Lesson 8 Formal Logic Drill pg. 31

[g_lawyered]

Hi,
I've read the full Formal Logic PDF and am working on the drills and reviewing with Answer Key. On #4 of the Drill: Additive Inference Drill, I realized I incorrectly wrote the diagram and the inference I made is incorrect but I don't understand how the inferences on the Answer Key are correct.

diagram: S T U V

Inference 1: T V
I don't understand why this inference is made because I thought we couldn't have 2 Inferences. Also because this is a compound statement I wasn't clear on which variable was in the middle to be able to make inference about. In this case T is a closed variable. How can we determine to use T to make inference? I don't think the PDF specified on the point of how or which closed variables will/won't lead to inference. Can someone elaborate on this method?

Inference 2: U S
I don't understand why this inference is made because since this is a compound statement I wasn't clear on which variable was in the middle to be able to make inference about. From the PDF explanation (pg. 27) I understood to start with open variables which in this case are S and V. So I'm confused on how can we determine to use closed variable U to make inference? I don't think the PDF specified on the point of how or which closed variables will/won't lead to inference. Can someone elaborate on this method?

Inference 3: S V
To clarify, this inference is made because in Compound Statements the open variables can be incorporated in the "Some" train to make inferences, correct?

Thanks in advance!

[Dave Killoran]

Hi G,

First, take a look at my reply to another of your questions here: https://forum.powerscore.com/lsat/viewtopic.php?t=33158 in this drill!

Second, let's look at some of your comments/questions:

Inference 1: T V
I don't understand why this inference is made because I thought we couldn't have 2 Inferences.

I don't see two Some inferences in a row here to make this one, so I'm not following you

Also because this is a compound statement I wasn't clear on which variable was in the middle to be able to make inference about. In this case T is a closed variable. How can we determine to use T to make inference? I don't think the PDF specified on the point of how or which closed variables will/won't lead to inference. Can someone elaborate on this method?

Closed variables are not the ones we start with to make inferences but that doesn't mean they should be ignored! For example, look at this relationship from above:

T U V

Using the Some Train, the inference there is T V. which is exactly as the answer key notes Just because T was in the middle meant we didn't initially start there, but you can use those for inferences just like any other variable.

Inference 2: U S
I don't understand why this inference is made because since this is a compound statement I wasn't clear on which variable was in the middle to be able to make inference about.

It's from this portion:

S T U

Ride the Some Train from U to S to see how the inference is made

Inference 3: S V
To clarify, this inference is made because in Compound Statements the open variables can be incorporated in the "Some" train to make inferences, correct?

Correct!


Formal Logic is tough so keep working hard, but start with the Some train first—it's key! Thanks!

[g_lawyered]

Hi Dave,
After re-reading the "Some" Train now it makes things a bit clearer. I think my struggle was using "Some" Train in compound statements (especially on how to use the closed variables). If I understood your explanation correctly, we can break apart part of the compound statements from 1 of the open variables and use the "Some" Train to make inference.
So in this case, the original diagram: S T U V

We break apart the open variable S and start with T to read: T U V. Then we use the "Some" Train to infer: T V

We also break apart the open variable V and start with U to read: S T U. Then we use the "Some" Train to infer: U S.

Is this the standard way to use closed variables to make inferences? Want to make sure I have this down to practice on other drills.
Thanks again!

[Dave Killoran]

YES, right on the money! Well done