LSAT Prep Concept: Sufficient & Necessary Formulas - No As Are Bs

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Okay, I know we’ve done this many a time, however, Sufficient & Necessary is the most commonly tested form of reasoning on the LSAT Test. So I think it deserves many, many posts dedicated to it so that you can continue to incorporate it into your LSAT prep. I want to go over some more variations of Sufficient & Necessary sentences to diagram. What do you think? You love it? Me too!

Let’s start out with one we know, the classic if/then ALL statement:

All mammals have fur.

M ===> F

If it doesn’t have fur, then it is not a mammal.

not F ===> not M

Now let’s go for a twist on this type of sentence.

No mammals are cold-blooded.

A good way to read the above sentence that could help you diagram it is: if it is a mammal, then it is not cold-blooded.

M ===> not CB

If it is cold blooded, then it is not a mammal.

CB ===> not M

The “No As are Bs” structure of the Sufficient & Necessary sentence is a tricky one for many LSAT prep students. A lot of times students will incorrectly diagram this sentence out as not A ===> B. This is why LSAC uses the “No As are Bs” structure a lot. It is tricky. But, if you take a simple logical statement that is true, for instance the above sentence of no mammals are cold-blooded, then you’ll see why it must be M ===> not CB, rather than not CB ===> M.

If the correct answer was to be not CB ===> M, then that means it must be true that if an animal is not cold-blooded, then it is a mammal.  However, this is not necessarily true. The animal can be a bird. Birds are not cold-blooded and not mammals. See? But, the sentence “if an animal is a mammal, then it is not cold-blooded,” must be true. Every mammal is warm-blooded, even the weird ones; for example, the platypus, the echidna, and the pangolin! Don’t know what a pangolin is? Check the image for this post out and see how far the mammalian world stretches!

Easy, right? So let’s do a couple more for practice:

1. All oranges are fruit.

O ===> F

not F ===> not O

2. No bananas are vegetables.

B ===> not V

V ===> not B

3. All gorillas are primates

G ===> P

not P ===> not G

4. No crocodiles are mammals.

C ===> not M

M ===> not C

Okay, you know what to do now...practice, practice, practice! So grab your practice LSAT questions and begin!

Happy Studying!

Naz signature Updated on Aug 18, 2016