directly clash with a stated or inferred fact.

Worked Example: Conditional Chain

Worked Example: Quantifier Clash

1

Premise 1: No gallery paintings are post-1900. \( (\text{Painting} \to \text{Not post-1900}) \)

2

Premise 2: All gallery watercolors are paintings. \( (\text{Watercolor} \to \text{Painting}) \)

3

Chain: \( \text{Watercolor} \to \text{Painting} \to \text{Not post-1900} \). Therefore: "A watercolor in the gallery was created after 1900" must be false.

Worked Example: Disjunction Trap

The stimulus establishes an exclusive or: each person is in exactly one department. Jordan is in marketing, so Jordan is not in finance.

✓ Could be true

"Jordan manages a team in marketing." Consistent: nothing contradicts this.

✕ Must be false

"Jordan works in both marketing and finance." Violates the exclusive-or rule.

The Negation Shortcut

A powerful technique: negate each answer choice. If the negation of a choice is something the stimulus must support, then the original choice must be false.

Common Contradiction Patterns

All vs. Some Not

Stimulus: \( \text{All A are B} \). Must be false: "\( \text{Some A are not B} \)."

None vs. Some

Stimulus: \( \text{No A are B} \). Must be false: "\( \text{Some A are B} \)."

Exclusive Or

Stimulus: \( \text{Either A or B, not both} \). Must be false: "\( \text{Both A and B} \)."

Common Mistakes to Avoid

✕ Confusing "unlikely" with "impossible"

An answer can be extremely improbable but still logically possible. Only select answers that are impossible.

✕ Ignoring the scope

The stimulus may apply to a specific group. Don't assume broader claims: "All gallery paintings" does not mean "all paintings everywhere."

✕ Picking "must be true" answers

Some students reverse the task and pick what must be true instead of what must be false. Re-read the stem.

Summary: Decision Framework

Answer Status	Relationship to Stimulus	Action
Must Be True	Logically follows from stimulus	Eliminate
Could Be True	Compatible but not required	Eliminate
Could Be False	Not certain either way	Eliminate
Must Be False	Contradicts the stimulus	Select ✓

Question 1 of 5

Stimulus: All members of the chess club are honor students. No honor student has ever received a failing grade. Marcus is a member of the chess club.

▼ Tap to read passage

If the statements above are true, which of the following must be false?

Marcus is in the chess club, so he is an honor student. No honor student has received a failing grade. Therefore, Marcus has never received a failing grade: the opposite claim must be false.

Question 2 of 5

Stimulus: Every employee in Department X speaks either French or German, but no employee speaks both. Lena is in Department X and speaks French.

▼ Tap to read passage

Which of the following cannot be true?

The stimulus says no employee speaks both French and German. Since Lena speaks French, she cannot also speak German. Speaking English is not addressed, so (A) could be true.

Question 3 of 5

Stimulus: No graduate of Hillside Academy has failed the state licensing exam on their first attempt. Priya graduated from Hillside Academy last year.

▼ Tap to read passage

If both statements above are true, which of the following must be false?

Priya graduated from Hillside Academy. No Hillside graduate has failed on their first attempt. Therefore, Priya cannot have failed on her first attempt: this must be false.

Question 4 of 5

Stimulus: Every novel on the recommended list was published after 2010. "The Quiet Forest" is on the recommended list. All books published after 2010 are available in digital format.

▼ Tap to read passage

Which of the following cannot be true?

Chain: \( \text{On the list} \to \text{published after 2010} \to \text{available digitally} \). "The Quiet Forest" is on the list, so it must be available digitally. Claiming it is not available digitally contradicts this chain.

Question 5 of 5

Speaker A: The new traffic regulation will reduce accidents because it limits speed in residential areas, and lower speeds have always correlated with fewer accidents.

Speaker B: The new traffic regulation will not reduce accidents. Most accidents in residential areas are caused by distracted driving, not excessive speed.

▼ Tap to read passage

Speaker A and Speaker B disagree about whether the regulation will reduce accidents. Based on their statements, which of the following must be false?

Speaker A claims the regulation will reduce accidents; Speaker B claims it will not. They explicitly hold opposing views, so it is impossible that both agree the regulation will reduce accidents.

Must BeFalse

What Does "Must Be False" Mean?

The Logical Landscape of Truth Values

Recognizing the Question Stem

The Core Rule: Contradiction Test

Formal Logical Structure

Must Be False vs. Could Be False

How to Apply the Strategy

Worked Example: Conditional Chain

Worked Example: Quantifier Clash

Worked Example: Disjunction Trap

The Negation Shortcut

Common Contradiction Patterns

Common Mistakes to Avoid

Summary: Decision Framework

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

Must Be
False