Logic on the LSAT: Two Types

This article is the fourth in a lesson series on LSAT Arguments. This article will discuss the two main types of logic used on the LSAT test. Ah, Logic on the LSAT.. what could be more fun than logic on the LSAT? 

This lesson series covers the following articles:

LSAT Arguments: An Introduction
Structure of LSAT Arguments
Finding LSAT Assumptions
Logic on the LSAT: Two Types
Practice LSAT Arguments

Logic on the LSAT

You will find two types of logic on the LSAT, formal and casual. Formal logic is almost mathematical. Formal premises tend to be simpler, yet more extreme in their wording. The following is an example of formal logic.

All clasps are red. Red items go in box number 3. Therefore, clasps go in box number 3.

This type of reasoning is short and straightforward. In this example, nothing is left out or assumed. If you accept the premises to be true, then the conclusion must also be true. The clasps must go in box number 3. With a little practice, many TestSherpa students find formal logic to be fairly simple. You will use formal logic almost exclusively in the Analytical Reasoning section of the LSAT test. Unfortunately, since most arguments in real life are not stated so simply, the LSAT does not use a lot of formal logic for the arguments in Logical Reasoning. In fact, you’ll be lucky if you get one or two in a section.

Most of the arguments in the Logical Reasoning section are casual. Casual arguments are typical of arguments you read or hear everyday. They aren’t mathematical and they aren’t airtight. They are expressed using less formal language and often have unstated assumptions. The same argument above could also appear as a casual argument.

Working the night shift can be so frustrating. I told Peter that the clasps go in box number 3, but he won’t listen to me. Everyone knows we only use red clasps, and the foreman told us to put red parts in box number 3.

Even though this argument is stated casually, it helps to paraphrase it in your own words, possibly in a more formal fashion. It’s easier to think about the argument in your own words.
 
Clasps in Box 3 . Why? Clasps are red and red things go in box 3.

Note that the first sentence is just padding, and we left it out of the paraphrase. Formal thinking TestSherpa students even write a paraphrase next to the question in the test booklet:

Red=3 and Clasp=Red, therefore Clasps=3

You don’t have to be so formal, and not every question requires you to write a quick paraphrase. It’s just easier to think about and remember your own paraphrase than it is to dig through a long casual argument several times.

In the next and final article in the series — brace yourself, it’s a long one — we’ll see some Practice LSAT Arguments.