Logical reasoning question types - Cannot be true questions

Learning Outcomes

By working through this article, you will be able to accurately identify and answer 'cannot be true' questions in the LSAT Logical Reasoning section. You will learn to recognize relevant question stems, apply elimination techniques, avoid common logical missteps, and distinguish these questions from related inference types. This targeted knowledge will underpin your confidence in tackling these high-yield question types on exam day.

LSAT Syllabus

For LSAT, you are required to understand 'cannot be true' questions as a distinct Logical Reasoning type. In your revision, pay close attention to:

• How 'cannot be true' question stems signal what the exam expects you to do
• The use of elimination strategy to dismiss possible or true answer choices
• The skill of identifying which statement or inference is directly contradicted by the stimulus
• The distinction between 'cannot be true', 'must be true', and 'could be true' questions
• The role of sufficient and necessary conditions in incorrect inferences

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. Which of the following questions is most likely a 'cannot be true' LSAT Logical Reasoning question?
   1. Which one of the following must be true?
   2. Which one of the following can be properly inferred?
   3. Which one of the following CANNOT be true?
   4. Which one of the following is most strongly supported?
2. What is the best strategy when you encounter a 'cannot be true' question?
   1. Pick the answer directly matching the passage
   2. Eliminate answers that could be true, and select the one contradicted by the passage
   3. Select the longest answer choice
   4. Pick the answer with the strongest wording

3. True or false: 'Cannot be true' questions sometimes require you to identify a choice that is never consistent with ALL the stimulus information.

4. In 'cannot be true' questions involving conditional logic, which error is commonly tested?
   1. Mistaking necessary for sufficient
   2. Affirming the consequent
   3. Denying the antecedent
   4. All of the above

Introduction

'Cannot be true' questions are a specific type of Logical Reasoning item on the LSAT, requiring you to find an answer that directly conflicts with information in the stimulus. These are negative inference questions: you must select the response that, if taken as true, would violate a fact or logical consequence set out in the stimulus.

Recognizing these questions is essential—they often use clear language such as "cannot be true," "would be inconsistent with," or "could not be correct." They contrast with 'must be true' questions, which demand you identify what necessarily follows.

Success on this question type depends on understanding both the logic of the argument (including formal logic and conditional relationships) and applying process-of-elimination rigorously to the answer choices.

Key Term: cannot be true question
A Logical Reasoning question type where your task is to select the answer directly contradicted or made impossible by the information in the stimulus. Could also be worded as "would be incompatible with," "could not occur," or "would violate the information above."

Identifying Cannot Be True Questions

Question stems for this type are often direct. Look for wording such as:

• "Which one of the following cannot be true?"
• "Each of the following could be true EXCEPT:"
• "Which one of the following would most seriously conflict with the information above?"

Be careful: 'could be true EXCEPT' means the correct answer is the only one that cannot be true, so restate the question to clarify your task.

The correct answer to a cannot be true question must always be wholly inconsistent with the stimulus.

Key Term: elimination strategy
The technique of rejecting answer choices that could potentially be true or are compatible with the information given, leaving the one choice that must always be false.

How to Approach Cannot Be True Questions

There are two core steps:

1. Understand the Stimulus: Focus on identifying key facts and logical connections (especially conditional statements).
2. Use Elimination: Dismiss any answer that could possibly be true given the information—even if unlikely. Only select a choice that would logically contradict or violate a premise or valid inference from the stimulus.

One common structure for these questions involves conditional (if-then) logic. Many incorrect answers will exploit typical errors associated with conditional reasoning.

Key Term: sufficient and necessary conditions
The logical relationship where the sufficient condition (if-part) guarantees the necessary (then-part), but not the reverse. LSAT commonly tests mistakes such as assuming the necessary condition is sufficient to trigger the sufficient condition.

Common Patterns and Errors

Typical traps in cannot be true questions include:

• Mistaking an answer that is merely unsupported for one that is contradicted
• Overlooking restrictions when applying conditional reasoning
• Focusing only on facts, not on logical inferences (contrapositives, etc.)

You must select the answer that absolutely violates at least one piece of information, whether stated directly or proven by logical inference.

Worked Example 1.1

A group offers membership only to attorneys. Pierre joins the group.

Which one of the following cannot be true? a) Pierre is an attorney. b) Pierre is an attorney and manager. c) Pierre is not licensed to practice law. d) Pierre is not an attorney.

Answer:
Choice (d) cannot be true. Since only attorneys may join and Pierre joins, he must be an attorney. Any answer stating otherwise directly contradicts the group's rules.

Worked Example 1.2

If all successful projects require teamwork, and the Nova project succeeded, which one of the following cannot be true? a) The Nova team worked together b) The Nova project had strong leadership c) The Nova project succeeded without teamwork d) The Nova project included external consultants

Answer:
Choice (c) cannot be true. The conditional "all successful projects require teamwork" means any success must involve teamwork. So, it's impossible that the project succeeded without teamwork.

Worked Example 1.3

If Jamal attends the event, then Kierra must also attend. Kierra does not attend.

Which one of the following cannot be true? a) Jamal attends b) Neither Jamal nor Kierra attend c) Only Kierra attends d) Kierra attends but Jamal does not

Answer:
Choice (a) cannot be true. If Kierra does not attend, then it's impossible for Jamal to attend, due to the contrapositive of the original rule.

Dealing with Conditional Logic

These questions often exploit misinterpretation of conditionals.

If a rule says "If X, then Y", the contrapositive is "If not Y, then not X". In a cannot be true question, an answer that affirms X while contradicting Y will always be incorrect—because it is impossible.

Carefully look for hidden conditional structures and always apply contrapositives.

Exam Warning

Be careful—many incorrect answers on 'cannot be true' questions are those that are simply not supported, not directly contradicted. The only correct choice is one which is incompatible with the passage; do not mistake absence of evidence for clear contradiction.

Revision Tip

To double check, try plugging the answer choice into the facts and see if it results in a clear contradiction. If you can create a possible scenario, even if contrived, it is not the correct answer.

Summary Table: Comparison with Related Question Types

Key Point Checklist

This article has covered the following key knowledge points:

• 'Cannot be true' questions require you to select an answer impossible given the passage's facts or logic
• The correct answer is always directly contradicted or logically impossible based on provided information
• Elimination strategy: rule out any answer that could possibly be true, select only the one that violates the passage
• Recognizing and correctly handling sufficient and necessary condition rules is essential
• Do not confuse unsupported answers with impossible ones

Key Terms and Concepts

• cannot be true question
• elimination strategy
• sufficient and necessary conditions