Logical Reasoning
LSAT Assumption Questions
Find the unstated premise the argument depends on
What It Is
Every argument has gaps -- unstated beliefs the author takes for granted. Assumption questions ask you to find the premise the argument requires but never states. Two types: Necessary Assumption (must be true for argument to work) and Sufficient Assumption (if true, guarantees the conclusion follows).
How to Identify the Question Type
- The argument assumes which of the following?
- The conclusion follows logically if which of the following is assumed?
- Which of the following is an assumption required by the argument?
The Negation Test
For Necessary Assumption: negate each answer choice. If negating it destroys the argument, that is your answer. If the argument survives without it, eliminate it. This is the most reliable tool for this question type.
Attack Strategy
- Step 1: Identify the conclusion and premises
- Step 2: Find the gap -- what concept in the conclusion was not established by the premises?
- Step 3: The assumption bridges this gap
- Step 4: For Necessary Assumption: apply the Negation Test to confirm
- Step 5: For Sufficient Assumption: look for an answer that, added as a premise, makes the conclusion airtight
Example Problem
Stimulus: 'Sales of electric vehicles have tripled in five years. Therefore, environmental awareness among consumers has increased.' Gap: The argument assumes EV purchases are driven by environmental concern, not cost savings, tax credits, or technology appeal. A necessary assumption: EV buyers are primarily motivated by environmental concerns.
Common Wrong Answer Traps
- Answers that strengthen the argument but are not strictly required
- Answers introducing concepts not present in the original argument
- Answers that are true in the real world but irrelevant to this specific gap
Top Tips
- The assumption will always bridge the exact gap between premises and conclusion
- Never bring in outside information -- the assumption must come from the argument's own logic
- Sufficient Assumption answers tend to be broader and more conditional than Necessary Assumption answers