Deductive vs. Inductive Reasoning

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 the LSAT often tests formal logic with must be true or must be false questions. We are dealing with certainty.

We’ve dealt with if then statements. Now we’re adding a level of complexity with some, all, and most.

Next LSAT: January 26

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 law school programs are part-time.

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

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

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

Rule #3: Some A are B and Some B are C
Some A are B: Some law school 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 law school programs.

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

Invalid Inference: Some A are C
Some law school programs are poetry degrees.

Invalid Inference: Some C are A
Some poetry degrees are law school programs.

Why are the last two 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 law school 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 law school programs.

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

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

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

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

Some/All Video Walkthrough Questions

This can be a confusing question, but all it’s really asking you to do is just use substitution (athletes for bankers).

Next LSAT: January 26