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dalaal on February 22 at 05:50PM

Regarding argument completion drills

In Argument Completion Drills, I did not quite understand the following conclusion: P1: X -> not Y P2: X -> Y Con: X does not exist How did you reach this conclusion, what is the logic behind it? Another question is regarding the following drill: P1: A exists P2: Not B -> not C P3: A -> C Is the conclusion only the fact that B exists, or should we say both B & C exist? On the LSAT, are conclusions only the end result since perhaps the fact that C exists could be considered as a subsidiary conclusion?

1 Reply

on February 23 at 02:03AM

Hello @Dalaal,

I think that the first one is kind of a trick question. X comes with two necessary conditions that are contradictory: Y and not Y. This means that X is impossible, it cannot exist. Let's try taking the contrapositive of the premises.

P1: Y - -> not X
P2: not Y - -> not X

If Y exists or Y doesn't exist, either way we have the same conclusion: X does not exist.


P1: A exists
P2: not B - -> not C
P3: A - -> C

Taking the contrapositive of P2, we have C - -> B.
A - -> C - -> B
A exists, therefore B exists.

For the purpose of these drills, we want to use all of the premises. Because of this, I can tell that [B exists] is the answer they are looking for. It is the main conclusion.

However, you are correct that [C exists] is another possible conclusion. I would call it a sub-conclusion! Good thinking. Sometimes a logical reasoning question will ask you to identify a subsidiary conclusion, so it is important to understand what this means.