LSAT Necessary and Sufficient Assumptions 4 — Questions and Answers
Question 1: An argument has premises P1 and P2 and conclusion C. A sufficient assumption SA is one where:
- P1 + P2 → SA
- P1 + P2 + SA → C (Correct answer)
- SA alone → C without needing P1 or P2
- C → SA must be true
Correct answer: P1 + P2 + SA → C
A sufficient assumption works together with the existing premises to guarantee the conclusion—it fills the gap that makes the argument valid.
Question 2: Which scenario demonstrates a necessary but NOT sufficient assumption?
- An assumption whose negation destroys the argument and whose affirmation guarantees the conclusion
- An assumption whose negation destroys the argument but whose affirmation does not alone guarantee the conclusion (Correct answer)
- An assumption that strengthens the conclusion without being required
- An assumption that is explicitly stated in the argument
Correct answer: An assumption whose negation destroys the argument but whose affirmation does not alone guarantee the conclusion
A necessary assumption is required for the argument but may not be enough on its own to make the conclusion certain.
Question 3: Argument: 'Organic farming avoids synthetic pesticides. Avoiding synthetic pesticides benefits local ecosystems. The Hendersons farm organically. Therefore, the Hendersons benefit local ecosystems.' This argument:
- Requires a necessary assumption about the Hendersons' motives
- Is already deductively valid with no unstated assumptions needed (Correct answer)
- Requires a sufficient assumption linking organic farming to ecosystem health
- Contains a logical flaw that no assumption can fix
Correct answer: Is already deductively valid with no unstated assumptions needed
The premises chain together logically to yield the conclusion, making this a valid deductive argument with no hidden gaps.
Question 4: A student negates an assumption and finds the argument becomes 'significantly weaker but not completely destroyed.' This suggests the assumption is:
- Clearly necessary
- Probably a strengthener, not a necessary assumption (Correct answer)
- Clearly sufficient
- Irrelevant to the argument
Correct answer: Probably a strengthener, not a necessary assumption
A necessary assumption, when negated, must completely undermine the argument—if it only weakens it, the statement is more like a strengthener.
Question 5: Argument: 'Violent video games desensitize players to violence. Desensitization to violence increases aggressive behavior. Therefore, playing violent video games increases aggressive behavior.' A necessary assumption is:
- All video games contain some violent content
- The desensitization caused by video games is the same type that increases aggression (Correct answer)
- Aggressive behavior always leads to violent crime
- Players are aware they are being desensitized
Correct answer: The desensitization caused by video games is the same type that increases aggression
The argument links two types of desensitization, so it must assume they are the same or equivalent for the chain to hold.
Question 6: In LSAT Logical Reasoning, 'Justify the Conclusion' questions are essentially asking for a:
- Necessary assumption
- Sufficient assumption (Correct answer)
- Causal explanation
- Principle that weakens the argument
Correct answer: Sufficient assumption
'Justify the Conclusion' is another label for Sufficient Assumption questions—the correct answer makes the conclusion logically follow.
Question 7: Argument: 'Professor Stein publishes frequently in top journals. Professors who publish frequently in top journals are respected researchers. Therefore, Professor Stein is a respected researcher.' Which best describes this argument's assumptions?
- It requires the necessary assumption that Stein enjoys research
- It requires no unstated assumption—it is deductively valid as stated (Correct answer)
- It requires a sufficient assumption about journal rankings
- It requires both a necessary and a sufficient assumption
Correct answer: It requires no unstated assumption—it is deductively valid as stated
The argument is a valid syllogism: the premises directly entail the conclusion with no hidden gaps.
LSAT Necessary and Sufficient Assumptions 4
An argument has premises P1 and P2 and conclusion C.
A sufficient assumption SA is one where: