Supply the missing premise that makes the conclusion follow logically:P: A → not BP: ?C: B → C

Mehran February 12, 2016

Of course! Let's use an abstract example first:

P1: X ==> Y
P2: ?

C: Y

So here we are concluding that Y exists based off the premise that "if X, then Y."

But notice here that the premise "if X, then Y" is not sufficient to allow us to conclude that Y exists. Something is missing here.

In order to conclude that Y exists from the premise, "if X, then Y" we would need to know that X also exists because X is the sufficient condition that would guarantee the existence of the necessary condition (i.e. Y).

So the answer to this drill is X (or that X exists).

Let's now move from the abstract to an actual LSAT question:

"Maria won this year's local sailboat race by beating Sue, the winner in each of the four previous years. We can conclude from this that Maria trained hard."

The question stem on this question states, "The conclusion follows logically if which one of the following is assumed?"

This is a Strengthen with Sufficient Premise question, which is asking us to identify the missing premise that would 100% guarantee the conclusion.

So this is just like the missing premise drills in the abstract.

Let's break down the argument:

P1: Maria won by beating Sue, a four-time winner. (B4TW)
P2: ?

C: Maria trained hard. (TH)

Notice that just based on the premise that Maria won by beating a four-time winner, we would not be able to properly conclude that Maria trained hard.

Something is missing here. We need to find an answer choice that connects Maria winning by beating a four-time winner to training hard.

The correct answer here is, "Maria could beat a four-time winner only if she trained hard."

We know that "only if" introduces a necessary condition so we can diagram this statement as follows:

B4TW ==> TH
not TH ==> not B4TW

P1: B4TW
P2: B4TW ==> TH

C: TH

Notice that the missing premise here guarantees the conclusion by making the premise (i.e. B4TW) sufficient for the conclusion (i.e. TH).

Hope this helps! Please let us know if you have any other questions.

Sarah-Powell October 17, 2018

Do you have any videos going over a missing premise question?

nizhoni September 2, 2019

Please include some videos going over missing premise drills or questions.

wendylevinson January 31, 2020

I really would like an explanation of the abstracts, bc at this point, I am frustrated to the point of tears! I am not unfamiliar with logic and logic terminology, but that does not seem to be helping in these kinds of problems. I was taught to use the ~ sign to indicate "not," and during my 2 logic courses,we never did problems that included "some," it was either have or have not, These problems with the dual premise or attempting to figure out the conclusion would really be helpful if you went through a few either in a video going step-by-step, or even a message board post, but. I really need help here!