Formal Logic on the LSAT Formal Logic on the LSAT

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This is a post in a series that focuses on the LSAT. Each post in this series contains an excerpt from our new Guide to Formal Logic on the LSAT; this focuses on formal logic on the LSAT. If you would like to download the full guide, please use the form at the bottom of this post.

In LR, formal logic questions are questions that involve a long series of claims about the relationships among various groups. Writing out complex formal arguments takes time, so, for most students, it makes sense to save questions involving complex arguments for the end of the section and work on easier problems first.

Example: All chickens are birds, and most birds are diurnal. Some diurnal creatures hibernate during the winter, but not all hibernating creatures are diurnal.

Many students find these questions quite challenging; they involve a complex set of relationships which are almost impossible to keep track of mentally. However, with the diagramming techniques we’ll cover below, you’ll be able to simplify these arguments and answer questions on them correctly.

Understanding these relationships starts with understanding the basic terminology and how the LSAT uses these words.

All: Every single one. No exceptions. All dogs are mammals. All people in Brooklyn are in New York City.

Note that for the purposes of diagramming conditional statements, the word “all” is synonymous with “if.” All dogs are mammals = Dog –> Mammal.

Most: More than half, so anywhere from 50.1%- to 100%. Most people in the US live in urban areas. Most dogs have four legs. Most dogs are mammals.

That last example is a little weird and merits a comment. In normal conversational English, when we say “most,” we mean 50.1%-99%, not 100%. For example, if you said, “well, I like most of my family members,” you would be strongly implying that you don’t like a few jerks in the group. But the LSAT doesn’t do that. On the LSAT, “most” means more than half and as many as all. So it’s weird but technically correct to say, “It is true that all dogs are mammals. It is also true that most dogs are mammals.

 

EXAMPLES

  1. Most of the jurors are tired, and all tired people make errors in judgment.
  2. All of the newspapers covered the story, as did most radio stations.

SOLUTIONS

  1. Jurors most Tired. Tired —> Errors in Judgment
  2. Newspaper –> Covered Story. Radio most Covered Story.

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