In this chapter we’re going to teach formal logical to better answer inference questions.

CHALLENGE: Identify Inferences

The LSAT tests formal logic through Must Be True questions. They’re identifiable through their distinctive question stems:

• Which of the following must be false?
• Which of the following is most strongly supported?
• If the statements above are true, which of the following must also be true?
• Which of the following may be correctly inferred?
• Which of the following inferences (inference means the same thing as “must be true” on the test) is best supported by the statement made above?
(Conclusions differ from inferences in that conclusions are the result of premises and inferences must be true if the premises are true).

How to solve

1. Diagram out the statements of the argument (if you can’t get a quick grasp of it in your head).
2. Make valid inferences from these statements (transitive property and contrapositive). Note any false inferences, if any, such as the Fallacy of the Converse and Fallacy of the Inverse. To understand these concepts, we’ll be deep diving into formal logic in this chapter.
3. In Must be True questions you’ll find four choices that can sometimes be wrong and one that can never be wrong (that’s the correct choice!). Go through every answer choice systematically and check if it is ALWAYS true. These questions should always be tackled using PoE (process of elimination) method.  If you can find a reasonable situation when it is not always true, eliminate it. Gradually eliminate answer choices until you have one left.

B. Cannot Be True

The opposite of Must be True is Cannot be true. In Cannot be True questions you will find four choices that are correct and one choice that can never be correct. So, you just run through the choices with Process of Elimination to knock the four workable ones and select the one answer that can never work.

These Must be true-type questions force you to follow highly specific logical rules. Most logical reasoning questions use soft and fuzzy inductive reasoning (with observations, lots of unstated assumptions, etc). Deductive reasoning is what a computer would do: hard, logic-following rules. You often use deductive reasoning in the legal field.

1. You were on the neighbor’s property. (Premise)
2. It is against trespassing laws to be on the neighbor’s property. (Premise)
3. You are therefore guilty of trespassing. (Conclusion)

Not much wiggle room there! That’s why these are must be true or must be false questions. We are dealing with certainty. To solve these questions we need to first provide a short course in formal logic and bring you up to speed. These are logical rules that you’ll need to understand for both the Logical Reasoning and Logic Games sections.

Deductive vs. Inductive Reasoning

Rule #1: All A are B
All LSATs are hard tests.

Valid Inference: All non-B’s are non-A’s.
A test that is not hard is not a LSAT.

Valid Inference: No non-B is an A.
No test that is not hard is a LSAT.

Invalid Inference: No non-A’s are B’s.
No non-LSAT tests are hard.

Invalid Inference: All B are A.
All hard tests are LSAT tests.

Rule #2: Some A are B
Some programs are part-time programs.

Valid Inference: Some B are A
Some part-time programs are MBA programs.

Invalid Inference: Some A are not B
Some MBA programs are not part-time.

Invalid Inference: Some B are not A
Some part-time programs are not MBA programs.

Rule #3: Some A are B and Some B are C
Some A are B: Some MBA programs are part-time programs.
Some B are C: Some part-time programs are poetry degrees.

Valid Inference: Some B are A
Some part-time programs are MBA programs.

Valid Inference: Some C are B
Some poetry degrees are part-time programs.

Invalid Inference: Some A are C
Some MBA programs are poetry degrees.

Invalid Inference: Some C are A
Some poetry degrees are MBA programs.

Why are C and D invalid? Although A and C share B, they don’t necessarily share the part of B that links both of them.

Rule #4: Some A are B and all B are C
Some A are B: Some MBA programs are accounting programs.
All B are C: All accounting programs are math-intensive programs.

Valid Inference: Some B are A
Some accounting programs are MBA programs.

Valid Inference: Some A are C
Some MBA programs are math-intensive programs.

Valid Inference: Some C are A
Some math-intensive programs are MBA programs.

Invalid Inference: All C are A
All math-intensive programs are MBA programs.

Invalid Inference: All C are B
All MBA programs are math-intensive programs.