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Why “Unless” Matters on 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 understanding the use of “unless” on the LSAT. If you would like to download the full guide, please use the form at the bottom of this post.


Some of the trickiest conditionals on the test involve the word “unless;” yet, somehow, most English-speakers can operate conditionals in everyday life using this word flawlessly.

Here are step-by-step instructions on how to diagram “unless”
  1. Take whatever term follows the “unless” and put it to the right of the arrow
  2. Take the rest of the conditional and put it to the left of the arrow

(3.) Negate (or Un-negate) the term on the left.

Let’s do two together:

Unless I’m mistaken, that painting was not here last time.

  1. [other term] –> I’m Mistaken
  2. That Painting Was Not Here –> I’m Mistaken
  3. That Painting Was Here –> I’m mistaken

In step 3, you can see the sensible conclusion – if that painting were here, then I would be mistaken. We can then infer the contrapositive: if I’m not mistaken, then the painting was not here. Let’s look at one more.



  1. The party will be a success unless we run out of dip.
  2. Unless the train is late, we won’t arrive in time to see Margerie.


  1. ~Success –> Ran Out of Dip. ~Run out of dip —> Success
  2. See Margerie –> Train is Late. ~Train is Late –> ~See Margerie.

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