Logical Reasoning – Assumptions
LSAT India – Study Notes
1. Simple Explanation
In a Logical Reasoning (LR) argument, assumptions are the hidden premises that the author relies on to make the conclusion seem inevitable.
- Without these assumptions, the argument’s conclusion would no longer follow from the stated evidence.
- In LR questions, you’re usually asked to identify the single assumption that is required for the argument to hold, or to eliminate a statement that would make the argument invalid.
2. Key Points
| # | Point | Why It Matters |
|---|---|---|
| 1 | Definition | The assumption is an unstated premise that connects the evidence to the conclusion. |
| 2 | Location | Usually hidden in the gap of a “Assumptions” question or implied by the wording of the argument. |
| 3 | Types of Assumptions | 1. Cause (X causes Y) 2. Correlation → Causation 3. Exclusion (No other cause) 4. Uniformity (Assuming all cases behave like the example) 5. Magnitude (Assuming a particular magnitude of effect) |
| 4 | Logical Form | E ⇒ C (Evidence → Conclusion). Assumption A is needed so that E ∧ A ⇒ C holds. |
| 5 | Common Traps | • The answer choice that adds information rather than bridging the gap. • The answer choice that contradicts the conclusion. |
| 6 | Strategy | 1. Read the argument fully. 2. Identify the conclusion. 3. List the explicit premises. 4. Ask yourself: “What else must be true for the conclusion to follow?” 5. Match that need to the answer choices. 6. Eliminate options that are irrelevant or contradictory. |
| 7 | Answer‑Choice Structure | Typically 5 options: A, B, C, D, E. The correct one is the only statement that, when added, makes the argument logically valid. |
| 8 | Time Management | Spend ~45 s per assumption question. Use the “quick elimination” technique to narrow to 2–3 options before detailed analysis. |
3. Worked Example
Prompt
Because the city council voted unanimously to approve a new zoning ordinance, the city’s downtown area will experience a dramatic increase in residential construction.
The ordinance specifically allows mixed‑use buildings in the downtown district, a change that has not been previously enacted in any other district of the city.
Question:
Which of the following, if true, is an assumption that the argument relies on?
A. The new mixed‑use zoning ordinance will not be repealed in the next election.
B. The downtown area currently has no residential buildings.
C. All zoning ordinances in the city are enacted by a unanimous vote.
D. Mixed‑use buildings are more profitable for developers than single‑family homes.
E. The city has already experienced a similar dramatic increase in construction after a prior ordinance.
Solution
-
Identify the conclusion:
The city’s downtown area will experience a dramatic increase in residential construction.
-
Identify the explicit premise:
The ordinance specifically allows mixed‑use buildings in the downtown district.
-
Ask what else is needed:
The ordinance allows mixed‑use buildings, but that alone does not guarantee construction will increase. We need to assume that developers will actually build mixed‑use buildings in the downtown area. -
Match to answer choices:
- A – talks about repeal, irrelevant.
- B – irrelevant.
- C – irrelevant.
- D – Mixed‑use buildings are more profitable … → If developers find them more profitable, they’re more likely to build them. This is the missing link.
- E – irrelevant.
-
Select the correct answer: D.
Explanation
The argument assumes that the economic incentive (profitability) drives developers to build mixed‑use structures, thereby causing the residential construction increase. Without that assumption, the ordinance alone would not guarantee construction.
4. Quick Revision Summary
| Step | What to Do | Checklist |
|---|---|---|
| 1. Locate the conclusion | Find the final claim the author wants you to believe. | ✅ |
| 2. List all explicit premises | Write down every stated fact. | ✅ |
| 3. Identify the logical gap | Ask: “What must be true, besides the premises, for the conclusion to follow?” | ✅ |
| 4. Scan answer choices | Look for the one that supplies that missing link. | ✅ |
| 5. Eliminate | Cross out: • Statements that add irrelevant data. • Statements that contradict the conclusion. |
✅ |
| 6. Confirm | Ensure the chosen answer makes the argument valid and no other choice does the same. | ✅ |
Tips for the Exam
- Practice Speed: Work on a timer; aim for 45 s per question.
- Use “Why?”: For each premise, ask “Why would this premise lead to the conclusion?” The answer will often hint at the assumption.
- Watch for “Natural” Answers: In many cases, the assumption is the most natural or intuitive reason why the conclusion would follow.
- Avoid “Edge” Answers: Choices that mention extreme cases or exceptions are rarely the assumption unless the argument explicitly relies on them.
End of Notes
Good luck, and keep practicing!