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marinar96 on December 2, 2019

Missing premises

Is there a specific process on how to identify missing premises? The video does not really cover this and I am having trouble working on the Missing Premises Drills assignment.

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Skylar on December 8, 2019

@mresende, maybe I can help!

The Missing Premise Drills are designed to familiarize you with the pattern of logic discussed in this lesson. You can think of LSAT exam questions as word problems to which these drills are simplified versions. We have removed the words that students often find confusing and boiled it down to the basics- just the variables.

Your goal in the Missing Premise Drills is to connect everything you are given in order to make a new deduction about what is missing. First you should identify contrapositives and then you should proceed to look to connect variables.

I'll walkthrough the first drill to show you what I mean by this.

The first drill gives us:
P1: Y -> not A
P2: ?
C: Y -> B

Let's start by finding the contrapositives of the statements we are given,
P1: Y -> not A. The contrapositive of this is A -> not Y.
C: Y -> B. The contrapositive of this is not B -> not Y.

We now have:
P1: Y -> not A
A -> not Y
P2: ?
C: Y -> B
not B -> not Y

So, we need to find a way to connect P1 to C. We see that both P1 and C have an S->N statement where Y is the S condition. Therefore, we can connect the two N conditions to say: not A -> B.
P1: Y -> not A
P2: not A -> B
C: Y -> not A -> B
Y -> B

We could approach this from the contrapositive instead, if that comes more naturally to you. Here, we notice that the contrapositive of P1 and the contrapositive of C are both S->N statements where not Y is the N condition. Therefore, we can look to connect the two S conditions to say: not B -> A.
P1 contrapositive: A -> not Y
P2: not B -> A
C contrapositive: not B -> A -> not Y
not B -> not Y

So, the correct answer for missing P2 is:
not A -> B
not B -> A

Notice that these two statements are the contrapositive of each other. Since contrapositives have the same meaning, it does not matter which you put first as your answer as long as you have both.

Does this make sense? Please reach out with any other questions and best of luck with your studies!