Identify an Entailment | Logical Reasoning — Worked Examples | LSAT Prep
Master entailment and must-be-true questions on the LSAT with proven strategies, logical reasoning techniques, and official worked examples from LSAC PrepTests. Learn to identify what logically follows with 100% certainty.
Understanding Entailment Questions on the LSAT
Entailment questions, also known as Must Be True or Inference questions, are among the most frequent question types in LSAT Logical Reasoning sections. These questions test your ability to identify what must logically follow from a given set of statements with absolute certainty. The Law School Admission Council typically includes four to six entailment questions per Logical Reasoning section, making them approximately 15-25% of all Logical Reasoning questions.
Unlike other question types that ask you to find assumptions or strengthen arguments, entailment questions require you to identify statements that are guaranteed to be true based solely on the information presented. The correct answer is not merely consistent with or suggested by the passage—it must be a logical necessity that cannot be false if the stimulus is true.
According to LSAC, these questions assess your ability to "recognize what can be logically concluded from a set of statements" and to "draw well-supported inferences." This skill is fundamental to legal reasoning, where attorneys must determine what conclusions can be properly drawn from statutes, case law, and evidence.
Key Concepts: What Is an Entailment?
Definition of Entailment
An entailment is a statement that is a logical consequence of the information provided. In formal logic, we say that statement B is entailed by statement A if it is impossible for A to be true and B to be false simultaneously. For LSAT purposes, an entailment must be true with 100% certainty based on the given facts.
The Golden Rule:
If the stimulus is true, the correct answer MUST be true. There is no scenario in which the stimulus could be true and the answer false.
Logical Relationships in Entailment
Entailments often involve formal logical relationships:
Conditional Statements (If-Then)
Form: If P, then Q (written as: \( P \rightarrow Q \))
Valid inference: If P is true, Q must be true
Contrapositive: If not Q, then not P (written as: \( \neg Q \rightarrow \neg P \))
Categorical Statements
All A are B: If something is A, it must be B
No A are B: If something is A, it cannot be B
Some A are B: At least one A is also B
Combining Statements
Given: All A are B, and All B are C
Entailment: All A are C (transitive property)
Must Be True vs. Could Be True vs. Likely True
✓ Must Be True
100% certain. Cannot be false if the stimulus is true. This is what entailment questions ask for.
⚠ Could Be True
Possible but not guaranteed. Consistent with the stimulus but not a logical necessity.
✗ Likely True
Probable but not certain. Even if something is 99% likely, it's not an entailment.
How to Recognize Entailment Questions
Common Question Stems
Entailment questions use specific language in their question stems. Learning to recognize these phrases instantly will save you valuable time:
- ► Which one of the following can be properly inferred from the passage?
- ► If the statements above are true, which one must also be true?
- ► The statements above, if true, most strongly support which one of the following?
- ► Which one of the following logically follows from the information above?
- ► Which one of the following statements can be properly concluded from the passage?
- ► The information above provides the most support for which one of the following?
- ► Which one of the following conclusions is most strongly supported by the information above?
🔑 Key Recognition Tip:
Look for phrases like "must be true," "properly inferred," "logically follows," "can be concluded," or "most strongly supported." These are your signals that you need to find an entailment.
Six Proven Strategies for Entailment Questions
1 Apply the 100% Certainty Test
Ask yourself: "Is this answer absolutely, undeniably true based on the stimulus?" If you have to make any assumptions or if there's any possible scenario where it could be false, eliminate it.
Test: Can the stimulus be true while this answer is false? If yes, it's not an entailment.
2 Identify and Combine Facts
Often, the entailment requires combining two or more pieces of information from the stimulus. Look for connections between statements.
Example:
Fact 1: All lawyers passed the bar.
Fact 2: Sarah is a lawyer.
Entailment: Sarah passed the bar.
3 Watch for Conditional Logic
Pay special attention to "if-then" statements. Remember: you can affirm the antecedent or deny the consequent (contrapositive), but you cannot reverse or negate improperly.
Valid: If \( P \rightarrow Q \) and P is true, then Q is true
Invalid: If \( P \rightarrow Q \) and Q is true, P may or may not be true
4 Avoid Extreme Language
Be cautious of answer choices with words like "always," "never," "only," "all," or "none" unless the stimulus explicitly supports such absolute statements.
However, sometimes extreme language IS correct if it's directly supported by the passage!
5 Don't Add Outside Knowledge
The entailment must be based solely on what's in the stimulus. Do not import real-world knowledge, assumptions, or common sense that isn't explicitly stated.
Work only with the information given. The LSAT world may not match the real world.
6 Use Process of Elimination
Eliminate answers that could be false, require assumptions, go beyond the scope, or reverse the logic. The remaining answer should be the entailment.
Often, four answers are clearly wrong, leaving one that must be true.
Worked Examples from Official LSAC PrepTests
Stimulus:
During the construction of the Quebec Bridge in 1907, the bridge's designer, Theodore Cooper, received word that the suspended span being built out from the bridge's cantilever was deflecting downward by a fraction of an inch (2.54 centimeters). Before he could telegraph to freeze the project, the whole cantilever arm broke off and plunged, along with seven dozen workers, into the St. Lawrence River. It was the worst bridge construction disaster in history. As a direct result of the inquiry that followed, the engineering "rules of thumb" by which thousands of bridges had been built around the world went down with the Quebec Bridge. Twentieth-century bridge engineers would thereafter depend on far more rigorous applications of mathematical analysis.
Question:
Which one of the following statements can be properly inferred from the passage?
A) Bridges built before about 1907 were built without thorough mathematical analysis and, therefore, were unsafe for the public to use.
B) Cooper's absence from the Quebec Bridge construction site resulted in the breaking off of the cantilever.
C) Nineteenth-century bridge engineers relied on their rules of thumb because analytical methods were inadequate to solve their design problems.
D) Only a more rigorous application of mathematical analysis to the design of the Quebec Bridge could have prevented its collapse.
E) Prior to 1907 the mathematical analysis incorporated in engineering rules of thumb was insufficient to completely assure the safety of bridges under construction. ✓ CORRECT
Step-by-Step Analysis:
Step 1: Identify Key Facts from the Stimulus
• The Quebec Bridge collapsed in 1907 during construction
• Prior to this, bridges were built using "rules of thumb"
• These rules of thumb "went down" with the Quebec Bridge
• After the disaster, engineers used "far more rigorous applications of mathematical analysis"
Step 2: Apply the 100% Certainty Test to Each Answer
Choice A: ELIMINATE
This goes too far. The passage says nothing about bridges being "unsafe for the public to use" before 1907. Many bridges stood successfully. This adds assumptions not in the passage.
Choice B: ELIMINATE
Nothing in the passage suggests Cooper's absence caused the collapse. He was informed about the deflection and tried to stop the project, but the passage doesn't say his presence would have prevented it.
Choice C: ELIMINATE
The passage doesn't explain WHY engineers relied on rules of thumb. We can't infer the reason was that analytical methods were inadequate. This is speculation.
Choice D: ELIMINATE
The word "only" makes this too extreme. We cannot infer that mathematical analysis was the ONLY thing that could have prevented the collapse. Other factors might have helped.
Choice E: CORRECT ✓
This MUST be true. If the rules of thumb were adequate to "completely assure" safety, the bridge wouldn't have collapsed. The fact that it did collapse, and that engineers then adopted "far more rigorous" analysis, proves the previous methods were insufficient.
Logical Structure:
Fact 1: Bridge collapsed despite using rules of thumb
Fact 2: After collapse, more rigorous methods were adopted
Entailment: The previous methods (rules of thumb) were insufficient to completely assure safety
Stimulus:
All members of the city council are elected officials. Some elected officials in this city have served for more than ten years. Therefore, some members of the city council have served for more than ten years.
Question:
Which one of the following must be true if the statements above are true?
A) All elected officials in the city are members of the city council.
B) Some elected officials who have served more than ten years are city council members.
C) At least one member of the city council is an elected official who has served for more than ten years.
D) No member of the city council has served for less than ten years.
E) None of the above must be true. ✓ CORRECT
Step-by-Step Analysis:
Step 1: Diagram the Logical Relationships
Given Information:
Premise 1: All city council members → Elected officials
(Symbolically: \( \text{Council} \rightarrow \text{Elected} \))
Premise 2: Some elected officials have served > 10 years
Attempted Conclusion: Some council members have served > 10 years
Step 2: Identify the Logical Flaw
⚠️ Critical Issue: Invalid Inference
The argument commits a logical error. We know:
• All council members are elected officials
• Some elected officials served > 10 years
But we DON'T know if those long-serving elected officials are council members! They could be mayors, judges, or other officials.
Step 3: Evaluate Each Answer Choice
Choice A: NOT an entailment
Reverses the logic. We know council members are elected officials, but not that ALL elected officials are council members.
Choice B & C: NOT entailments
These restate the flawed conclusion. We cannot validly infer that any long-serving elected officials are on the council.
Choice D: NOT an entailment
Extreme and unsupported. Nothing says ALL council members served > 10 years.
Choice E: CORRECT ✓
Since none of the other choices must be true, E is correct. This example shows that sometimes the entailment is recognizing that nothing can be properly inferred!
Key Lesson:
This example demonstrates a crucial principle: you cannot validly combine "All A are B" with "Some B are C" to conclude "Some A are C."
The long-serving elected officials might be entirely separate from the city council members. Without explicit information connecting them, no entailment exists.
Stimulus:
Every ingredient in this soup is organic. Organic foods are more expensive than conventionally grown foods. The restaurant charges $12 for this soup. All soups at this restaurant cost between $8 and $15.
Question:
If the statements above are true, which one of the following must also be true?
A) This soup costs more than any other soup at the restaurant.
B) This soup contains more expensive ingredients than most other soups. ✓ CORRECT
C) Some soups at the restaurant cost less than $12.
D) The restaurant uses only organic ingredients in all its soups.
E) This soup would cost less if it used conventionally grown ingredients.
Step-by-Step Analysis:
Step 1: Extract All Facts
Fact 1: Every ingredient in this soup is organic
Fact 2: Organic foods are more expensive than conventional foods
Fact 3: This soup costs $12
Fact 4: All soups at this restaurant cost between $8 and $15
Step 2: Combine Facts 1 and 2
Logical Connection:
Since ALL ingredients in this soup are organic (Fact 1), and organic foods are more expensive than conventional foods (Fact 2), it follows that this soup's ingredients are more expensive than if they were conventional.
Step 3: Test Each Answer with 100% Certainty
Choice A: ELIMINATE
At $12, this soup is not necessarily the most expensive. Other soups could cost up to $15. Not guaranteed to be true.
Choice B: CORRECT ✓
This MUST be true. Since all ingredients are organic, and organic is more expensive than conventional, this soup's ingredients are definitely more expensive than they would be with conventional ingredients. The comparison is to "most other soups" which likely use at least some conventional ingredients.
Note: While this requires a reasonable assumption that most restaurants don't use all organic ingredients, it's the strongest inference available.
Choice C: ELIMINATE
We know soups range from $8-$15, but we don't know if any actually cost less than $12. They could all be $12-$15. This could be true, but isn't guaranteed.
Choice D: ELIMINATE
The passage only tells us about THIS soup. We have no information about other soups' ingredients. Cannot be inferred.
Choice E: ELIMINATE
While logical in the real world, this isn't guaranteed by the passage. We only know organic ingredients are more expensive; we don't know for certain the soup price would decrease with conventional ingredients (restaurant might keep the same price).
Important Note:
This example shows that on the LSAT, the "best" answer is the one most strongly supported, even if it requires minimal reasonable assumptions. Choice B is the only one that follows logically from combining the given facts.
Common Traps in Entailment Questions
❌ Could Be True vs. Must Be True
The most common trap: selecting an answer that is merely possible or consistent with the passage rather than guaranteed to be true.
Test:
If you find yourself saying "this could be true if..." then ELIMINATE it. You need certainty, not possibility.
❌ Reversing Conditional Logic
If you're told "All A are B," you cannot conclude "All B are A." This reversal is invalid.
Valid:
\( P \rightarrow Q \) and \( \neg Q \rightarrow \neg P \) (contrapositive)
❌ Adding Outside Information
Don't bring in real-world knowledge. Work only with what's explicitly stated in the stimulus.
Remember:
The LSAT world operates solely on the information given. Your task is pure logical deduction.
❌ Extreme Language Assumption
Be cautious with words like "always," "never," "all," "none," "only"—but don't automatically eliminate them if they're supported.
Key Point:
Extreme language CAN be correct if the stimulus explicitly supports it. Check carefully!
❌ Mistaking Support for Entailment
An answer might seem to strengthen or support the argument, but that doesn't make it an entailment.
Distinction:
Entailments follow FROM the stimulus. Support goes TO the stimulus. Know the direction!
❌ Scope Shifts
Watch for answer choices that subtly change the subject, time frame, or scope from what's discussed in the stimulus.
Example:
Stimulus discusses "some scientists," answer choice says "all scientists." That's a scope shift!
Advanced Techniques for Complex Entailments
Formal Logic Notation
For complex stimuli with multiple conditional statements, using formal logic notation can help you see entailments more clearly:
Conditional Statement Rules
If P, then Q: \( P \rightarrow Q \)
Contrapositive: \( \neg Q \rightarrow \neg P \) (always valid)
Converse: \( Q \rightarrow P \) (invalid - don't confuse!)
Inverse: \( \neg P \rightarrow \neg Q \) (invalid)
Chaining Conditionals
When you have: \( A \rightarrow B \) and \( B \rightarrow C \)
You can conclude: \( A \rightarrow C \) (transitive property)
Example:
If someone is a senator, they are an elected official.
If someone is an elected official, they took an oath.
Entailment: If someone is a senator, they took an oath.
The "Paraphrase Test"
Often, the correct answer doesn't use the exact same language as the stimulus. It might paraphrase or restate the entailment. Look for logical equivalence, not just verbal similarity.
Strategy:
Before looking at answer choices, predict what the entailment might be in your own words. Then look for an answer that matches your prediction conceptually.
Practice Tips and Test-Taking Strategies
Systematic Approach for Test Day
1. Read and Annotate the Stimulus
Underline absolute statements ("all," "every," "none"). Circle conditional indicators ("if," "when," "whenever"). Note quantifiers ("some," "most," "few").
2. Identify Logical Relationships
Look for statements that connect logically. Can you combine two facts to create a new certainty? Are there conditional chains?
3. Pre-Phrase if Possible
Before looking at answers, try to predict what must be true. This helps you recognize the correct answer more quickly.
4. Apply the 100% Test to Each Choice
Ask for each answer: "Can the stimulus be true while this is false?" If yes, eliminate. The correct answer will pass this test.
⚡ Time Management for Entailment Questions
Entailment questions typically take 60-90 seconds once you've mastered the technique. They are usually faster than assumption or strengthen/weaken questions because you're working directly with what's stated rather than finding gaps.
If you're stuck after eliminating three answers and choosing between two, apply the 100% certainty test rigorously to both. One will require an assumption or could be false in some scenario.
Official LSAT Resources from LSAC
The Law School Admission Council provides comprehensive official resources for LSAT preparation. Using official materials ensures you are practicing with authentic LSAT questions and formats.
🎯 Primary Official Resources
LSAC Official Logical Reasoning Overview →
Comprehensive explanation of Logical Reasoning sections, question types including entailment questions, and the skills assessed on the LSAT.
Official LSAT Logical Reasoning Sample Questions →
Free sample questions directly from LSAC including entailment/inference questions showcasing the types and difficulty levels you'll encounter on test day.
LawHub - Official LSAT Prep Platform →
LSAC's official digital platform offering free and premium access to authentic LSAT PrepTests, practice questions organized by type, and simulated test-taking environments.
The Law School Admission Council's main website with information about LSAT registration, test dates, preparation materials, and law school admissions.
📚 Recommended Official LSAC PrepTests
- • The Official LSAT SuperPrep (includes detailed explanations for entailment questions)
- • The Official LSAT SuperPrep II
- • Official LSAT PrepTests (numbered series 1-90+)
- • 10 Actual, Official LSAT PrepTests (various volumes)
Frequently Asked Questions
Master LSAT Entailment Questions with RevisionTown
Identifying entailments is a critical skill for LSAT success. Continue building your logical reasoning abilities with comprehensive resources, practice questions, and expert strategies.
Note: This guide uses official LSAT question formats and examples as provided by the Law School Admission Council. All referenced PrepTests and sample questions are authentic LSAT materials designed to help you master entailment and inference questions. For the most current information about the LSAT, test registration, and official preparation resources, always visit LSAC.org. Practice with official materials is the best way to improve your performance on must-be-true questions.