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Setup and Rule Diagrams

LSAT Master
Posts: 247
Joined: Wed Jan 10, 2018 1:11 am
Points: 248

I was just wondering if my representation of the first and second rules are correct:
Jaclyn → Lorena (Contrapositive: Lorena → Jaclyn)
Lorena → Jaclyn (Contrapositive: Jaclyn → Lorena)

So the first rule indicates that both can't be on stage, and the second rule indicates that at least one is on stage (since both can't be on stage as stated in the first rule). So this is a bi-conditional in the form of A or B, but not both (A or B but not otherwise) which means either A is selected without B, or else B is selected without A:

Jaclyn ↔ Lorena
Jaclyn ↔ Lorena

Would this be right? So Jaclyn or Lorena is always in, and Jaclyn or Lorena is always out.
Brook Miscoski
PowerScore Staff
PowerScore Staff
Posts: 422
Joined: Thu Sep 13, 2018 6:38 am
Points: 421


You could do the rules as follows:

1. J :dblline: L
2. -L :dblline: -J

Where a positive is onstage and a negative (or cross-through) is offstage.

I don't see any value in talking about biconditionals. It's certainly not an "if and only if" statement.

Where the value is, is in recognizing that either J is onstage or J is offstage. The first rule establishes that J and L can't be onstage together, the second rule establishes that J and L can't be offstage together. That means that one of them is onstage and the other one is offstage in every valid scenario. Therefore, your final inference is correct.