LSAT Prep Concept: The “UNLESS” Sufficient & Necessary Sentence

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The Logical Reasoning section is a golden section chock full of points that can be gathered up and added to your very important raw score. Therefore, I think for our LSAT prep today we should practice on yet another form of the Sufficient & Necessary sentence, the UNLESS sentence!

Let’s take for example:

1. Naz will be tired, unless she sleeps.

A trick I use that always helps me is to put an arrow through the unless, like so: unless >. Thus, whatever is to the left of the “unless” is the sufficient condition, and therefore, goes before the “unless,” and whatever is to the right of the “unless” is the necessary condition, and therefore, goes after the “unless.” BUT, you also need to negate the sufficient condition a.k.a. whatever is to the left of the “unless.” Let’s try it out:

not T ===> S

Thus, the sentence reads: If Naz is not tired, then she has slept.

Contrapositive: If she has not slept, then Naz is tired.

not S ===> T

Okay let’s try another one:

2. The cat will starve, unless Peter feeds her.

So remember, you need to put an arrow through the unless and negate whatever comes before it, like so:

The cat will starve, unless  ===> Peter feeds her.

So it will read: If the cat does not starve, then Peter has fed her.

not S ===> PF

Contrapositive: If Peter does not feed her, then the cat will starve.

not PF ===> S

Easy right? So let’s try out a different “UNLESS” sentence. The same rules apply. It just looks a little bit different:

 3. Unless Naz eats, she will get a headache.

Now, I know you might be scratching your head and thinking, well nothing comes before the “unless.” If you are faced with this situation, you need to consider the part of the sentence that is not immediately to the right of the “unless” as the sufficient condition. Therefore, “Naz eats” is what is to the right of the unless and “she will get a headache” is what is figuratively to the left of the “unless.”

Unless  ===> Naz eats, she will get a headache.

The sentence reads: If Naz does not get a headache, then she has eaten.

not H ===> E

Contrapositive: If Naz has not eaten, then she will get a headache.

not E ===> H

I hope that helped you a bit on the UNLESS form of the Sufficient & Necessary statement. Now onward! Practice your Logical Reasoning sections, my LSAT prep friends!

Happy Studying!

Naz signature Updated on Aug 18, 2016