LSAT Cannot Be True

Last updated: May 2, 2026

Cannot Be True questions are one of the highest-leverage areas to study for the LSAT. This guide breaks down the rule, the elements you need to recognize, the named traps that catch most students, and a memory aid that scales to test day. Read it once, then practice the same sub-topic adaptively in the app.

The rule

In a Cannot Be True question, you treat every statement in the stimulus as true and look for the one answer choice that is logically incompatible with those statements. The other four choices are all things that could be true given the stimulus — they don't have to be true, they just have to be possible. Your only job is to spot the impossible one, which means the answer either directly contradicts a stated fact or is forced into contradiction by chaining the conditionals.

Elements breakdown

Lock the stimulus as fixed truth

Treat every premise as a non-negotiable fact for the duration of the question. You do not evaluate whether the argument is good; you accept it.

  • Read every clause as given
  • Do not question the premises
  • Note conditional cues: if, every, no, only
  • Note quantifier cues: all, some, most, none

Diagram the conditional and quantifier relationships

Translate the stimulus into a small map: which categories imply which, which are mutually exclusive, what the contrapositives are.

  • Write each conditional as A then B
  • Take contrapositives immediately
  • Mark mutually exclusive categories
  • Mark overlap claims (some X are Y)

Common examples:

  • Every senior researcher has published a monograph: senior_researcher then published
  • No senior researcher joined recently: senior_researcher then not_recent_joiner

Reframe the stem in your own words

Translate 'CANNOT be true' into 'four of these are possible, one is impossible — find the impossible one.'

  • Restate the stem before reading choices
  • Distinguish CANNOT from MUST be false (same thing)
  • Distinguish CANNOT from PROBABLY false

Test each choice for compatibility

For each answer, ask: can I construct any scenario, consistent with the stimulus, in which this is true? If yes, eliminate it.

  • Try to invent a possible world
  • Watch for chains: choice plus premise forces contradiction
  • Eliminate must-be-trues — they could be true
  • Eliminate could-be-trues even if unlikely

Confirm the contradiction explicitly

Once you find a candidate, articulate the chain that produces the contradiction. If you cannot name the contradicting clause, you have not yet found the answer.

  • Cite the specific premise it violates
  • Walk the chain step by step
  • Re-check the eliminated four are all possible

Common patterns and traps

The Direct Contradiction

The wrong-but-tempting version of this pattern feels like a paraphrase; the correct version is an answer that flatly negates a clause in the stimulus. The stimulus said 'No X is Y' and the answer says 'An X is Y.' These are the easiest Cannot Be True answers to spot once you slow down and read the choice next to the relevant premise.

An answer that asserts the existence of a member of a category the stimulus said had no members, or that ascribes a property to an entity the stimulus said lacks that property.

The Contrapositive Violation

The stimulus contains a conditional, and the answer asserts a scenario that triggers the antecedent while denying the consequent — or, equivalently, denies the contrapositive. Students miss this because they read the conditional in only one direction. Always take the contrapositive when you diagram.

An answer that puts an item into the 'if' category of a conditional but also into a category the conditional's contrapositive would have ruled out.

The Two-Step Chain Contradiction

The contradiction does not appear in a single premise; it surfaces only when you chain two or three conditionals together. The answer is consistent with each premise read in isolation but impossible once the chain runs. This is the single most common pattern in harder Cannot Be True items.

An answer where you must chain premise 1's conclusion into premise 2's antecedent (and possibly premise 3's antecedent) before the contradiction with the final premise becomes visible.

The Quantifier Overreach

The stimulus establishes a 'some' or 'most' relationship and combines it with a numerical or majority claim. The wrong answer overreaches to 'all' or 'none' in a way the count constraint forbids. Watch for premises that constrain totals (fewer than half, no more than three) — they convert quantifier overreaches into contradictions.

An answer asserting that every member of one group has a property, when combined with the stimulus's count constraint this would force more members into a second group than the stimulus permits.

The Could-Be-True Decoy

These are the answers students wrongly select. They feel implausible, off-topic, or unsupported by the stimulus, but the stimulus never actually forbids them. Remember: the test is compatibility, not support. An answer that introduces a new category the stimulus never mentioned is almost always a decoy, not the right answer.

An answer that brings in a fact about a person, year, or category the stimulus did not address, leaving you with no premise to chain against it.

How it works

Here is how this plays out. Suppose the stimulus says: every member of the orchestra plays at least one string instrument; no string-instrument player at Greenfield Hall has performed in a televised concert. Now an answer reads: 'A member of the orchestra performed in a televised concert at Greenfield Hall.' Chain it: orchestra member implies string-instrument player; string-instrument player at Greenfield Hall implies has not performed on television. So this person both has and has not performed on television — impossible. That is your answer. Notice that an answer like 'Some string-instrument players are not in the orchestra' is fine — the stimulus never said the orchestra contains all string-instrument players. And an answer like 'A guitarist at Greenfield Hall has appeared on television' depends on whether guitar counts as a string instrument, which the stimulus does not settle, so it could be true. The whole game is compatibility, not likelihood.

Worked examples

Worked Example 1

If the statements above are true, which of the following CANNOT be true?

  • A Some Institute employees who have published peer-reviewed monographs are not senior researchers.
  • B An Institute employee who joined within the past three years is a senior researcher. ✓ Correct
  • C Some senior researchers at the Institute have published more than one monograph.
  • D No senior researcher at the Institute joined within the past three years.
  • E Some Institute employees have neither joined recently nor published a monograph.

Why B is correct: The second premise states flatly that no senior researcher at the Institute joined within the past three years. Choice B asserts the existence of an employee who both joined within the past three years and is a senior researcher, which directly negates that premise. No chaining is even needed — the contradiction is one step.

Why each wrong choice fails:

  • A: The stimulus says all senior researchers have published a monograph, not that all monograph-publishers are senior researchers. The third premise actually makes A likely true, and 'likely true' is a kind of 'could be true.' (The Could-Be-True Decoy)
  • C: Premise one requires 'at least one' monograph; that does not cap the number at one. Senior researchers may have published several monographs without contradicting anything. (The Could-Be-True Decoy)
  • D: This is essentially a restatement of the second premise, so it must be true. Anything that must be true could be true, so it cannot be the answer to a Cannot Be True question.
  • E: The stimulus places no constraints on employees who are neither senior researchers nor recent joiners. Such employees may exist with any combination of publication histories, including zero monographs. (The Could-Be-True Decoy)
Worked Example 2

If the statements above are true, which of the following CANNOT be true?

  • A Some bicycles registered in Verdance after 2020 lack reflective hubs.
  • B A Tilsen-300 was registered in Verdance in 2018. ✓ Correct
  • C Some bicycles equipped with reflective hubs were registered after 2020.
  • D Every bicycle equipped with reflective hubs has passed the municipal safety inspection.
  • E The Tilsen-300 was first sold in Verdance in 2015.

Why B is correct: Chain the conditionals. If a Tilsen-300 were registered in Verdance in 2018, then by the first premise it would have reflective hubs; by the second premise, having reflective hubs requires having passed the safety inspection; but the third premise states no Tilsen-300 has ever passed the inspection. The chain forces a contradiction, so this scenario cannot be true.

Why each wrong choice fails:

  • A: The stimulus only requires reflective hubs on bicycles registered before 2020. Bicycles registered after 2020 are unconstrained on this point and may or may not have reflective hubs. (The Could-Be-True Decoy)
  • C: Reflective hubs are required on pre-2020 bicycles but not forbidden on post-2020 bicycles. Some post-2020 bicycles may certainly have reflective hubs. (The Could-Be-True Decoy)
  • D: This is a restatement of the second premise, so it must be true. A must-be-true claim is a could-be-true claim and so cannot be the right answer.
  • E: Being first sold in Verdance in 2015 does not entail being registered in Verdance. The stimulus only constrains registered bicycles, so a 2015 sale date is compatible with everything stated. (The Could-Be-True Decoy)
Worked Example 3

If the statements above are true, which of the following CANNOT be true?

  • A Every applicant who was offered admission scored above 90.
  • B Every applicant scored above 90. ✓ Correct
  • C An applicant who scored 75 was not offered admission.
  • D More than half of this year's applicants scored below 60.
  • E Some applicants who were offered admission scored exactly 90.

Why B is correct: If every applicant scored above 90, then by the first premise every applicant would have been offered admission. That collides with the third premise, which says fewer than half of applicants were offered admission. The two-premise chain rules out the universal claim in B.

Why each wrong choice fails:

  • A: The first premise guarantees admission for those above 90 but does not require that everyone admitted scored above 90. Still, nothing in the stimulus forbids a year in which only above-90 scorers were admitted, so A could be true. (The Could-Be-True Decoy)
  • C: A score of 75 falls between 60 and 90, a range the stimulus says nothing about. The committee may admit or reject such an applicant freely, so C is possible. (The Could-Be-True Decoy)
  • D: Nothing in the stimulus caps the number of applicants who scored below 60. Combined with the third premise (fewer than half admitted), this scenario is fully consistent. (The Could-Be-True Decoy)
  • E: A score of exactly 90 is not 'above 90,' so the first premise does not guarantee admission for those applicants — but it does not bar their admission either. The committee may have admitted some 90-scorers on other grounds. (The Could-Be-True Decoy)

Memory aid

Two-step check: (1) Restate as 'four possible, one impossible.' (2) For your candidate, name the exact premise it contradicts, in one sentence. If you cannot name it, keep looking.

Key distinction

CANNOT be true means logically impossible given the stimulus, not implausible or unsupported. An answer can feel weird, off-topic, or counterintuitive and still be the wrong answer if the stimulus does not rule it out. The right answer is the one the stimulus forbids, full stop.

Summary

Find the answer choice that the stimulus, taken as fixed truth, makes logically impossible — usually by direct contradiction or by chaining a conditional with its contrapositive.

Practice cannot be true adaptively

Reading the rule is the start. Working LSAT-format questions on this sub-topic with adaptive selection, watching your mastery score climb in real time, and seeing the items you missed return on a spaced-repetition schedule — that's where score lift actually happens. Free for seven days. No credit card required.

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Frequently asked questions

What is cannot be true on the LSAT?

In a Cannot Be True question, you treat every statement in the stimulus as true and look for the one answer choice that is logically incompatible with those statements. The other four choices are all things that could be true given the stimulus — they don't have to be true, they just have to be possible. Your only job is to spot the impossible one, which means the answer either directly contradicts a stated fact or is forced into contradiction by chaining the conditionals.

How do I practice cannot be true questions?

The fastest way to improve on cannot be true is targeted, adaptive practice — working questions that focus on your specific weak spots within this sub-topic, getting immediate feedback, and revisiting items you missed on a spaced-repetition schedule. Neureto's adaptive engine does this automatically across the LSAT; start a free 7-day trial to see your sub-topic mastery climb in real time.

What's the most important distinction to remember for cannot be true?

CANNOT be true means logically impossible given the stimulus, not implausible or unsupported. An answer can feel weird, off-topic, or counterintuitive and still be the wrong answer if the stimulus does not rule it out. The right answer is the one the stimulus forbids, full stop.

Is there a memory aid for cannot be true questions?

Two-step check: (1) Restate as 'four possible, one impossible.' (2) For your candidate, name the exact premise it contradicts, in one sentence. If you cannot name it, keep looking.

What's a common trap on cannot be true questions?

Confusing CANNOT BE TRUE with PROBABLY FALSE

What's a common trap on cannot be true questions?

Eliminating an answer because it MUST be true (must-be-true is a kind of could-be-true)

Related LSAT sub-topics

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