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Understanding Necessary in the Context of the LSAT

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 if/then statements as sufficient/ necessary. If you would like to download the full guide, please use the form at the bottom of this post.

We’ve talked already about basic “if/then” conditionals, which we diagrammed as

If –> Then

And, if we think about the conditionals we talked about in the last section, we might call them all SUFFICIENT conditions. In the example

Brooklyn –> New York City

Being in Brooklyn is SUFFICIENT for being in New York City. Do you need to be in Brooklyn in order to be in New York City? Of course not! New York City is very large and Brooklyn is only one part of it. However, it is necessary to be in New York City in order to be in Brooklyn. So another way of looking at a normal if/then conditional is

Sufficient –> Necessary


Diagram the following two examples:

  1. Todd needs an A on this midterm in order to pass the class.
  2. Anyone who takes the LSAT in June will take it in the afternoon.


  1. Pass the Class –> A on Midterm. (the A is necessary)
  2. LSAT in June –> Take in Afternoon. (June LSAT is sufficient for an afternoon examination)

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